NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Summary and Findings from

the NREL/DOE Hydrogen 

Sensor Workshop (June 8, 2011) 

W. Buttner, R. Burgess, M. Post, and C. Rivkin

National Renewable Energy Laboratory

Technical Report

NREL/TP-5600-55645

July 2012
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

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Contract No. DE-AC36-08GO28308

Summary and Findings from

the NREL/DOE Hydrogen 

Sensor Workshop (June 8, 2011) 

W. Buttner, R. Burgess, M. Post, and C. Rivkin

National Renewable Energy Laboratory

Prepared under Task No. HT12.7210

Technical Report

NREL/TP- 5600-55645

July 2012
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Executive Summary

On June 8, 2011, the U.S. Department of Energy (DOE) and the National Renewable Energy 

Laboratory (NREL) hosted a hydrogen sensor workshop attended by nearly 40 participants from 

private organizations, government agencies, and academic institutions. The participants 

represented a cross section of stakeholders in the hydrogen community, including sensor 

developers, end users, site safety officials, and code and standard developers. The goals were to 

identify critical applications for the emerging hydrogen infrastructure that require or would 

benefit from hydrogen sensors, to assign performance specifications for sensors deployed in each 

application, and to identify shortcomings or deficiencies (i.e., technical gaps) in the ability of 

current sensor technology to meet the assigned performance requirements. Current (e.g., onboard sensors for hydrogen forklifts) and emerging (e.g., residential) applications were included. 

The workshop was structured into two parts. The morning session consisted of topical talks that 

provided background information about various emerging hydrogen energy applications, the 

certification and listing processes, and about strategies for sensor deployment. Several critical 

key application areas were specifically identified, and for each application, breakout groups were 

formed to identify critical performance metrics, assign values to specifications, and identify 

shortcomings or deficiencies in current sensors to meet these requirements. Three sequential 

breakout sessions were held, which allowed workshop attendees to participate and provide input 

into multiple topical areas. Several breakout groups met in parallel, which restricted the size of 

each group to eight or fewer participants and enabled open discussions. Each breakout group was 

chaired by a topic expert. The breakout topics and chairs were:

• Indoor Fueling and Operations (chaired by Albert (Snapper) Pouché Jr., Defense 

Logistics Agency Susquehanna) 

o Facility Monitoring

o In-Dispenser Deployment of Sensors 

• Storage (chaired by Robert Burgess, NREL) 

• Production (chaired by Joe Cohen, Air Products) 

• Residential (chaired by Carl Rivkin, NREL) 

• Battery Backup (chaired by Curtis Ashton, CenturyLink) 

o Dedicated Controlled Environmental Vaults 

o Shared-Use Buildings

• Industrial Trucks (chaired by Aaron Harris, Nuvera Fuel Cells) 

• Vehicles (William Buttner, NREL)

Sensor requirements for each application were defined based on feedback from the breakout 

groups. Although application specific, many requirement metrics overlap applications. For 

example, response time is a critical parameter for all applications; however, specifications vary. 

Sensors deployed as facility monitors (in warehouses, battery backup rooms, etc.) need a 

moderate response time (e.g., 30 seconds), whereas sensors deployed within enclosures 
iv

surrounding pressurized hydrogen (i.e., in dispensers or on-board forklifts proximal to an 

enclosed storage tank) require a fast response time (e.g., 1 second). A 30-s response time is 

readily achieved with current technology, whereas the 1-s response time remains problematic; 

some newer sensor models have fast response times but compromised performance for other 

critical parameters (e.g., linear range, long-term stability). Other key sensor performance 

shortcomings were reviewed. Several of these parameters cross cut over multiple applications, 

but are not necessarily universal to all applications. Outstanding sensor shortcomings include: 

• Analytical performance parameters

o Response time (1 s) 

o Cross sensitivity/poisons 

• Operational parameters

o Cost of maintenance and calibrations

o Alarm thresholds

• Deployment parameters

o Code requirements 

o Placement

o Point sensors versus wide area monitoring. 

The workshop and findings represent a critical first step in defining the sensor requirements for 

the emerging hydrogen infrastructure. Recognizing that additional applications may be identified 

that were not addressed at the workshop, or that sensor requirements may evolve, a Hydrogen 

Sensor Task Group was organized by the Fuel Cell and Hydrogen Energy Association and 

NREL, and is open to all stakeholders in the hydrogen community. The task group meets 

regularly via a teleconference link and web conference format.
v 

Abbreviations and Acronyms

AHJ Authority Having Jurisdiction (typically for permitting)

ASIL Automotive Safety Integrity Level

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers 

CEV Controlled Environment Vault 

CSA Formerly Canadian Standards Association, CSA is now the official name 

CFD Computational Fluid Dynamics

CGS Combustible Gas Sensor 

cm3

 Cubic Centimeter

CO Carbon Monoxide 

DOE U.S. Department of Energy 

DOT Department of Transportation 

FCV Fuel Cell Vehicle

ft2

 Square Foot (Feet) 

ft3 Cubic Foot (Feet) 

g Gram

h Hour 

IEC International Electric Code

IEEE Institute of Electrical and Electronics Engineers

ISO International Organization for Standardization

kg Kilogram

LDL Lower Detection Limit

LEL Lower Explosion Limit, defined for safety purposes to be equivalent to LFL 

LFL Lower Flammable Limit

MEM Microelectromechanical System

min Minute 

MOX Metal Oxide Sensor 

MPa Megapascal

NFPA National Fire Protection Association

NREL National Renewable Energy Laboratory 

OSHA Occupational Safety and Health Administration 

P Pressure 

ppmv Concentration unit of a component in a gas mixture, expressed as a fraction of the 

volume of the component to the total gas volume scaled by a factor of 106 

psi Pounds per square inch 

RH Relative Humidity
vi 

T Temperature

TCD Thermal Conductivity Detector

UE Utility Enclosure 

UL Underwriters Laboratories

VDC Volts of direct current

vol% Concentration unit of a component in a gas mixture, expressed as a volume 

 percentage of the component to the total gas volume
vii 

Contents

Executive Summary ....................................................................................................................... iii

Abbreviations and Acronyms ......................................................................................................... v

Tables ............................................................................................................................................. ix

1 Background .............................................................................................................................. 1

1.1. Hydrogen Safety Sensor Metrics ...................................................................................... 2

1.1.1. Analytical .................................................................................................................. 3

1.1.2. Logistical................................................................................................................... 3

1.2. The 2007 and 2011 DOE/NREL Hydrogen Sensor Workshops ...................................... 4

1.3. Report Format .................................................................................................................. 7

2 Applications ............................................................................................................................. 8

2.1 Industrial Trucks (Forklifts) ............................................................................................. 8

2.1.1 Background ............................................................................................................... 9

2.2 Indoor Fueling Operations ............................................................................................. 14

2.2.1 Background ............................................................................................................. 15

2.2.2 Facility Monitoring (Area Monitors) ...................................................................... 16

2.2.3 Internal Dispenser Sensor Deployment (Contained Environment Monitor) .......... 21

2.3 Residential ...................................................................................................................... 26

2.3.1 Background ............................................................................................................. 26

2.3.2 Critical Gaps and Deficiencies ................................................................................ 29

2.3.3 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization ...................... 30

2.4 Production ...................................................................................................................... 31

2.4.1 Background ............................................................................................................. 31

2.4.2 Distributed Hydrogen Production ........................................................................... 32

2.4.3 Centralized Hydrogen Production ........................................................................... 41

2.5 On-Board Light-Duty Road Vehicles ............................................................................ 42

2.5.1 Background ............................................................................................................. 42

2.5.2 Critical Gaps and Deficiencies ................................................................................ 45

2.5.3 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization ...................... 46

2.6 Battery Backup ............................................................................................................... 46

2.6.1 Background ............................................................................................................. 47

2.6.2 Critical Gaps and Deficiencies ................................................................................ 49
viii

2.6.3 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization ...................... 52

2.7 Storage ............................................................................................................................ 53

2.7.1 Background ............................................................................................................. 54

2.7.2 Critical Gaps and Deficiencies ................................................................................ 55

2.7.3 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization ...................... 56

3 Summary ................................................................................................................................ 59

3.1 Review of Gaps and Deficiencies .................................................................................. 66

3.2 Sensor Specification and Gap Level by Application ..................................................... 68

References ..................................................................................................................................... 71

Appendix A Definitions ........................................................................................................... 73

Appendix B The Hydrogen Sensor Workshop ........................................................................ 77

Appendix C Hydrogen Sensor Codes and Standards .............................................................. 80
ix

Tables 

Table 1. Risk Levels for Different Hydrogen Concentrations ..................................................... 2

Table 2. DOE Target Specifications for Hydrogen Safety Sensor R&D* ................................... 5

Table 4. Hydrogen Sensor Metric Rankings for Indoor Fueling – Facility Monitoring ............ 18

Table 5. Hydrogen Sensor Metric Rankings for Indoor Fueling – In-Dispenser Deployment .. 23

Table 6. Hydrogen Sensor Metric Rankings for Residential Applications ................................ 28

Table 7. Hydrogen Sensor Metric Rankings for Distributed Hydrogen Production – Facility 

(Area Monitors) ............................................................................................................ 34

Table 8. Hydrogen Sensor Metric Rankings for Distributed Hydrogen Production – InProduction Unit ............................................................................................................ 39

Table 9. Hydrogen Sensor Metric Rankings for Automotive Fuel Cells – On-Board Safety 

Sensors .......................................................................................................................... 44

Table 11. Hydrogen Sensor Metric Rankings for Battery Backup Applications – 

Telecommunication CEVs ........................................................................................... 51

Table 12. Hydrogen Sensor Metric Rankings for Hydrogen Storage – Outdoor ......................... 57

Table 13. Hydrogen Sensor Metric Rankings for Hydrogen Storage – Indoor ............................ 58

Table 14. Critical Sensor Requirements for Industrial Truck Applications – On-Board 

Deployment .................................................................................................................. 61

Table 15. Critical Sensor Requirements for Indoor Fueling – Facility Monitoring ..................... 61

Table 16. Critical Sensor Requirements for Indoor Fueling – In-Dispenser Deployment ........... 62

Table 17. Critical Sensor Requirements for Residential Applications ......................................... 62

Table 18. Critical Sensor Requirements for Production – Facility Monitoring ........................... 63

Table 19. Critical Sensor Requirements for Production – In-Dispenser Deployment ................. 63

Table 20. Critical Sensor Requirements for On-Board Light-Duty Road Vehicles ..................... 64

Table 21. Critical Sensor Requirements for Battery Backup – Dedicated CEVs ........................ 64

Table 22. Critical Sensor Requirements for Battery Backup – Multiple-Use Buildings ............. 65

Table 23. Critical Sensor Requirements for Hydrogen Storage – Outdoor .................................. 65

Table 24. Critical Sensor Requirements for Hydrogen Storage – Indoor .................................... 66

Table 25. Sensor Metric Rankings by Application and Sensor Gap Level .................................. 69
1 

1 Background 

Hydrogen has been recognized by the U.S. Department of Energy (DOE) Vehicle Technologies 

Program as one of six alternative and renewable fuels for advanced vehicles [1]. Its use as a fuel 

already has commercially viable markets, which include stationary power systems (e.g., backup 

power) and fuel cell-powered industrial trucks (e.g., forklifts). Both markets are expanding, and 

this growth is driving the development of hydrogen support systems that include transport and 

production capability, on-site storage, and on-site dispensers [2]. The use of hydrogen as an 

alternative fuel will continue to grow, especially with the pending deployment of light-duty road 

vehicles [3]. Automobile manufacturers in North America, Europe, and Asia project a 2015 

release of commercial hydrogen fuel cell-powered road vehicles. These vehicles will be for 

general consumer applications, albeit initially in select markets but with much broader market 

penetration expected by 2025 [4]. The necessary support systems need to be developed to 

implement these vehicles. For the consumer market this includes not only expanded hydrogen 

production capacity, transport, on-site storage, and dispensing (fueling) infrastructure, but also 

other infrastructure elements unique to general public deployment such as parking garages 

compatible for hydrogen vehicles (residential as well as large public facilities) and maintenance 

facilities. It is critical that the hydrogen market and infrastructure develop safely and efficiently

to ensure the successful deployment of hydrogen as an alternative fuel. 

Safety is recognized as a critical factor for the successful implementation of hydrogen as a fuel, 

and numerous expert working groups have been organized to ensure hydrogen safety is properly 

addressed in the field [5, 6, 7]. A reliable safety system is composed of various elements that can

include intrinsic design features (e.g., material specifications, pressure control systems, venting 

systems), engineering controls (e.g., sample size minimization, deployment location) and the use 

of hydrogen sensors to detect unexpected releases. Hydrogen gas is odorless and colorless; thus, 

its detection requires a sensing device with an audible alarm or other indication for personnel and 

property safety. Hydrogen safety sensors are typically separate from the main operational 

features of a system, such as part of a dispenser control system or an internal monitor of a 

storage facility, and thus can provide a critical independent assurance of safety. Sensors can also 

operate on backup power or batteries, and thus remain operational during power outages, which 

may not necessarily be the case for all other safety design elements. In addition to audible alarms 

or other indicators, hydrogen sensors may be used to activate site ventilation and initiate system 

shutdowns. Hydrogen sensors can thus be viewed as an enabling technology for the safe 

implementation of the emerging hydrogen infrastructure. 

Furthermore, the use of hydrogen safety sensors in indoor hydrogen fueling operations was 

explicitly mandated by the National Fire Protection Association (NFPA 52), Vehicular Gaseous 

Fuel Systems Code and more recently by NFPA 2, Hydrogen Technologies Code. In addition to 

safety applications, hydrogen sensors will play a role in process control operations. Typically, 

process sensors would be deployed in highly enriched hydrogen atmospheres, possibly up to 

100% hydrogen. Their signals could provide necessary feedback to improve the desired output. 

Applications may include monitoring of composition, purity, and metering. Process control 

sensors have relevance for production, dispensing, and controlling the fuel cell. Of the two 

applications, safety sensors have more short-term relevance for the implementation of the 

hydrogen infrastructure, and this was the focus of the 2011 Department of Energey/National 

Renewable Energy Laboratory (NREL) Hydrogen Sensor Workshop and this report. 
2 

Safety sensors are regularly used to indicate a potentially hazardous situation. The flammable 

range is 4–75 vol% hydrogen in air. Accordingly, sensors are normally set to alarm below the 

hydrogen lower flammable limit (LFL) of 4 vol% in air. Different risk levels can be associated 

with different ambient hydrogen concentrations, which can be qualitatively described as trace

(0.1 vol% H2 in air), low (0.4 vol% H2 in air), medium (1 vol% H2 in air), elevated (4 vol% H2) 

and high (8 vol% H2 in air), as summarized in the Table 1, where the hydrogen concentration is 

given as fraction of the LFL and in concentration units (percent by volume in air – vol% or parts 

per million by volume in air – ppmv). This categorization, which can guide alarm thresholds for 

hydrogen sensors, indicates that for safety applications hydrogen at or above the LFL represents 

a serious risk, and that providing separate distinctions for higher hydrogen concentrations (such 

as the lower explosion limit [LEL] of 17 vol%) is unwarranted. Furthermore, safety regulations 

often treat LFL and LEL as synonymous, using the more conservative concentration (4 vol%) or 

fractions thereof as the action (alarm) level. Hydrogen safety sensor alarm levels are usually set 

below the LFL, typically at either the low (0.4 vol% H2 in air, which is equivalent to 10% of the 

LFL) or medium (1 vol% H2 in air, which is equivalent to 25% of the LFL) condition. However, 

to provide risk level data to site and emergency response personnel, it may be necessary to 

measure the hydrogen at concentrations above the LFL, especially if the concentration may 

either be increasing or remaining stagnant at or above the LFL. 

Table 1. Risk Levels for Different Hydrogen Concentrations 

Condition 

(risk) % of LFL [H2], vol%

(in air)

[H2], ppmv 

(in air)

Trace 2.5 0.1 1,000

Low 10 0.4 4,000

Medium 25 1.0 10,000

Elevated 100 4.0 40,000

High >200 >8.0 >80,000

1.1. Hydrogen Safety Sensor Metrics

The selection of a sensor technology (or any other analytical system) will be based on several 

performance parameters (or metrics) as they pertain to a specific application. One can envision a 

large number of metrics, which for the most part can be divided into analytical and logistic

parameters. The most important and obvious performance parameters pertain to the analytical 

characteristics of the device, which are those that are directly related to the ability of the chosen 

technology to perform the analytical measurements required by the application. The logistic 

needs of the application will impose additional parameters to the sensor requirements, which can 

be demarcated into deployment and operational metrics. The deployment and operational 

parameters do not pertain to a sensor’s ability to accurately detect hydrogen, but rather to other 

factors that ultimately determine the suitability of a technology for a specific application. The 

deployment parameters pertain primarily to the installation or incorporation of the sensor 

technologies into a specific system and are typically one-time, upfront considerations. 

Alternatively, the operational parameters pertain to operation and maintenance of the sensors 

once they have been installed and are ongoing or recurring considerations. Analytical, 

deployment, and operational parameters identified as important criteria for hydrogen safety

sensor specifications are as follows: 
3 

1.1.1. Analytical

• Analytical parameters. Selectivity, Lower Detection Limit (LDL), Analytical 

Resolution, Linear Range (and Dynamic Range), Accuracy, Response Time, Recovery 

Time, Repeatability, Drift, Environmental Effects (e.g., temperature [T], pressure [P], and 

relative humidity [RH]), Reversibility, Limits of Quantization (Limits of Determination), 

Saturation Stability, Sensitivity

1.1.2. Logistical

• Deployment parameters. Capital Cost, Installation Costs, Placement, Physical Size, 

Control Circuitry, Power Requirement, Electronic Interface, Pneumatic Design, Shelf 

Life, Maturity/Availability, Regulations (Codes), Alarm Set Points

• Operational Parameters. Operational Lifetime, Consumables, Calibration and 

Maintenance Requirements, Sample Size, Matrix Requirements, Signal Management, 

Orientation Effect, Device to Device Repeatability, Warm Up Time, Alarm Interface, 

Mechanical Stability

Definitions for and supplemental information about these parameters are provided in Appendix 

A. Typically a quantitative or numerical specification is assigned to each. For example, a 1-s 

response time refers to the performance metric “response time” having the specification of 1 s.

Additional parameters may be identified and added to the list. Additional parameters may be 

incorporated, especially for unique applications with specialized requirements (e.g., sensor 

installed for crash test studies, which will need to withstand forces associated with sudden 

impacts [8]).

In all cases, the importance of each parameter and its assigned specification is application 

specific, and the relative importance of a specific parameter varies by application. Thus, a critical 

parameter for one application may have negligible importance for another. As an example, a 

process control sensor may be expected to precisely and accurately control hydrogen 

concentration and thus would have stringent analytical resolution requirements with minimal 

importance assigned to the LDL; in contrast, safety sensors deployed as area monitors must have

an LDL at a fraction of the alarm threshold, but would not have severe requirements to 

quantitatively resolve small changes in hydrogen concentrations. 

The application defines the sensor requirements, which thus must be explicity assessed for each 

application. Since not every parameter listed will necessarily have strong relevance for a specific 

application, the project engineer must determined the relative importance of all metrics for each 

application. Thus, a universal (short) list of critical sensor metrics with assigned specifications 

relevant for all applications is of limited value, and may be counterproductive; a 1-s response 

time may add cost and complexity to a sensor and yet be of limited value for a given application. 

Accordingly, the focus of the 2011 DOE/NREL hydrogen sensor workshop was to identify 

crucial applications (current and emerging) associated with the hydrogen infrastructure and then 

to define critical metrics and specification requirements for the various specific applications. The 

goal was not to define an all-encompassing set of metrics to cover every scenario. 

Sensor cost factors are decoupled from the analytical parameters. Cost requirements – not 

analytical requirements – are deployment and operational issues. This decoupling is consistent 

with allowing the requirements of the application to define the necessary analytical performance 
4 

metrics without influence from programmatic and administrative inputs (e.g., budget 

constraints). Nor does the list favor one technology over another, which is also consistent with 

allowing the application to define the critical requirements as opposed to refining the 

requirements by the capability or limitations of the detector. Of course, facility engineers must 

often compromise between needs and capability; for example, a maintenance-free sensor is a 

typical desire, but code and manufacturer recommendations will often mandate periodic 

calibration. Facility engineers must also have reasonable expectations about the specifications 

assigned to critical metrics. Thus, a low-cost sensor with minimal maintenance may be a 

reasonable requirement, but a maintenance-free sensor with zero operational costs is probably 

not realistic.

Sensitivity is a term commonly used to describe a sensor performance, and as such it is included 

in the list of possible analytical parameters. It can be a vague performance parameter, however, 

and other metrics (e.g., LDL, analytical sensitivity, and linear range/dynamic range) actually 

precisely define specific properties often associated with sensitivity. For example, sensitivity is 

often misused as synonymous with the LDL. Sensitivity is the electronic response of a sensor to a 

given amount of an analyte and has units of concentration divided by electronic signal (e.g., the 

slope of a calibration curve). It can be misleading in that a sensor with a high sensitivity may 

have a noisy output and thus be less able to resolve analyte concentrations than a sensor with a 

lower sensitivity and less noise in its output (analytical resolution refers to the ability of a sensor 

to resolve analyte concentrations). An increased sensitivity means only a stronger electrical 

response for a given analyte concentration, and does not necessarily imply improved LDLs, 

analytical resolution, linear range, or other critical parameters.

Finally, as a point of clarification, sensor can have various meanings. Developers of sensing 

technology often use sensor to describe the sensing element. In this context, the sensing element

is the electrochemical, thermal conductivity, or other component that reacts with or responds to

the analyte gas (e.g., hydrogen) to generate an electrical response that then can be processed, 

directly or indirectly, into an electrical signal. The control circuitry and user interface allow 

practical use of this signal. In this report, sensor will refer to an instrumented system composed 

of a sensing element, control circuitry, and user interface that provides analytically useful 

information to the end user. 

1.2. The 2007 and 2011 DOE/NREL Hydrogen Sensor Workshops 

Numerous hydrogen sensor technologies are currently available and new model types are being 

developed and commercialized every year. In part, this is because hydrogen has been for many 

years a common chemical in the petroleum, food, glass, and other industries, and because of the 

projected market growth as hydrogen use as an alternative fuel increases. The availability of 

hydrogen safety sensors will be critical for the successful utilization of hydrogen as an

alternative fuel. These sensors, however, must meet necessary requirements. Several broad-range 

applications for hydrogen sensors were previously identified in the 2007 DOE workshop, which 

were the Fuel Producer/Supplier Environment and the End-User Environment [9]. Accordingly, 

DOE has published a short list of performance parameters with target specifications for hydrogen 

safety sensors to provide sensor developers sufficient guidance to meet the hydrogen community 

needs as viewed in 2007 [10] (see Table 2). These targets were viewed as 5-year goals to meet 

the projected 2012 needs. Most hydrogen sensors are based on a few basic platform types, each 

of which has unique operating principles that will ultimately control its performance [11, 12].
5 

Each technology type has its advantages and limitations, but none have been shown to 

simultaneously meet all of the target specifications prescribed by DOE in 2007. 

Table 2. DOE Target Specifications for Hydrogen Safety Sensor R&D* 

Parameter Specification 

(Value)

Measurement range 0.1 to 10 vol%

Operating temperature -30° to 80°C

Response time <1 s

Accuracy 5% of full scale

Gas environment Ambient air, 10% to 98% RH

Lifetime 10 years

Interference Resistance (e.g., hydrocarbons)

*Table 3.8.2 in [10] 

Since 2007, new markets have emerged and overtaken light-duty road vehicles as currently 

available and deployed commercial technologies; the most prominent are hydrogen-powered 

forklifts, and to a lesser extent, hydrogen-fueled stationary power systems. However, 

demonstration projects associated with road vehicles are ongoing and early demonstrations have 

been successfully completed. There is also extensive ongoing infrastructure implementation 

(e.g., installation of filling stations and associated support systems) to prepare for the 2015 

commercial release of hydrogen fuel cell vehicles (FCVs). With this growth in infrastructure, 

applications and new requirements are emerging for hydrogen fuel. Independent reviews on the 

ability of commercial sensors to meet the needs of some markets (e.g., automotive sensors) have 

been performed. 

A market survey of more than 50 commercial sensors provided an overview of their ability to 

meet the safety requirements for automotive and stationary applications [13] with a focus on a 

few analytical performance metrics. This survey relied primarily on vendor-supplied 

specification and performance data and did not include independent validation of sensor 

performance. Even so, some devices readily meet some – but not necessarily all – the application 

requirements.

Attempts to improve the specification of one parameter (e.g., response time) often compromise 

other metrics (e.g., linear range or device operational lifetime). It was therefore useful to revisit 

the 2007 sensor workshop to refine hydrogen sensor requirements for safety purposes for a range 

of specific hydrogen infrastructure applications. To achieve this, a second workshop, sponsored 

by DOE/NREL, was organized to provide an updated assessment of the hydrogen sensor 

requirements. This workshop was held on June 8, 2011 in Chicago (Rosemont), Illinois. The 

2007 targets were developed for generalized applications and thus were consolidated into a 

single short list of desired metrics, but as indicated above and explicitly demonstrated in the 

following sections of this report, sensor metrics are dependent on the specific application. Thus, 

rather than producing generalized targets for sensor performance, application-specific 

requirements were explored. Nearly 40 participants attended the workshop, representing a range 

of stakeholders in the hydrogen community. They included representatives from private, 

government, and academic institutions. Sensor developers, end users, standard developing 
6 

organizations, and site safety officials participated. A list of attendees and the workshop agenda 

is provided in Appendix B. 

The workshop was structured to include a series of topical talks by invited speakers that covered 

selected aspects of hydrogen sensors. The following topical talks were presented at the 2011 

DOE/NREL Hydrogen Sensor Workshop [14] 

• DOE Fuel Cell and Hydrogen Program; Role of Sensors – Scott McWhorter, Savannah 

River National Laboratory and DOE

• 2007 Workshop – Robert Burgess, NREL

• 2011 Workshop Background and Objectives; Technology Update – Robert Burgess, 

NREL

• Codes and Standards Sensor Requirements – Carl Rivkin, NREL

• What a Battery User Needs from a Hydrogen Sensing Unit – Curtis Ashton, CenturyLink

• Wide Area Sensor Needs – Nick Barilo, Pacific Northwest National Laboratory

• Hydrogen Fuel Detection based on Smart Sensor Systems – Gary Hunter, National 

Aeronautics and Space Administration – Glenn Research Center

• Indoor Fueling – Bob Skinner and Albert (Snapper) Pouché Jr., LMI

• Hydrogen Sensor Certification – Michael Alexander, Underwriters Laboratories (UL) 

Project Engineer

• H2 Sensor in Industrial Truck Applications – Needs assessment from Field Experience – 

Aaron Harris, Nuvera Fuel Cells

• Sensor Metrics – William Buttner, NREL

More importantly, several critical key application areas were identified that will require 

hydrogen sensors for safe operations. For each application, breakout groups were formed to 

identify critical performance parameters requirements, assign values to specifications, and 

identify gaps or shortcomings in current technologies to meet these requirements. Three 

sequential breakout sessions were held. Typically, each breakout group met in multiple sessions, 

albeit with different participants. In each session, multiple breakout groups met in parallel, which 

restricted the size of each group to eight participants and enabled open discussions. Sequential 

sessions allowed workshop attendees to participate and provide input in three topical areas. Each 

breakout group was chaired by a topic expert. The breakout topics and chairs were: 

• Indoor Fuel and Operations (chaired by Albert (Snapper) Pouché Jr., Defense Logistics 

Agency Susquehanna) 

• Storage (chaired by Robert Burgess, NREL) 

• Production (chaired by Joe Cohen, Air Products)
7 

• Residential (chaired by Carl Rivkin, NREL) 

• Battery Backup (chaired by Curtis Ashton, CenturyLink) 

• Industrial Trucks (chaired by Aaron Harris, Nuvera Fuel Cells) 

• Vehicles (William Buttner, NREL)

1.3. Report Format

Sections 2.1–2.7 summarize the outcomes of the breakout groups by major application. This 

assessment prioritizes the performance metrics by application and attempts to assign target 

specifications. Neither the workshop nor this report addresses specific sensor technology types, 

nor are the respective performance metrics among various sensor types covered. An assessment 

of the main sensor platform types has been presented elsewhere [10, 11]. Rather, the workshop 

organizers strived to specifically identify critical applications for the emerging hydrogen 

infrastructure that require or would benefit from hydrogen sensors, and, more importantly, to 

then assign performance specifications for sensors deployed in that application. 

A relative ranking of each identified performance metric was assigned and is presented by 

application in Tables 3 through 11. For each application, each metric was assigned a qualitative 

ranking of low, medium, or high importance, as indicated by a marker in the appropriate column. 

The position of the marker within the column refines the assessment; a left justified mark 

indicates a lower importance than a right or middle alignment within the column. Specification 

requirements are also listed.

An identification of gaps and deficiencies in the ability of current sensor technology to meet 

these requirements is included in the accompanying discussion. Each of the following sections 

provides a brief description of a specific application, a prioritization of sensor performance 

metrics and assignment of desired specifications, an assessment of current technology to meet 

requirements, and finally, proposed recommendations to address the identified shortcomings and 

gaps in sensor performance. The discussion for each application is presented in separate sections, 

which are complete and structured to be independent of the other application discussions. A 

summary (Section 3) discusses similarities and differences between sensor requirements for the 

various applications. The following assessments were based primarily on the findings of the 

sensor workshop, and in general, each section was reviewed and updated by the corresponding 

breakout session participants and chairs.

The workshop and findings represent a critical first step in defining the sensor requirements for 

safety in the emerging hydrogen infrastructure. Recognizing that additional applications may be 

identified that were not addressed at the workshop, or that sensor requirements may evolve, a 

Hydrogen Sensor Task Group was organized by the Fuel Cell and Hydrogen Energy Association 

and NREL, and is open to all stakeholders in the hydrogen community. The Hydrogen Sensor 

Task Group is now chaired by NREL (for information see the NREL Sensor Test Laboratory 

home page [15]). The task group is to meet quarterly via a teleconference link and web 

conference format.
8 

2 Applications

The 2011 hydrogen sensor workshop identified several distinct hydrogen applications and 

subtopics that require or would benefit from hydrogen sensors. Breakout groups were formed to 

review the sensor requirements for each application. The goals of the breakout groups were to 

formulate relative rankings of the importance of the various sensor performance parameters, 

assign specifications, and to review the ability of current sensor technology to meet these 

requirements. The following applications were specifically addressed in the workshop and are 

discussed in the following sections:

• Industrial Trucks 

• Indoor Fuel and Operations 

o Facility Monitoring (area monitors)

o Internal Dispenser Sensor Deployment 

• Residential 

• Production 

o Distributed Hydrogen Production 

o Centralized Hydrogen Production

• On-board Light Duty Road Vehicles 

• Battery Backup 

o Building 

o Controlled Environment Vaults 

• Storage 

2.1 Industrial Trucks (Forklifts)

• Application 

o Deployment of hydrogen sensors to detect leaks in on-board forklift fuel storage 

and delivery systems.

• Applicable standards 

o NFPA 2, Hydrogen Technologies Code

o Listing of sensor and sensor components 

o UL 60079-15, Electrical Apparatus for Explosive Gas Atmospheres or equivalent 

listing requirements for use in Class I, Division 2 hazardous locations 

o UL 2075, Gas and Vapor Detectors and Sensors or CSA C22.2, No. 152, 

Performance of Combustible Gas Detection Instruments or equivalent listing for 

sensor performance 
9 

o FM Global Class 6310 and 6320, Approval Standard for Combustible Gas 

Detectors

o UL 2267, Fuel Cell Power Systems for Installation in Industrial Electric Trucks

• Local Authority Having Jurisdiction (AHJ) invoking modified fire code 

2.1.1 Background

Fuel cell-powered industrial trucks (forklifts) are an 

emerging, near-term market that uses hydrogen as a 

fuel [1]. These forklifts are already commercially 

available and are being deployed in numerous 

private industrial operations. The currently 

deployed fleet typically has on-board hydrogen 

sensors as part of its safety systems, and the use of 

sensors is likely to continue with future generations 

of hydrogen-powered forklifts. Thus, this 

application represents an existing and growing 

market for hydrogen sensors. 

Forklift on-board hydrogen storage is approximately 1 kg, but this varies with forklift design.

Although uncommon, system failures leading to a full release of hydrogen from the storage tank 

have occurred [16]. In one instance, hydrogen was released during the fill that followed a 

maintenance operation on an on-board solenoid valve. A malfunction in the solenoid valve led to 

the release of the hydrogen within the tank. It should be noted that the hydrogen was safely 

dissipated, and there was no report of a fire or other event. However, there is reason for concern. 

A release of 1 kg of hydrogen uniformly distributed in a 25,000-ft3 facility would be only 

approximately 1.7% by volume, which is less than half of the hydrogen LFL (a 5000-ft2

warehouse with a 25-ft ceiling would be 125,000 ft3

). However, uniform distribution is highly 

unlikely and such a release may not be instantaneously distributed. A release of stored hydrogen, 

even in quantities less than a full tank, could lead to a potentially dangerous situation in the event 

of containment, pockets, or entrapment that are caused, for example, by structural features or 

operation in an enclosed space such as a small room or unloading/loading a truck trailer. 

Although flow regulators, shutoff valves, and bypass valves are incorporated into the forklift 

safety system, hydrogen sensors, which are separate from operation, provide an increased 

independent assurance of safety.

Hydrogen sensors can be deployed on board the forklift to detect and quickly alert personnel to 

hydrogen leaks. Sensor placement should be proximal to the storage tank and fuel cell, which are 

usually enclosed in a metal housing. Although the housing is not sealed and allows for passive 

ventilation, exchange of the inside gas with outside air is to some extent impeded, which may 

cause hydrogen to build up internally. Thus, this deployment may be considered a contained 

environment. Currently, there are no mandatory sensor requirements for on-board deployment, 

but manufacturers recognize the importance of sensors and generally have voluntarily 

incorporated them into the forklift safety system. Local AHJs may impose locally modified fire 

code and thereby mandate sensor use. Sensors listed as compliant to national standards, 

especially Class I, Division 2 listings, would be useful, although such a listing is not currently 

required. A sensor not listed to an accepted safety standard may need to be shut down when it is NREL PIX 16601
10 

in alarm because it is not approved for operation in a (potentially) combustible atmosphere. This 

in turn may require system (e.g., the forklift and all control elements) shutdown because of a lack 

of active detectors. 

In addition to electrical and other safety standards, there are performance-based standards for 

combustible gas sensors (CGSs) (e.g., UL 2075, Gas and Vapor Detectors and Sensors and CSA 

C22.2, No. 152, Performance of Combustible Gas Detection Instruments). Although these 

standards are not specific to hydrogen detection, some jurisdictions have already mandated that 

hydrogen safety sensors be listed as compliant to UL 2075, and at some point this may be more 

universally required. Typically, hydrogen sensors deployed on forklifts are configured for a 

single alarm level. When hydrogen is detected at 10% of the LFL, an audible alarm is activated. 

Two levels of response could be of interest. 

• A low-level response (e.g., 10% of the LFL or 0.4 vol% hydrogen) to activate the audible 

alarm and, if possible, to enhanced ventilation with a ventilation fan mounted around the 

storage tank. 

• A medium-level risk corresponding to 25% of the LFL (1 vol% hydrogen), which would 

shut down the forklift operation and isolate the fuel delivery system bya activating the

shutoff valves. 

Telemetric communication for remote monitoring of the output is not required. The gas detector 

should have electronic outputs that are in engineering units (e.g., vol% H2) or easily converted to 

engineering units. If an on-board control system monitors the state, operation, and performance 

of the forklift, the output of the sensor should interface with the control system. The interfacing 

would be performed by the forklift manufacturer. 

Leaks in an enclosure, even with ventilation ports, may quickly lead to situations of concern 

(e.g., hydrogen concentrations approaching the LFL), so a fast response time for the sensor is 

necessary. Although unlikely, the sensor may experience a direct hit of hydrogen by a leak jet. 

Accordingly, a brief exposure to pure hydrogen should not impact its analytical capability. The 

sensor should also not be affected by chemical interferences, including, but not limited to, carbon 

monoxide (CO) and sulfur compounds. Because silicone compounds such as sealants, lubricants, 

and pneumatic elements are used extensively, the sensor should also be resistant to silicone; 

these compounds are known poisons of many sensing element platform types, especially 

combustible bead sensors or metal oxide sensors (MOXs).

Failure caused by dust buildup on the sensor or sensor inlet was reported in the workshop. This

can lead to either a false positive, such as would occur with a catalytic bead sensor that heats up 

because of the thermal insulating effect of dust, or to false negatives because the active surface 

of the sensing elements is blocked. Thus, some means to control dust buildup on the sensing 

element or gas inlet to the sensing element may be necessary. Forklifts will often operate in 

warehouses with limited climate control or in freezer storage areas. Therefore, sensors ought to 

operate over a temperature range of -40–40°C and a humidity range of 5%–90%. 

If the forklift will be operated in or around refrigerated facilities, the sensor should also not show 

cross sensitivity to common refrigerants, including ammonia or chlorofluorocarbon compounds 

such as Freon. Although significant pressure fluctuations are not expected, sensors must be 

calibrated to local prevailing pressure levels, as barometric pressure depends on the installed 
11 

elevation (from approximately 1 bar at sea level to 0.8 bar at 2000 m). Routine maintenance, 

such as calibration, should be required perhaps once per year, although longer intervals are 

highly desirable. Sensors deployed for on-board operation will be subjected to mechanical 

stresses (e.g., vibrations). The target sensor cost has been set at $100, but lower costs are of 

course desired. 

Table 3 summarizes the sensor requirements and specifications for on-board forklift deployment. 

A relative ranking of each identified performance metric is presented and assigned a qualitative 

ranking of low, medium, or high importance, as indicated by the marker in the appropriate 

column. The position of the marker refines the assessment; a left-justified mark indicates a lower 

importance than a right or middle alignment.

Table 3. Hydrogen Sensor Metric Rankings for Industrial Truck Applications – 

On-Board Deployment 

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO, dust

LDL X 0.04 vol% (1% of LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X To LFL (0.1–4 vol% H2)

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 10 s

Recovery Time X 30 s

Repeatability X

Signal Drift X To avoid false positives and negatives at all alarm levels

Environmental Impacts X T and RH are more relevant than P

- T X 0 to +40°C (or -40°C for freezer storage areas)

- P X Must be calibrated for deployment altitude

- RH X 15%–95% RH

Reversibility X Postexposure recovery should be >95%

Limits Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Relevant for sensors mounted near tank

Deployment Parameters

Capital Cost X <$100

Installation Cost X

Physical Size X Small size compatible with mounting around tank

Control Circuitry X

Electronic Interface X Output in engineering units (e.g.,% H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf

Alarm Thresholds X Must indicate adverse condition

- Trace (1,000 ppmv) X Not required for on-board, trace background is possible

- Low (low risk) X 10% of the LFL, indicate adverse for single alarm level

- High (pending risk) X 25% of the LFL, shutdown system for multiple alarm levels

Regulations and Codes X Possible local AHJs

Deployment Placement X Within enclosure proximal to the tank

Operational Parameters
12 

Parameters Importance Specifications and Notes Low Medium High

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years

- Unit Replacement X 5 years

Consumables X None

Calibration Schedule X >once per year; longer time between calibrations is desired

Maintenance X Maintenance free except calibration

Sample Size X No critical restriction

Matrix Requirements X Normal air environment

Signal Management X 

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X Readout is not necessary

- Remote monitoring X Remote interrogation is not necessary

Device Repeatability X Plug in replacement is useful

Warm-Up Time X <1 h for initial installation, <1 min for all shutdowns

Alarm Interface

- Number of set points X One mandatory, two useful shutdowns

- Audible X Activate alarm at lowest set point

- Ventilation X Activate at 1st set point, if feasible

- Shutdown X Activate at 2nd set point (25% of LFL)

- Remote X Not necessary

Mechanical Stability X Vibration tests, specification TBD

Power Requirements X Moderate power requirements preferred (<0.5 W)

2.1.1.1 Critical Gaps and Deficiencies

The operational requirements for sensors are a major limiting factor for on-board sensor 

deployment. Costs, especially maintenance costs, are a concern. The overall sensor cost 

encompasses several elements, including the actual capital cost of the device, the expense 

associated with on-board installation, and the operational expenses (e.g., recurring in-the-field 

calibration and maintenance). Because capital cost and factory installation are typically one-time 

upfront expenses (per truck), they are not viewed as detrimental to sensor acceptance as the 

ongoing and recurring operational costs. There is, of course, always a desire for lower capital 

cost and simplified installation requirements, providing sensor reliability is not compromised. 

Operational expenses are recurring costs and are a major concern. Operational activity, such as 

routine maintenance, calibrations (or validation of calibration), and sensor replacement are 

typically performed manually in the field, and are very expensive because they require forklift 

downtime and trained personnel must do the work. 

Although the on-board hydrogen sensor is primarily in an indoor environment, it must operate in 

temperature and humidity extremes, especially when the sensor is co-deployed in an enclosure 

containing the fuel cell or storage tank. Some hydrogen forklift fleets operate outdoors, and thus 

would be subjected to more extreme temperature fluctuations. False alarms are caused by the 

physical operating environment, which can include temperature, pressure, and humidity 

fluctuations as well as interferents, poisons and dusts, so they remain major concerns. It is 

therefore unlikely that the calibration requirement will be eliminated for the sensor.

Another deployment consideration is that sensors often need to be certified (or listed) for 

deployment in a potentially combustible atmosphere. Some jurisdictions are requiring 
13 

performance-based certification as well. Although a nonlisted sensor can be used to detect 

hydrogen, it would not necessarily be safe to operate as the hydrogen concentration approaches 

the LFL. In the event the sensor must shut down, it would also be advisable to shut down the 

forklift. Some sensors are compliant with Class I, Division 2 (e.g., UL 60079-15) operation, but 

no sensor is currently compliant with performance-based standards (e.g., UL 2075 or CSA 

C22.2).

Sensor deficiency areas and gaps impeding deployment can be summarized as follows. A major 

deficiency represents a current critical issue that has been problematic (e.g., occurring with an 

unacceptable frequency). A secondary issue is important but is either not critical or has not been 

reported as occurring (e.g., false negatives have not been reported). 

• Major

o Analytical performance in the on-board environment (high T and RH) 

– No false positives, due to environment-induced effects

– Fast response/recovery times

o Maintenance and calibration frequency

– Long-life sensors (>5 years)

o Calibrations no oftener than once per year

o All other maintenance requirements less often than once per year

o Robust performance, especially against contaminants, poisons, and dust 

• Secondary 

o Capital cost for appropriate monitoring systems 

– Acceptable target: <$100/sensor)

o Listed sensors

– Performance-based approvals (e.g., UL 2075 or CSA C22.2, No. 152) 

– Approval for operation in a combustible environment (UL 60079) 

o Analytical performance

– Recovery from high hydrogen exposure 

– No false negatives

2.1.1.2 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

One proposed strategy to facilitate sensor deployment is to simplify and economize the 

operational requirements. Every deployed sensor must be calibrated (or have the calibration of 

the instrument validated) prior to truck delivery. Following deployment, trained personnel will

use on-site gas standards to perform or validate periodic calibrations. Robust sensors with long 

operational lifetimes would cut down replacement cost, and could, in principle, lower the cost of 

maintenance by increasing the time between calibrations. However, sensor performance can be 

affected by a variety of stresses, including chemical (e.g., exposure to poisons), mechanical (e.g., 

dust, thermal stresses, and vibration), and electrical factors (e.g., power surges, shutdowns, and 
14 

circuit failures). The extreme and fluctuating temperature and RH operating conditions are also 

contributing factors for sensor drift. Thus, even though robust sensor technology may reduce site 

maintenance, deployed sensors will still require periodic calibration. Manual calibration 

performed in the field is not a cost-effective solution. An automated, efficient sensor 

functionality test is required. 

A limited number of hydrogen sensors are currently listed for Class I, Division 2 operation (e.g., 

UL 60079-15), but no sensors are currently listed as compliant to performance standards (UL 

2075, for example). Sensors listed to safety standards could allow operation once alarm levels of 

hydrogen are detected, and listed components may eventually be required. A nonlisted sensor 

would have to shut down when it detects hydrogen near the LFL, probably at 25% of the LFL. 

When the sensor shuts down, other systems should also terminate operation. Listed sensors, 

which could continue monitoring at or beyond the alarm level, would facilitate operation by 

differentiating between a spurious response (e.g., a transient exposure of hydrogen from some 

internal source) and an internal system release of hydrogen. A listed sensor would also allow for 

continuous monitoring once a release is detected, thereby providing more reliable performance.

Thus, listing of sensors should be supported and facilitated. Currently, the sensor of choice for 

on-board applications has been the catalytic bead sensors (also called CGSs and MOXs), 

although no specific technology has been explicitly endorsed. Both are high-temperature devices, 

and thus, less susceptible to adverse impact from high-temperature operation, but some platforms 

are highly susceptible to drift induced by moisture fluctuations. 

Sensor capital costs are likely to decrease with demand as the hydrogen infrastructure develops. 

However, certain platforms and designs will be better impacted by economies of scale, especially 

those that are amenable to high-scale manufacturing afforded by technology developed by the 

microelectronics industry. Miniaturized devices show particular potential. Miniaturized sensors, 

mass-produced using methods developed for the electronics industry (e.g., microfabrication, 

microelectromechanical systems [MEMs], and thin-film deposition protocols) could provide 

low-cost, low-power devices, coupled with a rapid response; a 1-s response time is feasible.

However, the sensor must also still meet the analytical requirements for range (e.g., multiple 

alarm levels) and robustness requirements (insensitive to poisons, interferences, and drift).

Improved protection from dust and interferences via design changes should be pursued; this must 

be accomplished without compromise to the response/recovery time requirements.

2.2 Indoor Fueling Operations

• Applications 

o Facility (warehouse) monitoring around indoor fuel operations (area monitor) 

o Sensors installed inside dispensers (contained environment monitor) 

• Applicable standards 

o NFPA 2, Hydrogen Technologies Code

o Local AHJs invoking modified fire code 

o Listing of sensor and sensor components 

o UL 60079-15, Electrical Apparatus for Explosive Gas Atmospheres or equivalent 

listing requirements for use in Class I, Division 2 hazardous locations 
15 

o UL 2075, Gas and Vapor Detectors and Sensors or CSA C22.2, No. 152, 

“Performance of Combustible Gas Detection Instruments,” or equivalent listing 

for sensor performance 

o FM Global Class 6310 and 6320, Approval Standard for Combustible Gas 

Detectors

2.2.1 Background

Fuel cell-powered industrial trucks (forklifts) are 

already commercially available and deployed in 

numerous industrial and government facilities. Thus, 

the fundamental support technology is already 

available, although significant near-term 

infrastructure expansion can be expected as 

deployments increase. Forklift fueling requires 

support systems, including delivery of hydrogen or 

on-site hydrogen production, on-site storage (typically 

outdoors), transfer hardware, and hydrogen dispensers. Typically fill quantities for forklifts range 

from 0.25 kg to usually less than 1 kg. A 0.5 kg/min delivery rate corresponds to a flow rate of 

approximately 5,500 standard liters per minute. A 1 kg hydrogen release uniformly distributed in 

a large room (e.g., 25,000 ft3

) would be approximately 1.7% by volume, which is less than half 

of the LFL (a 5000-ft2 warehouse with a 25-ft ceiling would have a volume of 125,000 ft3).

However, uniform distribution throughout the warehouse is highly unlikely and such a release 

may not be instantaneously distributed. Even a relativley small release during dispensing could 

lead to a dangerous situation in the event of containment or entrapment within the dispenser. 

During fueling, the dispenser delivers hydrogen to the forklift for high-pressure storage in the 

on-board tank. The maximum nominal settled pressure of the forklift storage tank can be as high 

as 35 MPa. For comparison, light-duty road vehicle tanks will be filled to nominal settled 

pressures up to 70 MPa, although road vehicles will be fueled primarily with outdoor dispensers. 

Hydrogen is typically supplied to the dispensers via a transport system from outdoor storage 

tanks, which hold either liquid or gaseous hydrogen pressurized to 1.4–14 MPa (200–2000 psi). 

Thus, during a fill, the dispenser must pressurize hydrogen by up to a factor of 50. Within the 

dispenser are high-pressure hydrogen and the associated pneumatic elements (tubing, fittings, 

valves, compressors, etc.). 

Facility managers overseeing hydrogen forklift deployments generally prefer indoor hydrogen 

dispensing. Indoor fueling allows for easier access to the dispenser, hence much quicker fill 

times relative to outdoor fueling. Indoor fueling operations have been safely performed 

throughout the DOE-funded demonstrations; this safety record has continued through the early 

market entry of commercial fleets [2]. Indoor dispensers are typically mounted at the wall 

adjacent to the outdoor storage facility to minimize the volume of the indoor transfer lines. Thus, 

the indoor hydrogen systems required for forklift fueling are primarily associated with the 

dispenser. 

Although flow regulators, shutoff valves, and bypass valves are incorporated into the hydrogen 

dispenser, hydrogen sensors, which are independent of operation, provide increased assurance of 

safety. Further, sensors can operate on backup power or batteries and therefore can be 

Nuvera Image, used with Permission
16 

independent of the site power. Thus, even though the dispenser and its internal safety system 

may shut down during power outages, the hydrogen safety sensor can remain active through 

backup power systems, analogous to battery backup designs used in residential smoke and CO

detectors. Sensors thus play a critical role in hydrogen dispenser safety. 

Indoor hydrogen dispensing is an established and accepted operation, and thus represents an 

existing and growing application for the use of hydrogen safety sensors. Two main sensor 

applications are associated with indoor hydrogen dispensing: facility monitoring for external 

releases (e.g., an area monitor) and deployment in the dispenser (e.g., a contained environment 

monitor). These applications are treated separately below. 

2.2.2 Facility Monitoring (Area Monitors)

NFPA 2 mandates the use of chemical sensors around hydrogen operations, including indoor 

hydrogen fueling. Accordingly, hydrogen sensors are mounted proximal to the hydrogen 

dispenser (e.g., an area monitor). The sensor can be mounted either on the wall adjacent to and 

above the dispenser (the most common location) or from the ceiling above the dispenser. 

Typically a single sensor is used. Hydrogen sensors currently deployed proximal to indoor 

dispensers are typically configured for a single alarm level. If they detect hydrogen at 10% of the 

LFL (or 25%, depending on local requirements), an audible alarm is activated. This is the 

minimim requirement for the safety sensor deployed as an area monitor. Although currently 

multiple alarm levels are not typically used around indoor dispensers, two alarm levels could be 

of interest: 

• A low-level threshold at 10% of the LFL (0.4 vol% hydrogen) would activate the audible 

alarm and enhanced ventilation. 

• A medium-level response at 25% of the LFL (1 vol% hydrogen) would activate shutdown 

of the dispenser operation and isolate the fuel delivery system via activation of shutoff 

valves. 

It may be useful and at some point mandated by AHJs for the sensor to interface with the 

building ventilation systems, which would be activated at the low alarm level. Relays would be 

useful in this circumstance. 

Sensors that are compliant with national standards, especially Class I, Division 2 listings, are

advisable, and most indoor fueling facilities conform to this. A sensor not certified to operate in a 

combustible environment would need to shut down when such a condition is indicated (e.g., 

when the sensor is in an alarm state). This in turn may require shutdown of the dispenser and 

associated support systems because of a lack of active area hydrogen detectors. Although system 

shutdown is a conservative action in response to the detection of hydrogen at the alarm level, it 

may be an overreaction. A depowered sensor would not be able to indicate whether the condition 

was either becoming more hazardous with increasing hydrogen levels, stable at a hydrogen 

concentration above the alarm threshold but below the LFL, or transitory such that all spurious 

hydrogen has dissipated. 

Such uncertainty would mandate shutdown of the dispenser because the sensor indicates a 

potentially unsafe condition, which might be only a transitory event. Of course, system shutdown 

should occur whenever hydrogen continues to rise above at the medium action level (25% LFL). 
17 

In addition to electrical safety standards for Class I, Division 2 operation, there are performancebased standards (e.g., UL 2075 and CSA C22.2, No. 152). Some jurisdictions (e.g., the state of 

California) have already mandated that hydrogen safety sensors be listed as compliant to UL 

2075, and at some point this may be more universally required. 

The gas detector should have outputs that are in engineering units (e.g., vol% H2) or easily 

converted to engineering units. Although not required, telemetric communication capability 

could be useful for convenient remote interrogation. Several factors contribute to the desired 

sensor response time. System leaks can quickly lead to situations of concern, so a reasonably fast 

response time is desirable. Alternatively, the sensor is likely to be mounted remotely from the 

source, so diffusion processes will dominate the sensor response. A facility sensor response time 

of 10–30 s seems adequate. It is possible, although highly unlikely, that a release jet may directly 

contact the sensor. Accordingly, the sensor should be immune to or recovery quickly from a brief 

exposure to a high hydrogen concentration. This is not, however, a critical requirement.

The sensor should not be affected by common interferants, including but not limited to CO and 

sulfur compounds. Because silicone compounds are used extensively as sealants, lubricants, and 

pneumatic elements, the sensor should also be resistant to poisoning by silicone, unless site 

operations can specifically ensure that such compounds will not be present. However, such 

guarantees are unlikely for most industries. If operated in or around refrigerated facilities, the 

sensor should also not show cross sensitivity to common refrigerants, including ammonia or 

chlorofluorocarbon compounds such as Freon. In fact, fuel cell-powered forklifts serve some 

refrigerated warehouses that store perishable items. Such facilities are an ideal market for these

forklifts because they need emission-free vehicles around food products and, in contrast to 

batteries, fuel cells maintain their power at lower temperatures.

Most warehouses have some indoor climate control, but it may not be extensive. Thus, sensors 

ought to operate over broad temperature and humidity ranges. Working temperature ranges for 

the sensor should be consistent with the deployment facility, which can range from below -20°C 

for refrigerated facilities or unheated facilities in winter to +40°C for summer operation in a 

thermally unregulated facility. Of course, the selected sensor must be suitable to the prevailing 

condition and not necessarily for this temperature range. Although significant pressure 

fluctuations are not expected, sensors must be calibrated to prevailing pressure levels, because 

barometric pressure depends on the installed elevation (from approximately 1 bar at sea level to 

0.8 bar at 2000 m elevation). Routine maintenance such as calibration should be once per year or 

sooner per manufacturer recommendations, but longer times between routine maintenance are 

highly desirable, as this significantly lowers operational costs. The target sensor cost, which 

would include all control elements and user interfaces, can be set at $500, but lower costs are of 

course desired.

Table 4 summarizes the sensor requirements and specifications for facilities with indoor fueling 

operations. A relative ranking of each identified performance metric is presented and assigned a 

qualitative ranking of low, medium, or high importance, as indicated by the marker in the 

appropriate column. The position of the marker refines the assessment; a left-justified mark 

indicates a lower importance than a right or middle alignment. 
18 

Table 4. Hydrogen Sensor Metric Rankings for Indoor Fueling – Facility Monitoring 

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO, dust, ammonia , 

chlorofluorocarbons

LDL X 0.04 vol% (1% of LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X To LFL (0.1–4 vol% H2)

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 30 s

Recovery Time X 1 min

Repeatability X

Signal Drift X To avoid false positives and negatives at all alarm levels

Environmental Impacts T and RH are more relevant than P

- T X -20° to +40°C

- P X Must be calibrated for deployment altitude

- RH X 5%–95% RH

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Unlikely to experience 100% H2 (or very high H2)

Deployment Parameters

Capital Cost X <$500 for complete sensor system

Installation Cost X Part of capital cost

Physical Size X Size is not a significant issue for wall/ceiling mounting

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf

Alarm Thresholds X Must indicate adverse condition

- Trace (1,000 ppmv) X Not required but useful in ventilation system or extractive

- Low (low risk) X 10% of the LFL, to track intermediate or increasing levels

- High (pending risk) X 25% of the LFL, shutdown system for multiple alarm levels

Regulations and Codes X NFPA 2. Class I, Division 2 operation

Deployment Placement X Guidance on placement is needed

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years

- Unit Replacement X 5 years, 10 years desired

Consumables X None

Calibration Schedule X >once per year; longer time between calibrations is desired

Maintenance X Maintenance free except calibration

Sample Size X No critical restriction

Matrix Requirements X Normal air environment, possible dust concern

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X H2 concentration readout could be useful 

- Remote Monitoring X Remote interrogation is useful

Device Repeatability X Plug in replacement not required, but must be calibrated
19 

Parameters Importance Specifications and Notes Low Medium High

Warm-Up Time X <1 h for initial installation,15 min for all shutdowns

Alarm Interface X Must connect to control system 

- Number of set points X One mandatory, two useful shutdowns

- Audible X Activate at lowest alarm set point

- Ventilation X Activate at 1st set point if feasible

- Shutdown X Activate at 2nd set point (25% of LFL)

- Remote X Option to external alarm, and remote transmitters

Mechanical Stability X Vibration tests, specification TBD

Power Requirements X Moderate power requirements preferred (<0.5 W)

2.2.2.1 Critical Gaps and Deficiencies

The operational requirements for sensors are a major limiting factor for sensor utilization; overall 

costs are a dominant concern. Sensor costs encompass several elements, including the actual 

capital cost of the device, the expense associated with on-site installation, and the operational 

expenses (e.g., recurring in-the-field calibration and maintenance). Capital cost and site 

installation are typically one-time upfront expenses, so these costs do not impact as strongly as 

the ongoing operational costs. There is, of course, always a desire for lower capital costs and 

simplified installation requirements, providing the sensor reliability is not compromised. 

Operational activities, such as routine maintenance, calibrations (or validation of calibration), 

and sensor replacement are typically performed manually in the field, and are very expensive 

because trained personnel must do the work. A second concern pertains to actual deployment of 

the sensor. 

There is little guidance for establishing optimal sensor location and distribution. Multiple sensors 

may be required for large facilities and some operations may have specific requirements for 

sensor networks in large facilities [17]. It is not clear if the monitoring requirements of a large 

facility, especially one with mobile hydrogen-powered forklifts, would be better served by an 

array of point sensors or wide area monitoring technologies [18]. Wide area monitors currently 

are primarily experimental systems. Technical questions remain about their performance, and 

logistical questions remain about their commercial maturity for real-world applications. 

Currently, a single point sensor with an alarm set point at 10% of the LFL is typically used 

around each indoor hydrogen dispenser. Sensors will be mounted on the wall or near the ceiling 

around the dispenser, so actual hydrogen detection will be remote from the hydrogen source. 

There is thus some uncertainty about the appropriate alarm levels because of potential impacts of 

dilution as the vapor migrates from the source; this is exasperated by sensor deployed in 

ventilation systems or extractive sensor designs with multiple sampling points (e.g., a sensor 

with active sample collection from a remote location or locations using internal sampling).

Currently, alarm levels are typically set to 10% of the LFL (0.4 vol% hydrogen), although some 

jurisdictions may allow alarm thresholds at 25% of the LFL.

Another deployment consideration is the need for sensors that have been certified (or listed) for 

deployment in a potentially combustible atmosphere. Although a nonlisted sensor can be used to 

detect hydrogen, it would not be necessarily safe to operate as the hydrogen concentration 

approaches the LFL. In the event the sensor must shut down, it would be advisable to shut down 

the dispenser. Some sensors are compliant with Class I, Division 2 (e.g., sensors certified to UL 
20 

60079-15) operation, but no sensor is currently compliant to performance-based standards (e.g., 

UL 2075 or CSA C22.2).

Sensor deficiency areas and gaps inhibiting deployment can be summarized as follows. A major 

deficiency represents a current critical issue that has been problematic (e.g., occurring with an 

unacceptable frequency). A secondary issue is important but is either not critical or has not been 

reported as occurring.

• Major

o Maintenance and calibration are major shortcomings 

– Long-life sensors (>5 years)

– Manual calibrations no oftener than once per year (less often than one year 

or no calibration requirement is preferable) 

– All other maintenance requirements oftener than once per year

– Robustness against interferents, poisons, and dust 

o Acceptable target: <$500 per sensor 

o Deployment guidance 

• Secondary 

o Capital cost for appropriate monitoring systems 

2.2.2.2 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

One proposed strategy to facilitate sensor deployment is to simplify and economize the 

operational requirements. Currently, the sensor of choice for hydrogen monitoring around indoor 

dispensers has been the catalytic bead sensor (also called the CGS), although no specific 

technology has been explicitly endorsed by AHJs or facility engineers. Currently, manufacturers 

require periodic calibration to ensure sensor accuracy and functionality. Sensor performance can 

be affected by a variety of stresses, including chemical (e.g., exposure to poisons), mechanical 

(e.g., stresses from vibration and thermal fluctuations), and electrical factors (e.g., power surges, 

shutdowns, and circuit failures). 

Typically, calibrations are to be performed at least once per year, but could be more frequent 

based on local code requirements or manufacturer recommendations. Robust sensors with long 

operational lifetimes would reduce replacement cost, and could in principle lower the cost of 

maintenance by increasing the time between calibrations. Robust sensors should eliminate most 

nonroutine maintenance requirements. However, because a broad range of factors can affect 

sensor performance, the sensor calibration requirement most likely will not be eliminated, 

especially because local regulations or codes often specifically require a minimal calibration 

cycle. Thus, an automated calibration system would significantly lower operational costs 

associated with wide-scale sensor deployment for area monitors. Acceptance of such a system 

would be facilitated if it were available at a reasonable cost (<$100 per sensor). 

Sensor capital costs are likely to drop as demand increases with the emerging hydrogen 

infrastructure. However, certain platforms and designs will be better impacted with economies of 

scale, especially those that are amenable to high-scale manufacturing afforded by technology 
21 

developed by the microelectronics industry. Miniaturized devices show particular potential.

Miniaturized sensors, mass-produced using methods developed for the electronics industry (e.g., 

microfabrication, MEMs, or thin-film deposition protocols), could provide low-cost, low-power 

devices, coupled with a rapid response; a 1-s response time is feasible. However, the sensor must 

also still meet the analytical requirements including range (e.g., multiple alarm levels) and 

robustness requirements (insensitive to poisons, interferences, and drift). Commercial hydrogen 

sensor technologies are emerging for a number of thin-film and other microfabrication platform 

types. These show a fast response to hydrogen, but many tend to saturate at low concentrations 

and to exhibit a poor dynamic range. Their long-term stability and robustness to stresses (e.g., 

chemical, thermal, and moisture) should be more thoroughly studied. 

Guidance about sensor deployment (e.g., ceiling, walls, or in ventilation systems) and their

relationship to appropriate action levels is needed. Hydrogen may be released from a localized 

source (e.g., from the dispenser) or dispersed sources (forklifts). The sensor will likely be placed 

remote from the source, and the distance may vary depending on the specific source of the leak. 

Thus, the detected hydrogen gas concentration could be significantly attenuated. This is 

especially true for sensors deployed in ventilation systems, although sensors mounted on the wall 

adjacent to the dispenser are more common. Although mandated by code (e.g., NFPA 2), no 

specific guidance is provided about how to best position the sensor to detect a spurious hydrogen 

release. Nor is there rational guidance on the appropriate number of sensors to be installed in a 

facility. 

Hydrogen can concentrate remotely from the dispenser via natural prevailing dispersion 

processes or from mobile source (e.g., from the forklift). It is thus necessary to established 

whether a single point sensor mounted on the wall adjacent to the dispenser would ensure that a 

hydrogen release is detected, and if multiple monitoring points are required, whether a 

distributed array of point sensors would be more advantageous and a better approach than wide 

area monitoring technologies. Cost, performance, and level of commercial maturity must be 

considered, and point source sensors are certainly more commercially mature than wide area 

monitoring technologies. Computational fluid dynamics (CFD) modeling of various release 

scenarios (for example, low and high release rates) and sensor deployment should be performed 

to provide such guidance about position and appropriate alarm levels. This guidance should 

ultimately be incorporated into or cited by NFPA 2. 

2.2.3 Internal Dispenser Sensor Deployment (Contained Environment Monitor)

The hydrogen dispenser is the interface between the 

site hydrogen storage (typically outdoors) and the 

forklift. For facile fueling, forklift hydrogen fills are 

typically performed using indoor dispensers, although 

dispensers servicing the road vehicle fleet are

typically outdoors. Once communication between the 

dispenser and the vehicle is established, the actual fill 

times are as fast as 2 min. The following discussion 

focuses primarily on sensors installed inside 

dispensers installed for indoor fueling, but it is also relevant for dispensers deployed for outdoor 

operations. Although flow regulators, shutoff valves, and bypass valves are incorporated into the 

dispenser safety system, hydrogen sensors, which are independent of operation, provide 

NREL PIX 15984
22 

increased safety. Furthermore, sensors can also operate on backup power, and thus remain active 

even in the event of a power outage. Although not mandatory, hydrogen sensors are often 

installed within the dispenser, especially for systems that are used for indoor fueling. 

Hydrogen concentrations can quickly build up within a contained environment. For example, less 

than 0.8 g of hydrogen uniformly distributed within a container having a free volume of 8 ft3

would exceed the LFL. Without venting, this amount would be reached in less than 1 h with an 

average release rate of only 0.013 g/min (or approximately 2 cm3

/s), a rate sufficiently slow that 

it may not be detected by pressure sensors. There is typically a single alarm set point at 25% of 

the LFL for sensors installed within the dispenser. At 25% of the LFL the system shuts down and 

an audible alarm is activated. Although multiple alarm levels are not currently employed to 

internally monitor the dispenser, two alarm levels could be of interest: 

• A low-level threshold at 10% of the LFL (0.4 vol% hydrogen) would activate the audible 

alarm and enhanced ventilation. Relays could be used to activate internal ventilation of 

the dispenser to facilitate the purge. 

• A medium-level response at 25% of the LFL (1 vol% hydrogen) would shut down the 

dispenser and isolate the fuel delivery system via activation of shutoff valves. 

Sensors listed as compliant to Class I, Division 2 operation (e.g., sensors certified to UL 60079-

15) are required for deployment within dispensers and other contained environments. In addition 

to electrical safety standards for operation in potentially flammable environments, there are 

performance-based standards for gas sensors (UL 2075 and CSA C22.2, No. 152”). Some 

jurisdictions have already mandated that hydrogen safety sensors be listed as compliant to UL 

2075, and at some point this may be more universally required. The sensor should have 

electronic outputs that are in engineering units (e.g., vol% H2) or easily converted to engineering 

units. Although not required, telemetric communication capability could be useful. The sensor 

output may be interfaced to an internal communication system that is integral to the dispenser. 

Many manufacturers monitor commercial dispensers remotely, but this may not always be the 

case. For now, internal control systems that allow the dispenser manufacturer remote access to 

system controls and sensors fulfill the telemetric communication requirement. System leaks 

within the dispenser can quickly lead to high hydrogen concentrations, and a fast response time 

(1 s) is desirable and recommended. Within a contained environment, especially one surrounding 

high-pressure hydrogen, there is an increased likelihood that a release jet would directly contact 

the sensor. The sensor should therefore be immune to or recover quickly from a brief exposure to 

high concentration of hydrogen. The sensor should not be affected by common interferences 

including but not limited to CO and sulfur compounds. Because silicone compounds are used 

extensively as sealants, lubricants and pneumatic elements, the sensor should also be resistant to 

poisoning by silicone. The dispenser, if installed inside warehouses or other buildings, will have 

some indoor climate control, but it may not be extensive. Thus, sensors ought to operate over 

broad temperature and humidity ranges. 

Working temperature ranges for the sensor should be consistent with the deployment facility, 

which can range from 0° to +40°C operation in a facility with limited thermal regulation that 

includes some heating in winter but very limited air conditioning in summer. Lower temperatures 

may be encountered for dispensers in unheated (or refrigerated) environments. Of course, the 

selected sensor must be amenable to the prevailing condition and not for this whole range. 
23 

Although significant pressure fluctuations are unlikely, sensors must be calibrated to prevailing 

pressure levels, as barometric pressure depends on the installed elevation (from approximately 1 

bar at sea level to 0.8 bar at 2000 m elevation). Routine maintenance, such as calibration, should 

be required once per year or sooner per manufacturer recommendations, but longer times 

between routine maintenance would significantly lower operational costs. The target sensor cost, 

which would include all control elements and user interfaces, can be set at $100, but lower costs 

are of course desired. 

Table 5 summarizes the sensor requirements and specifications for in-dispenser sensor 

deployment. A relative ranking of each identified performance metric is presented and assigned a 

qualitative ranking of low, medium, or high importance, as indicated by the marker in the 

appropriate column. The position of the marker refines the assessment; a left justified mark 

indicates a lower importance than a right or middle alignment. 

Table 5. Hydrogen Sensor Metric Rankings for Indoor Fueling – In-Dispenser Deployment 

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO

LDL X 0.04 vol% (1% of LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X To LFL (0.1–4 vol% H2)

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 1 s

Recovery Time X 1 min

Repeatability X

Signal Drift X To avoid false positives and negatives at all alarm levels

Environmental Impacts T and RH are more relevant than P

- T X -10° to +40°C

- P X Must be calibrated for deployment altitude

- RH X 5%–95% RH

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X May be exposed to 100% H2 (or very high H2)

Deployment Parameters

Capital Cost X <$100 

Installation Cost X Part of capital cost

Physical Size X Must be physically compatible for in-dispenser deployment 

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf

Alarm Thresholds X Must indicate adverse condition

- Trace (1,000 ppmv) X Not required (trace background may be present)

- Low (low risk) X 10% of the LFL, to track intermediate or increasing levels

- High (pending risk) X 25% of the LFL, shutdown system for multiple alarm levels

Regulations and Codes X NFPA 2, Class I, Division 2 operation listing required

Deployment Placement X Guidance on placement is needed
24 

Parameters Importance Specifications and Notes Low Medium High

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years

- Unit Replacement X 5 years, 10 YEARS desired

Consumables X None

Calibration Schedule X >once per year; longer time between calibrations is desired

Maintenance X Maintenance free except calibration

Sample Size X No critical restriction

Matrix Requirements X Normal air environment 

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X Display not necessary

- Remote Monitoring X Remote interrogation is useful, output should be %H2 units

Device Repeatability X Plug in replacement useful, but must be calibrated

Warm-Up Time X <1 hour for initial installation,15 minutes for all shutdowns

Alarm Interface X Must connect to control system 

- Number of set points X One mandatory, two useful shutdowns

- Audible X Activate at lowest alarm set point

- Ventilation X Activate at 1st set point if feasible

- Shutdown X Activate at 2nd set point (25% of LFL)

- Remote X Option to external alarm, and remote transmitters

Mechanical Stability X Vibration tests, specification TBD

Power Requirements X Moderate power requirements preferred (<0.5 W)

2.2.3.1 Critical Gaps and Deficiencies

The operational requirements are a major limiting factor for extensive sensor deployment. 

Acceptance is severely affected by cost of maintenance, especially calibration requirements. 

Capital costs for the sensors are also a factor impacting acceptance. This factor is typically a onetime upfront expense, so it does not impact as strongly as the ongoing operational costs. 

Nevertheless, there is always a need for lower cost systems, providing the sensor reliability is not 

compromised. 

Sensor deficiency areas and gaps inhibiting deployment can be summarized as follows. A major 

deficiency represents a critical issue that has been problematic (e.g., occurring with an 

unacceptable frequency). A secondary issue is important but is either not critical or has not been 

reported as occurring.

• Major

o Maintenance and calibration are major shortcomings 

– Long-life sensors (>5 years)

– Manual calibrations no oftener than once per year (less often than once per 

year or no calibration requirement is preferable)

– All other maintenance requirements less often than once per year

– Robustness against poisons and dust 

– Response time of 1 s
25 

o Acceptable sensor target cost: <$100 per sensor)

o Deployment guidance 

• Secondary 

o Capital cost for appropriate monitoring systems 

2.2.3.2 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

One proposed strategy to facilitate sensor deployment would be to simplify and economize the 

operational requirements. Robust sensors with long operational lifetimes would cut down 

replacement cost, and could in principle lower the cost of maintenance by increasing the time 

between calibrations. Robust sensors should eliminate most nonroutine maintenance 

requirements, although sensor calibrations would still be required. Most manufacturers require 

periodic calibration to ensure sensor accuracy and functionality. Sensor performance can be 

affected by a variety of stresses, including chemical (e.g., exposure to poisons), mechanical (e.g., 

thermal stresses and vibration), and electrical factors (e.g., power surges, shutdowns, and circuit 

failures). Underlying codes may specifically require a minimal calibration cycle. Usually 

calibrations are to be performed at least once per year, but could be more frequent based on 

either local requirements or manufacturer recommendations. 

Because of the broad range of factors that can affect sensor performance, it is unlikely that the 

sensor calibration requirement will be totally eliminated. Thus, an automated calibration system 

would significantly lower operational costs associated with wide-scale sensor deployment for 

battery backup systems. Acceptance of such a system would be facilitated if it were available at a 

reasonable cost (<$100 per sensor per year, including initial capital cost). Currently, the sensor 

of choice for in-dispenser deployment has been the catalytic bead sensors (also called CGSs), but 

no specific technology has been explicitly endorsed. The CGS is a high-temperature sensor; a 

room temperature sensor may have safety advantages. Sensors listed to national standards will 

likely be required, especially to be compliant for Class I, Division 2 operation. 

Sensor capital cost is likely to decrease with demand as the hydrogen infrastructure develops.

However, certain platforms and designs will be better impacted with economies of scale, 

especially those that are amenable to high-scale manufacturing afforded by technology 

developed by the microelectronics industry. Miniaturized devices show particular potential.

Miniaturized sensors, mass-produced using methods developed for the electronics industry (e.g., 

microfabrication, MEMs, or thin-film deposition protocols), could provide low-cost, low-power 

devices, coupled with a rapid response; a 1-s response time is feasible. However, the sensor must 

also still meet the analytical requirements including range and robustness requirements 

(insensitive to poisons, interferences, and drift). Commercial hydrogen sensor technologies are 

emerging for a number of thin-film and other microfabrication platform types. Although these 

show a fast response to hydrogen, these devices tend to saturate at low concentrations and to 

exhibit a poor dynamic range. Their long-term stability and robustness to stresses (e.g., chemical, 

thermal, and moisture) should be more thoroughly studied. 

Guidance about sensor deployment and the relationship to appropriate action levels is needed. 

The number of internal dispenser sensors will be kept to a minimum, mostly likely one. The 

sensor must be able to respond to internal leaks, and thus should be designed into the dispenser 

rather than treated as an add-on. The dispenser should also be designed so as to not trap 

hydrogen and to channel internal gas releases to the sensor.
26 

2.3 Residential

• Application 

o Safety sensors installed in private dwellings 

• Applicable standards 

o Listed sensors

– UL 60079-15 or equivalent listing requirements for use in Class I, 

Division 2 hazardous locations 

– UL 2075 or CSA C22.2, No. 152 or equivalent listing for sensor 

performance 

o NFPA 2, Hydrogen Technologies Code 

o Local AHJs invoking modified fire code 

o Society of Automotive Engineers standards for hydrogen vehicles (e.g., J2579) 

2.3.1 Background

The target release date for commercial hydrogen light-duty road vehicles is set for 2015. It is 

projected that by 2025 there will be more than 1 million FCVs in the United States [4]. Thus, 

although there is as yet no market for residential hydrogen sensors, it is likely to be quite large 

within a few years. Hydrogen vehicles will be parked on private property, including inside 

residential garages. The design of on-board hydrogen storage systems, per SAE J2579, dictates 

performance-based requirements to limit worst-case leakage so that the LFL will not be reached 

in a minimum sized garage space. Although flow regulators, shutoff valves, and bypass valves 

will be incorporated into the vehicle fuel system to minimize released hydrogen volumes, 

hydrogen sensors in areas around the vehicle will increase safety. Safety sensors on board the 

FCV may be considered to provide indoor monitoring capability, but these sensors are designed 

to ensure passenger safety. Specifically, on-board sensors will monitor for hydrogen within the 

passenger and trunk compartments, and thus would not respond to hydrogen releases external to 

the vehicle. Because of safety concerns, it is preferred that hydrogen releases vent external to the 

vehicle rather than internally where they can quickly concentrate to hazardous levels; less than 1 

g of hydrogen uniformly distributed in 8 ft3

 would exceed the LFL. 

Sensors on board FCVs may also not be operational when the vehicle is turned off. Thus, if the 

vehicle is parked in a garage, the external venting of hydrogen would then be contained within 

the garage, and would most likely be undetectable with on-board vehicle sensors. These releases 

can best be detected with a dedicated sensor mounted inside the garage. Furthermore, ongoing 

NFPA 2 code development may require that hydrogen sensors be placed in garages that house 

hydrogen vehicles, although this is not yet the case. Even if not mandated by code, some 

homeowners may decide to install hydrogen sensors. The residential application thus represents 

an emerging and potentially very large market for hydrogen detectors. In many ways the need for 

residential hydrogen detector deployment is analogous to home CO detectors. There is a 

potentially large sales volume, but this market demands low-cost, low-maintenance technology 

with high reliability. Garage deployments can be more challenging because there is less 

environmental control and more potential exposures to chemicals than those encountered in 

residential living spaces. 
27 

At a minimum, a single alarm level that activates at 10% of the LFL (or possibly 25% of the 

LFL), would be required for residential hydrogen safety sensors. Two sensor alarm levels could 

be of interest for larger residential garages such as apartment building parking structures. These 

would be a low-level response (e.g., 10% of the LFL, 0.4 vol% H2) that would enhance 

ventilation and a medium-level response (e.g., 25% of the LFL, 1 vol% H2) that would activate 

an audible alarm. The second alarm level might also interface to the vehicle to shut down the fuel 

delivery system and isolate the bulk hydrogen from the release point. However, such multilevel 

alarms with active feedback to both home and vehicle control systems may be too complicated 

and expensive for general consumer use and acceptance.

Comparisons between residential hydrogen detectors and home CO detectors can be made.

Residential CO detectors are priced well below $100. Home CO detectors certified to UL 2034, 

Single and Multiple Station Carbon Monoxide Alarms have multilevel scenarios to activate an 

audible alarm or other indication. These scenarios require faster response times for higher 

concentrations of CO [19]. Specifically, home CO detectors will alarm at a low CO 

concentration (70 ppmv) providing it has been present for longer than 1 h and must alarm within 

4 h; a midlevel CO concentration (150 ppmv) must activate the alarm within 15–50 min, whereas 

a higher concentration of CO (400 ppmv), must activate the alarm between 4 and 15 min. This 

multilevel alarm threshold is based on the known toxicity properties associated with CO 

exposure. There is generally more tolerance to low concentrations of CO, thus allowing longer 

exposure times. Allowable exposure times become correspondingly shorter with higher 

concentrations, thereby shortening the instrument alarm activation time. Typically, however, 

only a single alarm type (e.g., audible) is incorporated into the detector, and it is not possible to 

ascertain which scenario triggered the alarm.

Alternatively, hydrogen exposure has no known adverse health effects, providing that it does not 

displace oxygen below 19.5 vol% [20]. This requires approximately 7 vol% hydrogen. Hydrogen 

becomes immediately hazardous when its concentration exceeds the LFL of 4 vol% in air, 

regardless of time at this concentration. Thus, hydrogen alarm levels should be set to an 

acceptable fraction of the LFL. At a minimum, a medium-level alarm threshold of 25% of the 

LFL (1 vol% H2 in air) is recommended as soon as hydrogen is detected at this concentration. 

Depending on jurisdiction, the alarm set point could be 0.4 vol% (10% of the LFL). Trace level 

detection (e.g., 1000 ppmv or less) is not likely to be necessary, nor would it be advisable, 

because residual hydrogen levels are likely from releases associated with standard fuel cell purge 

cycles that could occur during vehicle startup or idling. 

The basic consumer hydrogen detector would primarily provide an alarm and would not require 

interfacing to the ventilation control system (although this could be an option) or communicate 

with the vehicle fuel shutoff system. The audible alarm must, however, be detectable external to 

the garage, including (when appropriate) within the attached house (either audibly or by some 

other indication, such as flashing lights). It will be necessary to train FCV owners and their 

families to respond appropriately in the event of a hydrogen sensor alarm, which could include: 

• Avoid entering the garage until it has been demonstrated that it is safe to do so. 

• Evacuate the house if it is attached to the garage. 

• Contact emergency response personnel. 
28 

The sensor response time should be fairly fast, allowing alarms within 30 s. This should be 

adequate, as concentration buildup in a garage is likely to be slow – most releases will be slow 

and the migration of hydrogen to the sensor will be controlled by diffusion processes. Displays 

or other readout systems are optional, but if present, the output should be in engineering units 

(e.g., vol% H2). Exposures to higher hydrogen concentrations (e.g., 10 vol% or greater) should 

not adversely affect the analytical capability of the sensor, but exposure to pure hydrogen is 

unlikely. The sensor should not be affected by common interferences, including CO and sulfur 

compounds. Because silicone compounds are used extensively in consumer products such as 

sealants and lubricants, the sensor should also be resistant to poisoning by silicone vapors. 

Residential garages will have at best minimal climate control, so sensors ought to operate over a 

temperature range of -40 to +40°C and a humidity range of 10%– 95% RH. Less restricted 

ranges, especially for temperature, are allowed based on regional considerations. Although 

significant pressure fluctuations are not expected, sensors must be calibrated to prevailing 

pressure levels because barometric pressure depends on the installed elevation (from 

approximately 1 bar at sea level to 0.8 bar at 2000 m). Other than routine power checks to ensure 

battery backup, there should be no user-required maintenance procedures, including calibrations. 

Five-year reliability is necessary for the consumer market. The target sensor cost should be less 

than $100, but lower costs are of course desired. 

Table 6 summarizes the sensor requirements and specifications for facilities with indoor fueling 

operations. A relative ranking of each identified performance metric is presented and assigned a 

qualitative ranking of low, medium, or high importance, as indicated by the marker in the 

appropriate column. The position of the marker refines the assessment; a left justified mark 

indicates a lower importance than a right or middle alignment. 

Table 6. Hydrogen Sensor Metric Rankings for Residential Applications 

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO, household solvents and paints

LDL X 0.04 vol% (1% of LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X To LFL (0.1–4 vol% H2)

Accuracy X Within ±50% of reading for all working (T,P, RH) conditions

Response Time X 30 s

Recovery Time X 1 min

Repeatability X

Signal Drift X To avoid false positives and negatives at all alarm levels

Environmental Impacts T and RH are more relevant than P

- T X -40° to +40°C to cover both northern and desert climates

- P X Must be calibrated for deployment altitude

- RH X 5%–95% RH

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Unlikely to be exposed to 100% H2 (or very high H2)

Deployment Parameters

Capital Cost X <$100

Installation Cost X Easily mounted (no special requirements or costs)

Physical Size X Size not a significant issue for wall/ceiling mounting 
29 

Parameters Importance Specifications and Notes Low Medium High

Control Circuitry X Simple, must be part of integrated system for deployment

Electronic Interface X Output in engineering units (e.g., % H2) 

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Currently not available, but must be off-the-shelf by 2015

Alarm Thresholds X Must indicate adverse condition

- Trace (1,000 ppmv) X Not required (trace background may be present)

- Low (low risk) X 10% of the LFL, activate alarm (or 25%)

 - High (pending risk) X 25% of the LFL, activate alarm

Regulations and Codes X NFPA 2, possible performance specification

Deployment Placement X Guidance on placement is needed

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years

- Unit Replacement X 5 years, 10 years desired

Consumables X None

Calibration Schedule X No calibration during operational lifetime

Maintenance X Maintenance free 

Sample Size X No critical restriction

Matrix Requirements X Normal air environment 

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% or 25% of LFL, depending on AHJ)

- Displays X Display not necessary

- Remote Monitoring X Not required

Device Repeatability X Will be replaced as unit

Warm-Up Time X <1 h for initial installation, 15 min for all shutdowns

Alarm Interface To activate audible alarm, which is part of system 

- Number of set points X One mandatory

- Audible X Activate at alarm set point

- Ventilation X Not required 

- Shutdown X Not relevant

- Remote X Perhaps useful to warn against entry

Mechanical Stability X No special requirements for fixed deployment

Power Requirements X Moderate power requirements preferred (<0.5 W)

2.3.2 Critical Gaps and Deficiencies

Acceptance in a consumer market will be severely affected by the original capital cost of the 

hydrogen sensor. Using the home CO monitor as a model system, residential hydrogen sensors 

must cost less than $100, and preferably less than $50. Other costs, such as those associated with 

maintenance, especially calibration requirements, have to be negligible. The sensor must operate 

reliably (e.g., no false positives or false negatives) for several years. A 5-year deployment 

lifetime is a good target. 

Sensor deficiency areas and gaps inhibiting deployment can be summarized as follows. A major 

deficiency represents a current critical issue that has been problematic (e.g., occurring with an 

unacceptable frequency). A secondary issue is important but is either not critical or has not been 

reported as occurring.
30 

• Major

o Maintenance and calibration are major shortcomings 

– (Long-life sensors (>5 years)

o No calibrations for the life of the sensor (5 years) 

o All other maintenance requirements (e.g., backup batteries) >1 year

o Robustness against poisons and dust 

o Controlling the human response when confronted with a hydrogen sensor in alarm

o Availability of appropriate technology for the application (Level of maturity) for 

<$100 

o Listing and certification requirements

• Secondary 

o Deployment guidance 

2.3.3 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

Presently, hydrogen detectors are designed for residential applications. Industrial monitors are of 

course available, but these will not directly transfer to consumer applications because of cost and 

possible complex operational issues. One means to ensure availability of residential detectors is 

to support product development through grants or other incentives. Alternatively, as the private 

FCV fleet grows, so will the residential market. This will provide incentive to home safety 

system manufacturers (e.g., manufacturers of home CO and smoke detectors) to expand into this 

market. Market forces will tend to ultimately keep overall cost reasonable, but this must occur 

without corrupting performance. Economical product development will take time, however, and 

the cost of the first versions of the residential hydrogen detector should not be so high as to 

induce impediments to their deployment or to FCV acquisition. Thus, capital cost may initially 

impede deployment, but should ultimately become less significant. A second deployment 

concern pertains to guidance on the location of the hydrogen detector. Fortunately, preliminary 

research on this topic has been performed [21]. Hydrogen distribution in a normal garage is 

sufficiently fast and the deployment location has minimal impact on sensor response time, 

providing that the sensor was mounted above the potential hydrogen source. This work did not 

address thermal entrapment zones that are formed because of warmer temperatures near the 

ceiling than in the bulk, which preclude recommending deployment of sensors directly at the 

ceiling. Formalized guidance on sensor deployment, however, must be developed. 

It is not unlikely that home hydrogen detectors will require listing to performance-based 

standards, such as UL 2075, providing they are compatible for the residential market. Such 

listings have been imposed on home CO detectors (e.g., UL 2034). Certification is an expensive 

process, and, without inducements, will be pursued by sensor manufacturers only after the

development of a sizable market associated with commercial release of FCVs. The current 

standard needs to be reviewed, and edited if necessary for the residential market. Personnel in the 

NREL sensor test laboratory are on the UL 2075 Standards Technical Panel. 

Sensor capital cost is likely to decrease with demand as the hydrogen infrastructure develops. 

However, certain platforms and designs will be better impacted by economies of scale, especially 
31 

those that are amenable to high-scale manufacturing afforded by technology developed by the 

microelectronics industry. Miniaturized devices show particular potential, and commercial 

devices are being developed based on thermal conductivity devices (TCDs), MOX, CGSs, 

palladium thin films, and other platforms. However, some platforms are highly susceptible to 

drift induced by moisture fluctuations. Miniaturized sensors, mass-produced using methods 

developed for the electronics industry (e.g., microfabrication, MEMs, or thin-film deposition 

protocols), could provide low-cost, low-power devices, coupled with a rapid response; a 1-s 

response time is feasible. However, the sensor must also still meet the analytical requirements 

lifetime and robustness requirements (insensitive to poisons, interferences, and drift). Home CO 

alarms could form the basis of the residential hydrogen detector, albeit with modification of the 

sensor element to optimize for hydrogen measurements. Home CO detectors use primarily 

MOXs and electrochemical sensors (for higher end models). Both platforms can be used for 

hydrogen detection. Improvements in cross-sensitivity may be necessary; a means to compensate 

for temperature fluctuations must also be developed. Sensor lifetime will have to be increased to 

ensure a reliable 5-year deployment.

Finally, an FCV owner should receive training about hydrogen safety, which should include the 

importance of hydrogen detectors and what to do when they go into alarm. 

2.4 Production

• Applications 

o Distributed production (localized low to medium scale hydrogen production) 

o Centralized production (large scale, with hydrogen transported to utilization site) 

• Applicable standards 

o NFPA 2 Hydrogen Technologies Code 

o Local AHJs invoking modified fire code 

o Listing of sensor and sensor components 

o UL 60079-15 or equivalent listing requirements for use in Class I, Division 2 

hazardous locations

o UL 2075 or CSA C22.2, No. 152 or equivalent listing for sensor performance 

o FM Global Class Number 6310 and 6320 

o U.S. Department of Transportation (DOT) transport of flammable material 

2.4.1 Background

The hydrogen vehicle infrastructure requires several support systems and operations, including 

delivery or on-site production, on-site storage (typically outdoors), transfer hardware, and 

dispensing systems. In this section, issues pertaining to sensor requirements for hydrogen 

production are discussed. Two main methods of hydrogen production are reforming from 

methane (natural gas) and through water electrolysis; other experimental production methods 
32 

(e.g., algae, biomass, and photoelectrolysis over specialized catalyst) are being explored. There 

are two broad categories of hydrogen production: centralized large-scale and distributed small- to 

medium-scale production. Centralized production facilities are well established and have been 

operating for many years to fulfill the traditional hydrogen markets, especially hydrogen for the 

petroleum industry. The distributed production would be for on-site use (e.g., fueling stations 

with on-site electrolyzes or reformers), and daily capacity at 5000 kg or less. Central production 

technology has been developed for established industries and may ultimately be the main 

supplier of hydrogen providing the transportation or transport infrastructure is established; this 

would require increased capacity for trucking and ultimately the establishment of a hydrogen 

pipeline network. Alternatively, distributed production with localized small- to medium-scale 

capacity is likely to play a significant role in the early phases of the developing hydrogen 

infrastructure. This role is necessary in part because of the needed support systems for 

centralized productions (e.g., pipelines, transport systems), which are less developed than the 

support systems required for distributed production (e.g., availability of water and/or natural gas, 

electricity). The two production types will be treated separately.

2.4.2 Distributed Hydrogen Production

Site reformers and electrolysis units are already commercially available and deployed in 

numerous industrial facilities and fueling stations. These systems produce hydrogen for 

stationary fuel cells deployed for backup power and for road vehicles and may, in the future, 

provide hydrogen for forklifts and other applications. Distributed (or localized) hydrogen 

production is typically performed within structures to protect expensive hardware from the 

weather. The hydrogen can be produced for on-demand use by a stationary fuel cell or stored onsite for future use. Two main sensor applications are associated with distributed hydrogen 

production: (1) facility monitoring for external releases (e.g., an area monitor); and (2) 

deployment within the production unit (e.g., a contained environment monitor). These 

applications are treated separately below.

2.4.2.1 Distributed Hydrogen Production – Facility Monitoring

Distributed (or localized) hydrogen is currently being produced using on-site reformers or on-site 

water electrolysis units. These operations are 

typically within structures to provide some protection 

of the hardware from the weather and proximal 

activity. Accordingly, the hazards and risks associated 

with indoor on-site production are analogous to other 

indoor operations (see Section 2.2.1). Although flow 

regulators, shutoff valves, and bypass valves are 

incorporated into hydrogen production, hydrogen 

sensors, which are independent of operation, provide 

increased safety.

Hydrogen sensors can remain active in event of a power outage via battery backup systems. 

Sensors thus play a critical role in hydrogen production safety. Sensors are also mandated by 

code for indoor fueling (e.g., NFPA 2, Hydrogen Technologies Code, 2011 Edition). Local AHJs 

may impose locally modified fire codes and thereby regulate additional safety system designs for 

sensor deployment (e.g., nature of alarms, alarm levels, integration to facility ventilation 

systems). Accordingly, hydrogen sensors should be mounted proximal to the hydrogen 

NREL PIX 21392
33 

production unit; the sensor can be mounted on the wall adjacent to the production unit or on or 

near the ceiling above the production unit. Most sensors deployed at distributed production sites 

are set to activate an audible alarm only. The alarm set point is typically 0.4 vol% hydrogen, 

which is 10% of the LFL. This is the most efficient and cost-effective use of sensors, but 

multiple alarm levels could be useful. For example, an audible alarm could be activated at 10% 

of the LFL, but at 25% of the LFL the system shuts down. 

Sensors listed as compliant to national standards for instrument safety (e.g., Class I, Division 2 

listings) are recommended for deployment around production facilities. Performance-based 

standards (UL 2075 and CSA C22.2, No. 152) would also be useful. Some jurisdictions have 

already mandated that hydrogen safety sensors be listed as compliant to UL 2075 for indoor 

operations, and at some point this may be more universally required. The gas detector should 

have electronic outputs that are in engineering units (e.g., vol% H2) or easily converted to 

engineering units. Although not required, telemetric communication capability could be useful, 

especially to warn first responders and site engineers of a release before it becomes hazardous.

It may be useful and sometimes mandated by AHJs for the sensor to interface with ventilation 

systems. The ventilation system would be activated at the low alarm level for operations that 

have multiple alarm levels in the sensor. Therefore, relays would be useful to activate the alarm 

and to interface to the ventilation system. Several factors contribute to the desired sensor 

response time. System leaks can quickly lead to situations of concern, so a reasonably fast 

response time is desirable. Alternatively, however, because the sensor is likely to be mounted 

remotely from the source, diffusion processes will dominate its response. A response time of 30 s 

would seem adequate for a facility sensor. Herein, the response time can be defined as the time 

to reach 90% of the final indication that is achieved when the sensor is exposed to a steady flux 

of hydrogen at the designated alarm level (e.g., either 25% or 10% of the LFL if that is the alarm 

level). Although unlikely, the sensor may experience a direct hit of hydrogen by a leak jet. 

Accordingly, a brief exposure to pure hydrogen should not impact the analytical capability of the 

sensor.

The sensor should not be affected by common interferences, including but not limited to carbon 

monoxide and sulfur compounds. Reformer production units could also have methane, which 

will be an interferent on several sensor platform types (CGSs and MOXs, for example); on the 

other hand, the CGS could be used to monitor for both hydrogen and methane, although it would 

not be able to distinguish between these compounds. Water electrolysis units do not require 

methane or natural gas, although natural gas may be present for other purposes (e.g., facility 

heating). Because silicone compounds are used extensively as sealants, lubricants, and pneumatic 

elements, the sensor should also be resistant to silicone poisoning.

Structures housing production units will have some but not necessarily extensive indoor climate 

control, so deployed sensors ought to operate over broad temperature and humidity ranges. 

Working temperature ranges for the sensor should be consistent with the deployment facility, 

which can range from below -40°C for northern climates to +40°C for summer operation in a 

thermally unregulated facility. Of course, the selected sensor must be amenable to the prevailing 

condition and not for this whole temperature range. Although significant pressure fluctuations 

are not expected, sensors must be calibrated to prevailing pressure levels, as barometric pressure 

depends on the installed elevation (from approximately 1 bar at sea level to 0.8 bar at 2000 m 

elevation). Routine maintenance such as calibration should be once per year or sooner per 
34 

manufacturer recommendations, but longer times between routine maintenance would 

significantly lower operational costs. The target sensor cost, which would include all control 

elements and user interfaces, can be set at $500, but lower costs are of course desired.

Table 7 summarizes the sensor requirements and specifications for facilities with indoor fueling 

operations. A relative ranking of each identified performance metric is presented and assigned a 

qualitative ranking of low, medium, or high importance, as indicated by the marker in the 

appropriate column. The position of the marker refines the assessment; a left justified mark 

indicates a lower importance than a right or middle alignment. 

Table 7. Hydrogen Sensor Metric Rankings for Distributed Hydrogen Production – 

Facility (Area Monitors)

Parameters Importance Specifications and Notes

Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO

LDL X 0.04 vol% (1% of LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X To LFL (0.1–4 vol% H2)

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 30 s

Recovery Time X 60 s

Repeatability X Not critical

Signal Drift X To avoid false positives and negatives at all alarm levels

Environmental Impacts X T and RH are more relevant than P

- T X -40°C to +40°C

- P X ust be calibrated for deployment altitude

- RH X 15%–95% RH

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Relevant for sensors mounted near tank

Deployment Parameters

Capital Cost X <$500

Installation Cost X

Physical Size X Small size compatible with mounting around tank

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf

Alarm Thresholds X Must indicate adverse condition

- Trace (1,000 ppmv) X Not required for on-board, trace background is possible

- Low (low risk) X 10% of the LFL, indicate adverse for single alarm level

- High (pending risk) X 25% of the LFL, shut down system for multiple alarm levels

Regulations and Codes X Possible local AHJs

Deployment Placement X Must avoid pockets

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X years

- Unit Replacement X 5 years
35 

Parameters Importance Specifications and Notes

Low Medium High

Consumables X None

Calibration Schedule X >once per year; longer time between calibrations is desired

Maintenance X Maintenance free except calibration

Sample Size X No critical restriction

Matrix Requirements X Normal air environment

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X Readout is not necessary

- Remote Monitoring X Remote interrogation is not necessary

Device Repeatability X Plug in replacement is useful

Warm-Up Time X <1 h for initial installation,15 min for all shutdowns

Alarm Interface

- Number of set points X One mandatory, two useful shutdowns

- Audible X Activate alarm at lowest set point

- Ventilation X Activate at 1st set point, if feasible

- Shutdown X Activate at 2nd set point (25% of LFL)

- Remote X Not necessary

Mechanical Stability X Vibration tests, specification TBD

Power Requirements X Moderate power requirements preferred (<0.5 W)

2.4.2.1.1 Critical Gaps and Deficiencies

The operational requirements for sensors are a major limiting factor for extensive sensor 

deployment. Acceptance is severely affected by cost of maintenance, especially calibration 

requirements. Small drifts in the sensor background often lead to the need for on-site calibration.

Capital costs for the sensors are also a factor impacting acceptance. This is typically a one-time 

upfront expense, so it does not impact as strongly as the ongoing operational costs. Nevertheless, 

there is always a need for lower cost systems, providing the sensor reliability is not 

compromised. A second operational concern pertains to actual deployment of the sensor. There 

is some ambiguity in defining optimal location of deployment and required distribution of the 

sensors; it is likely that multiple sensors will be required, depending on the size and complexity 

of the facilities. Although alarm levels were defined earlier in this section, actual detection will 

be remote from the hydrogen source. There is thus some uncertainty in the appropriate alarm 

levels because of potential impacts of dilution as the vapor migrates from the source. This is 

especially true for the mounting of sensors in ventilation systems in which gases are collected 

from multiple locations.

The major gaps inhibiting optimal sensor utilization in on-site hydrogen production facilities can 

be summarized as:

• Major 

o Maintenance and calibration are major shortcomings 

– Long-life sensors (>5 years)

o Manual calibrations no oftener than once per year (less often than once per year or 

no calibration requirement is preferable)

o All other maintenance requirements less often than once per year
36 

o Robustness against poisons and dust 

o Unacceptable sensor drift (necessitating recalibration or system shutdowns)

• Secondary 

o Capital cost for appropriate monitoring systems 

o Acceptable target: <$500 per sensor

2.4.2.1.2 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

One proposed strategy to facilitate sensor deployment is to simplify and economize the 

operational requirements. Robust sensors with long operational lifetimes would cut down 

replacement cost, and could in principle lower the cost of maintenance by increasing the time 

between calibrations. Robust sensors should eliminate most nonroutine maintenance 

requirements, although sensor calibrations would still be required. Many manufacturers require 

periodic calibration to ensure sensor accuracy and functionality. Sensor performance can be 

affected by a variety of stresses, including chemical (e.g., exposure to poisons), mechanical (e.g., 

thermal stresses and vibration), and electrical factors (e.g., power surges, shutdowns, and circuit 

failures). Underlying codes may specifically require a minimal calibration cycle. Usually 

calibrations are to be performed at least once per year, but could be more frequent based on 

manufacturer recommendations. Because of the broad range of factors that can affect sensor 

performance, it is unlikely that the sensor calibration requirement will be eliminated. Thus, the 

development of automated calibration system would significantly lower operational costs 

associated with wide-scale sensor deployment for battery backup systems. Acceptance of such a 

system would be facilitated if it were available at a reasonable cost (<$100 per sensor).

Sensor capital costs are likely to decrease as demand increases as the hydrogen infrastructure 

develops. Currently, the sensor of choice for indoor monitoring has been the catalytic bead 

sensor, although no specific technology has been endorsed. However, certain platforms and 

designs will be better impacted with economies of scale, especially those that are amenable to 

high-scale manufacturing afforded by technology developed by the microelectronics industry. 

Miniaturized devices show particular potential. Miniaturized sensors, mass-produced using 

methods developed for the electronics industry (e.g., microfabrication, MEMs, or thin-film 

deposition protocols), could provide low-cost, low-power devices, coupled with a rapid response; 

a 1-s response time is feasible. However, the sensor must also still meet the analytical 

requirements including range (e.g., multiple alarm levels) and robustness requirements 

(insensitive to poisons, interferences, and drift). Commercial hydrogen sensor technologies based 

on thin-film and other microfabricated platforms are emerging. Although these devices show a 

fast response to hydrogen, they tend to saturate at low concentrations and to exhibit a poor 

dynamic range. Their long-term stability and robustness to stresses (e.g., chemical, thermal, and 

moisture) should be more thoroughly studied. 

Guidance about sensor deployment (e.g., ceiling, walls, or in ventilation systems) and the 

relationship to appropriate action levels is needed. The sensor will likely be placed remote from 

the source, and the distance may vary depending on the specific source of the leak. Because the 

sensor will be remote, the detected hydrogen gas concentration could be significantly attenuated. 

This trend is especially true for sensors deployed in ventilation systems, although sensors 

mounted on the wall adjacent to the dispenser are more common. Although mandated by code 

(e.g., NFPA 2), no specific guidance on the means to best position the sensor to capture a 
37 

hydrogen generation event is provided. Nor is there rational guidance on the appropriate number 

of sensors to be installed in a facility. For example, will a single point sensor mounted on the 

wall adjacent to the dispenser provide adequate assurance of detecting a hydrogen release?

Hydrogen can concentrate remote from the dispenser via natural prevailing dispersion processes 

or from mobile source (e.g., from the forklift). Would a distributed array of point sensors be 

more advantageous and a better approach than wide area monitoring technologies? Cost, 

performance, and level of commercial maturity must be considered. CFD modeling of various 

generation rates (low and high) and sensor deployment should be performed to specifically 

provide such guidance on position and appropriate alarm levels [22]. This guidance should 

ultimately be incorporated into or cited by NFPA 2. 

2.4.2.2 Distributed Hydrogen Production – In-Unit Sensor Deployment

Distributed (or localized) hydrogen production is currently being performed using on-site 

reformers or on-site water electrolysis units. In addition to area monitors, hydrogen safety 

sensors may be installed in the enclosure surrounding the production unit. A sensor within the 

enclosure represents a contained environmental application. A small hydrogen release could 

quickly lead to a dangerous situation. Although flow regulators, shutoff valves, and bypass 

valves are incorporated into the dispenser safety system, hydrogen sensors, which are 

independent of operation, increase safety. Furthermore, the sensor can also operate on backup 

power, and thus remain active even in the event of a power outage. Although not mandatory by 

national standards, local jurisdictions and customers have chosen to have hydrogen sensors 

installed in production units. To avoid buildup of dangerous gases, the enclosures are oftens 

purged with ambient air, although this is more common with reformers. If a sensor is deployed, it 

will typically monitor for hydrogen in this air purge. The internal environment differs between 

reformers and electrolysis units, so sensors are 

subjected to different stresses (environmental, 

chemical, and physical). Enclosures for both 

technologies will tend to be at elevated temperature 

and humidity levels than ambient. High humidity is 

especially true for electrolysis units. Because

oxygen is an unused by-product coproduced with 

hydrogen, enclosures around electrolysis units may 

also have an enriched oxygen atmosphere relative 

to ambient. Reformers may have CO, carbon 

dioxide, and the feed gas within the enclosure. 

Sensors listed as compliant to national standards (e.g., Class I, Division 2 listings) are required 

for deployment within production units and other contained environments. In addition to 

electrical safety standards for Class I, Division 2 operation, there are sensor performance-based 

standards (UL 2075 and CSA C22.2, No. 152). Some jurisdictions have already mandated that 

hydrogen safety sensors be listed as compliant to UL 2075 for indoor operations, and at some 

point this may be more universally required. The sensor should have electronic outputs that are 

in engineering units (e.g., vol% H2) or easily converted to engineering units. Many end users 

require telemetric communication capability (or interface to a control system that can be 

accessed remotely).

Many manufacturers monitor their commercial production units (especially those providing 

hydrogen to stationary fuel cells). Internal control systems that allow the manufacturer remote NREL PIX 23191
38 

access to system controls and sensors fulfill the telemetric communication requirement, but this 

may not always be the case. System leaks in the production unit can quickly lead to high 

hydrogen concentrations, and a fast response time is desirable. Accordingly, a response time of 1 

s is recommended. In a contained environment there is an increased likelihood that a release jet 

would directly contact the sensor. The sensor should therefore be immune to or recover quickly 

from a brief exposure to a high concentration of hydrogen. 

The sensor should not be affected by common interferences including but not limited to CO and 

sulfur compounds (this requirement is less critical for electrolysis units). Carbon dioxide levels 

will be elevated in reformer units; in electrolysis units, the oxygen level will be elevated. Thus, 

depending on deployment, the sensors should be immune to higher levels of these two gases. 

Because silicone compounds are used extensively as sealants, lubricants, and pneumatic 

elements, the sensor should also be resistant to poisoning by silicone. Working temperature 

ranges for the sensor should be consistent with the production unit environment, which can range 

up to +60°C or hotter. Although significant pressure fluctuations are unlikely, sensors must be 

calibrated to prevailing pressure levels, as barometric pressure depends on the installed elevation 

(from approximately 1 bar at sea level to 0.8 bar at 2000 m elevation). Routine maintenance, 

such as calibration, should be no more frequent than once per year. Longer times between 

routine maintenance would significantly lower operational costs. Calibration, as with other 

service activities on the production units, is performed by trained personnel. To minimize costs, 

nonroutine maintenance activity must be eliminated.

A reformer developer reported that sensors currently deployed in reformers tend to show 

unacceptable drift and thus require significantly more frequent calibration than that specified by 

the manufacturer. This often results in unscheduled sensor maintenance. One reformer developer 

has set the maximum allowable baseline drift between calibration cycles to a signal 

corresponding to <10% of the lower alarm level (that is, the drift in the sensor output should 

correspond to <0.25 vol% hydrogen). Because baseline drift cannot be differentiated from a 

response due to hydrogen, physical inspection and on-site calibration have been required once a 

minimal drift threshold has been detected. Often such drifts are due to a change in the sensor 

baseline as opposed to actual hydrogen, and thus can be classified as false positives. 

Accordingly, deployed sensors are being perceived as not reliably meeting an acceptable stability 

requirement, so there has been significant resistance to their use in production units. The 

occurrence of false positives leads to a loss of confidence in sensor technology. The target sensor 

cost, which would include all control elements and user interfaces, can be set at $500, but lower 

costs are of course desired.

Table 8 summarizes the sensor requirements and specifications for in-dispenser sensor 

deployment. A relative ranking of each identified performance metric is presented and assigned a 

qualitative ranking of low, medium, or high importance, as indicated by the marker in the 

appropriate column. The position of the marker refines the assessment; a left justified mark 

indicates a lower importance than a right or middle alignment. 
39 

Table 8. Hydrogen Sensor Metric Rankings for Distributed Hydrogen Production – 

In-Production Unit

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO

LDL X 0.04 vol% (1% of LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X To LFL (0.1–4 vol% H2)

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 1 s

Recovery Time X 1 min

Repeatability X

Signal Drift X To avoid false positives and negatives at all alarm levels

Environmental Impacts T and RH are more relevant than P

- T X -10° to +40°C

- P X Must be calibrated for deployment altitude

- RH X 5%–95% RH

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X May be exposed to 100% H2 (or very high H2)

Deployment Parameters

Capital Cost X <$500 (complete sensor system); <$100 if part of controls 

Installation Cost X Part of capital cost

Physical Size X Must be physically compatible for in-dispenser deployment 

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf

Alarm Thresholds X Must indicate adverse condition

- Trace (1,000 ppmv) X Not required (trace background may be present)

- Low (low risk) X 10% of the LFL, to track intermediate or increasing levels

- High (pending risk) X 25% of the LFL, shutdown system for multiple alarm levels

Regulations and Codes X NFPA 2. Class I, Division 2 operation listing required

Deployment Placement X Guidance on placement is needed

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years

- Unit Replacement X 5 years, 10 years desired

Consumables X None

Calibration Schedule X >once per year; longer time between calibrations is desired

Maintenance X Maintenance free except calibration

Sample Size X No critical restriction

Matrix Requirements X Normal air environment 

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X Display not necessary

- Remote Monitoring X Remote interrogation is useful, output should be vol% H2 units
40 

Parameters Importance Specifications and Notes Low Medium High

Device Repeatability X Plug in replacement useful, but must be calibrated

Warm-Up Time X <1 h for initial installation,15 min for all shutdowns

Alarm Interface X Must connect to control system 

- Number of set points X One mandatory, two useful shutdowns

- Audible X Activate at lowest alarm set point

- Ventilation X Activate at 1st set point if feasible

- Shutdown X Activate at 2nd set point (25% of LFL)

- Remote X Option to external alarm, and remote transmitters

Mechanical Stability X Vibration tests, specification TBD 

Power Requirements X Moderate power requirements preferred (<0.5 W)

2.4.2.2.1 Critical Gaps and Deficiencies

The operational requirements and capital cost are major limiting factors for extensive sensor 

deployment. Acceptance is severely affected by cost of maintenance, especially calibration 

requirements. Capital cost is typically a one-time upfront expense, so it does not impact as 

strongly as the ongoing operational costs. Nevertheless, there is always a need for lower cost 

systems, providing reliability is not compromised. 

Sensor deficiency areas and gaps inhibiting deployment can be summarized as follows. A majo” 

deficiency represents a current critical issue that has been problematic (e.g., occurring with an 

unacceptable frequency). A secondary issue is important but is either not critical or has not been 

reported as occurring.

• Major

o Maintenance and calibration are major shortcomings 

– Long-life sensors (>5 years)

o Manual calibrations no oftener than once per year (less often than once per year or 

no calibration requirement is preferable)

o All other maintenance requirements less often than once per year

o Robustness against poisons and dust 

o Drift

o Acceptable target: <$100 per sensor. 

• Secondary 

o Capital cost for appropriate monitoring systems 

2.4.2.2.2 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

One proposed strategy to facilitate sensor deployment is to simplify and economize the 

operational requirements. Robust sensors with long operational lifetimes would reduce 

replacement cost, and could in principle lower the cost of maintenance by increasing the time 

between calibrations. Robust sensors should eliminate most nonroutine maintenance 

requirements, although sensor calibrations would still be required. Many manufacturers require 

periodic calibration to ensure accuracy and functionality. Sensor performance can be affected by 
41 

a variety of stresses, including chemical (e.g., exposure to poisons), mechanical (e.g., thermal 

stresses and vibration), and electrical factors (e.g., power surges, shutdowns, and circuit failures). 

Underlying codes may specifically require a minimal calibration cycle. Usually calibrations are 

to be performed at least once per year, but could be more frequent based on manufacturer 

recommendations.

Because of the broad range of factors that can affect sensor performance, it is unlikely that the 

sensor calibration requirement will be eliminated. Thus, automated calibration systems would 

significantly lower operational costs associated with wide-scale sensor deployment for battery 

backup systems. Acceptance of such a system would be facilitated if it were available at a 

reasonable cost (<$100 per sensor). Currently, the sensor of choice for in-dispenser deployment 

has been the catalytic bead sensors (also called CGSs), but no specific technology has been 

explicitly endorsed. This is a high-temperature sensor; a room temperature sensor may have 

safety advantages. It is likely that sensors listed to national standards, both for electrical safety 

(e.g., Class I, Division 2) and performance will be required.

Sensor capital cost is likely to decrease with demand as the hydrogen infrastructure develops. 

However, certain platforms and designs will be better impacted by economies of scale, especially 

those that are amenable to high-scale manufacturing afforded by technology developed by the 

microelectronics industry. Miniaturized devices show particular potential, and commercial 

devices are being developed based on TCDs, MOXs, CGSs, palladium thin films, and other 

platforms. However, some platforms are highly susceptible to drift induced by moisture 

fluctuations. Miniaturized sensors, mass-produced using methods developed for the electronics 

industry (e.g., microfabrication, MEMs, or thin-film deposition protocols), could provide lowcost, low-power devices, coupled with a rapid response; a 1-s response time is feasible.

However, the sensor must also still meet the analytical requirements lifetime and robustness 

requirements (insensitive to poisons, interferences, and drift). In addition, their long-term 

stability and robustness to stresses (e.g., chemical, thermal, and moisture) should be more 

thoroughly studied, especially for the production unit environment, which is significantly harsher 

(higher T, RH, and enriched levels of potential interferents) than typical ambient environments. 

Guidance about sensor deployment and the relationship to appropriate action levels is needed. 

The number of internal dispenser sensors will be kept to a minimum, mostly likely one. The 

sensor must be able to respond to internal leaks, and thus should be designed into the dispenser 

rather than treated as an add-on. The dispenser should also be designed so as to not trap 

hydrogen and to channel internal gas releases to the sensor. 

2.4.3 Centralized Hydrogen Production

Centralized (or localized) hydrogen production is typically being performed using on-site 

reformers, and is noted for high throughput production (>1,000 kg/day capacity, typically larger).

Centralized production is an established process serving the petroleum and other markets. As 

such, centralized production has its own safety design requirements. Sensors have been included 

in these operations for many years. Appropriate safety systems have been developed by private 

industry; the centralized production of hydrogen has an excellent safety record. It is beyond the 

scope of this document to induce changes in established safety programs with excellent safety 

records. No further discussions about hydrogen safety sensors for centralized production are 

warranted at this time.
42 

Centralized production does have other concerns as it is applied to the emergence of the 

hydrogen infrastructure. Namely, transport of hydrogen from production site to the filling 

stations (or other end users) needs development. Trucks can be used to transport pressurized or 

liquefied hydrogen, or it can be moved via a pipeline network. Trucking (road or rail) is ongoing 

and meets current needs. Ultimately, a hydrogen pipeline network will be implemented, much as 

the natural gas pipeline system. DOT oversees domestic regulations for trucking and piping of 

gaseous compounds. Sensors could be deployed for both categories of transport. However, the 

trucking application is currently well served, and the pipeline application is nearly nonexistent. 

This topic was not covered in the DOE/NREL workshop, and thus would best be addressed in 

the NREL-Fuel Cell and Hydrogen Energy Association sensor user group. 

2.5 On-Board Light-Duty Road Vehicles

• Application 

o (Primary) hydrogen sensors to measure hydrogen in compartments (trunk, 

passenger, engine) 

o (Secondary) exhaust and internal controls 

• Applicable standards 

o Automotive Safety Integrity Levels (ASIL) Safety requirements are set in 

International Organization for Standardization (ISO) Standards 

o ISO 26262 for automotive 

o IEC 61508 for demonstration of safety level integrity 

o DOT regulations for hydrogen vehicles 

2.5.1 Background

The target release date for commercial hydrogen fuel cell-powered light-duty road vehicles 

(automobiles) is set for 2015. This would be the first large-scale commercial introduction of 

hydrogen fuel technology for consumer applications. The automotive fuel cell market is 

emerging in Asia, Europe, and North America. 

Compliance to international standards (or 

international harmonization of national standards) is 

highly desirable, as product profitability will be 

predicated on market size, especially in the initial 

years of release. An international market would be 

considerably larger than the expected U.S. domestic 

market. Thus, although there is as yet no consumer 

market, it is likely to be quite large within a few 

years.

The hydrogen sensor may be viewed as a safety-critical item for first production vehicles, so it 

must be reliable. Numerous strategies, including redundancymay be used to ensure reliability. 

Although flow regulators, shutoff valves, and bypass valves will be incorporated into the vehicle 

fuel system to minimize the risks associated with hydrogen releases, hydrogen sensors around 

and inside the vehicle will increase safety. Hydrogen sensors may also have relevance as part of 

a process control system (fuel cell operation) and emission monitors. Safety sensors are, 

NREL PIX 16601
43 

however, the most critical application for on-board sensor deployment, and will be focus of this 

discussion. To ensure safety, and to facilitate consumer acceptance, hydrogen safety sensors are 

being deployed on board hydrogen FCVs. The primary application will be to verify the absence 

of hazardous levels of hydrogen in the enclosed compartments, which include the passenger 

compartment, the trunk area (or the area proximal to the on-board storage tanks), and the front 

(engine) compartment. Sensors will likely be deployed in each identified compartment of the 

first generation of commercial hydrogen-powered road vehicles. 

Unlike stationary applications, a range of deployment environments is likely for mobile 

applications. The sensor must thus accommodate extreme cold weather conditions associated 

with northern climates, dry, hot areas associated with the Southwest, and hot humid climates of 

the Southeast. Although passenger compartments may have regulated temperatures and 

humidity, such controls do not operate in other compartments, nor would such controls operate 

when the vehicle is off. In-vehicle temperatures will range from very low (-40°C) to extremely 

high (+40°C or higher) temperatures. The internal temperature of a car parked in the exposed sun 

is likely to exceed +40°C, especially in the southwestern United States. 

Humidity fluctuations will be at least as great as those of ambient conditions and thus will likely 

range from dry to near condensing. Sensors deployed near or in line with exhaust vents will be 

subjected to high humidity levels, because water is produced by the fuel cell. Barometric 

pressure changes will also be encountered with variations in altitude. Although alarm levels (e.g., 

the LFL) are reported as the fraction of the total gas composition (a relative gas concentration), 

many sensor technologies are sensitive to the partial pressure of the target analyte (an absolute 

concentration). 

Typically sensors are assumed to operate at 1 bar pressure, and the conversion from fractional 

units (e.g., 4 vol%) to absolute units (e.g., partial pressure of 0.04 bar for 4 vol% hydrogen at an 

ambient pressure of 1 bar) is straightforward. It is also typical to calibrate sensors at 1 bar.

However, operation at a pressure of 0.8 bar, 4 vol% hydrogen would have a partial pressure of 

0.032 bar, which could lead to an attenuated sensor response. Barometric changes of 0.3 bar 

would be encountered when driving from sea level (1 bar) to 10,000 feet (0.7 bar), the altitude of 

Leadville, Colorado. In the United States, it is also possible to drive on public highways at

altitudes higher than 14,000 feet (0.6 bar). Thus, pressure fluctuations may be encountered, and 

in absence of compensation, these may impact sensor accuracy and performance. 

Hydrogen sensors deployed on board light-duty road vehicles will need to monitor for and 

quickly respond to hydrogen leaks. A response time of less than 30 s would be adequate. The 

range of the sensor should be at least up to the LFL of hydrogen of 4 vol%, and the sensor should 

be able to distinguish to 25% accuracy the hydrogen concentration throughout this range. The 

ability to exceed the range (to 10 vol% hydrogen) would be useful. The sensor should output an 

audible alarm in the event of a hazardous condition. A single alarm threshold of 1 vol% 

hydrogen (25% of the LFL) would be adequate. 

Although a direct hit by a hydrogen jet is unlikely, it is a possibility, so exposure to a high 

hydrogen concentration should not adversely affect the analytical capability of the sensor; its

output may saturate during exposure to high hydrogen levels, but it should recover quickly (<1 

min) to produce analytically accurate signals once the hydrogen concentration comes back into 

range. Nor should the sensor be affected by common chemical interferences, including CO and 
44 

sulfur compounds. Because of silicone compounds are used extensively as sealants, lubricants, 

and pneumatic elements, the sensor should also be resistant to poisoning by silicone. The sensor 

will interface to the vehicle‘s on-board computer system, and may be used to implement a 

shutdown procedure in the event of a detected hydrogen release. On-board applications also have 

some restrictions on thesensor‘s size. Analogous to other automotive chemical sensors (e.g., the 

oxygen sensor), routine maintenance such as calibration should not be required for the expected 

life of the sensor, which should be at least 5 years. The target sensor cost can be set at $25 and 

would include required control circuitry and output interfaces. Lower costs are of course desired.

Table 9 summarizes the sensor requirements and specifications for on-board vehicle deployment. 

A relative ranking of each identified performance metric is presented and assigned a qualitative 

ranking of low, medium, or high importance, as indicated by the marker in the appropriate 

column. The position of the marker refines the assessment; a left justified mark indicates a lower 

importance than a right or middle alignment. For safety applications, ASIL requirements will 

guide the sensor deployment. For a lower rating (Level A), ASIL requirements can be met with 

redundancy or other means to ensure operability, but for the higher rating (B), an extremely 

small failure/hour must be demonstrated via a thorough engineering design review and risk 

analysis of the whole sensor system. On-board compartment monitors must meet ASIL Level A 

requirements.

Table 9. Hydrogen Sensor Metric Rankings for Automotive Fuel Cells – 

On-Board Safety Sensors 

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO

LDL X 0.1 vol%, which is 10% of proposed alarm set point)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X 0 to 4 vol% H2, may be extended to 10% hydrogen

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 30 s

Recovery Time X 60 s

Repeatability X

Signal Drift X To avoid false positives and negatives for >5 years

Environmental Impacts T and RH are more relevant than P

- T X -40 to +40°C

- P X 0.6 to 1.1 bar (to account for altitude changes during drive)

- RH X 5%–95% RH

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Relevant for sensors mounted near tank

Deployment Parameters

Capital Cost X <$25, including control circuitry

Installation Cost X Must be compatible with automotive requirements

Physical Size X Small size compatible with mounting in vehicle cabins

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years
45 

Parameters Importance Specifications and Notes Low Medium High

Commercial Maturity X Must be off-the-shelf; does not exist at desired cost

Alarm Thresholds X

- Trace (1,000 ppmv) X NA

- Low (low risk) X 10% of the LFL, indicate adverse for single alarm level

- High (pending risk) 25% of the LFL, shutdown system for multiple alarm levels

Regulations and Codes X ASIL, SAE

Deployment Placement X Desire for one sensor to meet all in-vehicle requirements

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years

- Unit Replacement X 5 years

Consumables X None

Calibration Schedule X No routine calibrations

Maintenance X No routine maintenance 

Sample Size X No critical restriction

Matrix Requirements X Normal air environment

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X Readout is not necessary

- Remote monitoring X Remote interrogation is not necessary

Device Repeatability X Plug in replacement is useful

Warm-Up Time X <1 hour for initial installation,15 minutes for all shutdowns

Alarm Interface

- Number of set points X One mandatory

- Audible X Activate alarm at lowest set point

- Ventilation X Not necessary

- Shutdown X Not necessary

- Remote X Not necessary

Mechanical Stability X Vibration tests, specification TBD

Power Requirements X Moderate power requirements preferred (<0.01 W)

2.5.2 Critical Gaps and Deficiencies

The on-board sensor has several critical analytical requirements. Sensor technology must operate 

over wide temperature and humidity ranges. The vehicle will also likely be subjected to 

variations in barometric pressure. A deployed sensor must be immune to changes in 

environmental parameters, but it must meet these analytical requirements without faulting on the 

necessary operational and deployment requirements. The operational requirements of a 5-year, 

maintenance-free deployment for sensors are major limiting factors for on-board sensors. False 

alarms caused by the physical operating environment (interferents, dusts) will be a concern.

Deployment requirements remain an issue. Multiple sensors may be initially required, but the 

goal is to minimize the number of sensors to a single unit. This will be achieved, in part, through 

engineering enhancements and risk analysis as historical deployment data are accumulated.

Capital costs of the sensors are also a concern; a target price of $25 (or less) for sensor and 

electronics can be achieved only through economy-of-scale manufacturing. Sensor reliability 

cannot be compromised. Another possible deployment consideration is the need for sensors that 

are safe for deployment in an atmosphere that may contain hazardous levels of hydrogen. 
46 

Although a nonlisted sensor can be used to detect hydrogen, it would not necessarily be safe to 

operate as the hydrogen concentration approaches the LFL. 

• Major

o Maintenance and calibration are major shortcomings 

– Long-life sensors (>5 years)

o No calibrations for the life of the sensor (5 years) 

o All other maintenance requirements less often than once per year

o Robustness against poisons and dust 

o Acceptable target: <$25 per sensor)

o Availability of appropriate technology for the application (level of maturity)

• Secondary 

o Capital cost for appropriate monitoring systems 

o Deployment (multiple sensors to meet current requirements)

2.5.3 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

Hydrogen detectors designed for automotive applications are available, but have not yet been 

totally demonstrated in all environmental regimes. Currently, the sensor of choice for on-board 

applications has been the TCD sensor, and to a lesser extent the MOX sensor, although no 

specific technology has been endorsed. However, many models of these platform types are 

susceptible to drift induced by moisture fluctuations and, to a lesser extent, by temperature. 

Smart sensor systems are under development. These show promise to compensate for 

temperature and humidity fluctuations. This development should be supported, and may need to 

expand to include robustness to pressure variations. As the FCV fleet grows, so will the market. 

This will provide an incentive to manufacturers to expand into this market and increase 

production capabilities. Market forces will tend to keep overall costs reasonable, but this must 

occur without corrupting performance. Sensor capital cost is likely to decrease with demand as 

the hydrogen vehicle market develops. However, certain platforms and designs will be better 

impacted by economies of scale, especially those that are amenable to high-scale manufacturing 

afforded by technology developed by the microelectronics industry. Miniaturized devices show 

particular potential. Miniaturized sensors, mass-produced using methods developed for the 

electronics industry (e.g., microfabrication, MEMs, or thin-film deposition protocols), could 

provide low-cost, low-power devices, coupled with a rapid response; a 1-s response time is

feasible. However, the sensor must also still meet the analytical requirements lifetime and 

robustness requirements (insensitive to poisons, interferences, and drift). 

2.6 Battery Backup

• Application 

o (Primary) Hydrogen sensors for use in CEVs with battery backup 

o (Secondary) Supplemental safety system for battery backup systems deployed in 

multiple use buildings 

• Applicable Standards
47 

o Institute of Electrical and Electronics Engineers/American Society of Heating, 

Refrigerating and Air-Conditioning Engineers (IEEE/ASHRAE) Draft Guide for 

the Ventilation and Thermal Management of Batteries for Stationary Applications

o OSHA 1910.146 Permit Required Confined Spaces-General Environmental 

Controls 

o Local AHJs invoking modified fire code (NFPA, International Fire Code) 

o Listing of sensor and sensor components 

– UL 60079-15 or equivalent listing requirements for use in Class I, 

Division 2 hazardous locations 

– UL 2075 or CSA C22.2, No. 152 or equivalent listing for sensor 

performance 

– FM Global Class Number 6310 and 6320 

2.6.1 Background

Battery backup systems are deployed to ensure continuity of power during electrical outages. 

Many large- and small-scale industries and operations, from leaders in the telecommunications 

industry to startups, rely on battery backup. Accordingly, various levels of safety designs are 

necessary, although these should be based on IEEE/ASHRAE Guide for the Ventilation and 

Thermal Management of Batteries for Stationary Applications and local AHJ requirements.

Backup applications primarily use lead acid batteries, which 

can generate hydrogen as a by-product. Power backup systems 

can be quite large, encompassing several thousand battery 

units. Backup systems are routinely deployed, often in 

multiuse buildings. In addition, distributed, unmanned CEVs 

equipped with battery backup systems are extensively 

deployed in the telecommunications industry. Backup power 

systems in CEVs and multiuse buildings represent two distinct 

applications for hydrogen safety and hydrogen sensor 

requirements.

The IEEE/ASHRAE Guide for the Ventilation and Thermal Management of Batteries for 

Stationary Applications does not require hydrogen sensors for battery backup systems installed 

in buildings. Accordingly, sensors are not extensively deployed. Maintenance costs, especially 

those associated with calibrations, are a main reason the industry does not mandate their use.

However, sensors are an optional supplement to the safety system, and AHJs may impose locally 

modified fire codes to mandate hydrogen sensors around battery backups. Nevertheless, the 

primary code does not mandate use of hydrogen sensors because the normal charging process 

does not produce significant levels of hydrogen. Only when the system is overcharged do 

relatively high and potentially dangerous levels of hydrogen occur. Hydrogen generation can be 

prevented by intelligent charging systems. 

Further safeguards may include monitoring the integrity of select cells in a bank of batteries. A 

high or low cell voltage during charging may indicate a failed cell (e.g., partially shorted) in a 

bank of cells. Engineering controls such as active ventilation minimize the probability of 

hydrogen gas buildup. Typically six equivalent air changes per hour meet human occupancy 

needs. These are much more than the minimum air changes per hour in most instances to prevent CenturyLink Image, Used with permission
48 

excessive hydrogen buildup) and are implemented to preclude mandatory sensor deployment. 

There have been, however, situations where supplemental measures were overridden (e.g., the 

ventilation system was turned off as a cost-saving measure) that led to deleterious hydrogen 

events.

Hydrogen sensors could supplement safety designs for battery backup systems deployed in 

buildings. Guidance about deployment (e.g., where the sensor should be placed) would facilitate 

this. For example, guidelines about placement (e.g., on or near the ceiling versus in the air intake 

of the ventilation system) is needed. The corresponding alarm levels associated with the 

deployment design needs to be considered. For example, a lower alarm level may be advisable 

for sensor deployment in a ventilation system because of hydrogen dilution by air flow.

The telecommunications industry also has a network of battery backup systems deployed in 

distributed CEVs. CEVs are semisealed, mostly subsurface enclosures, and are considered 

confined spaces. Thus, they are regulated by OSHA 1910.146, Permit-Required Confined Space, 

which requires verifiable assurance that oxygen and combustible gases are at safe levels prior to 

human entry. Protocols to ensure appropriate oxygen levels and the absence of combustible gases 

may include one or more of the following: (1) active ventilation; (2) use of chemical detectors 

just prior to entry; and (3) continuous monitoring using deployed sensors. The

telecommunications industry often continuously monitors the internal CEV atmosphere. CEVs 

may be partially below ground level. For CEVs, with battery backup systems, both methane and 

hydrogen are combustible gases of concern. A single sensor for methane and hydrogen is 

desired, which indicates that CGSs – and possibly TCDs or MOXs – are the technologies of 

choice.

As with all chemical sensors, periodic calibration is required (at least once per year, perhaps 

more frequently as dictated by local codes and sensor specifications); the mandatory calibration 

is a major expense associated with sensor deployment. The design and operation of CEVs define 

other requirements for hydrogen sensors. CEVs in the telecommunications industry may have 

thermal regulation, but typically minimal humidity control, so although major temperature 

changes are unlikely, the sensor should not be impacted by humidity fluctuations. Although 

significant pressure fluctuations are not expected, sensors must be calibrated to prevailing 

pressure levels, as barometric pressure depends on the installed elevation (from approximately 1 

bar at sea level to 0.8 bar at 2,000 m). The CEV environment (temperature, ventilation, signal 

management, etc.) is managed by commercial control systems, such as but not limited to designs 

by Quest Controls, Sierra Monitor, or Honeywell. These systems have an estimated target cost of 

$2500. The hydrogen sensor is only one component of the overall system. Thus, target sensor 

costs can be set at $100, which would include control interface circuitry. 

Because hydrogen buildup will occur only via side reactions with battery backup systems, which 

would be a slow process, fast response and recovery times are not essential. For most CEVs, a 

single point source hydrogen sensor is adequate because normal thermal equilibration processes 

will ensure near uniform distribution of hydrogen at or near the ceiling at a rate faster than any 

expected generation rate, and CEVs are relatively small structures. The main requirement of a 

sensor would be to alert personnel to a buildup of hydrogen toward the LFL. Typically two 

levels of response are of interest. Both response levels are well below any hydrogen 

concentration that poses danger, and may be defined as a low-level response (e.g., 10% of the 
49 

LFL) to activate enhanced ventilation and a medium-level response (e.g., 25% of the LFL) that 

would warrant manual inspection. 

The telecommunications industry guarantees service, and thus strongly avoids unnecessary 

shutdowns, even at the medium level (manual inspection of the CEV will be promptly 

performed). Thus, the automated deactivation or shutdown of the CEV operation based solely on 

the readout of a hydrogen sensor is not desired. Of course, a disconnect of a CEV battery backup 

system remains an option if multiple safety systems indicate a potentially dangerous situation 

(e.g., a high differential between ambient CEV aisle temperature and battery post temperature 

could indicate thermal runaway of the batteries). Low-level detection (<1000 ppmv or 0.1 vol%) 

is not required. The gas detector should have electronic outputs that are in engineering units 

(e.g., vol% H2) or easily converted to engineering units. However, direct telemetric 

communication of the output is not a required sensor instrument option, as this is a core 

capability of the telecommunications industry. Although remote monitoring of the control system 

is routine in the telecommunications industry, other industries utilizing battery backup systems 

may not have extensive telecommunications expertise. Thus, remote monitoring capability might 

be a useful option. 

Walk-in cabinets (sometimes known as utility enclosures or UEs in the telecommunications 

industry) are a cross between a CEV and an aboveground hut. These UEs are approximately halfburied. They are thus not considered confined spaces; however, they are commonly equipped 

with similar environmental control systems to CEVs, including combustible gas monitoring. 

2.6.2 Critical Gaps and Deficiencies 

The operational requirements and capital costs for sensors are major limiting factors for 

extensive deployment. Acceptance is severely affected by cost of maintenance, especially 

calibration requirements. Capital cost is typically a one-time upfront expense, so it does not 

impact as strongly as the ongoing operational costs. Nevertheless, there is always a need for 

lower cost systems, providing reliability is not compromised. A second operational concern 

pertains to actual deployment. There is some ambiguity in defining optimal deployment location 

and corresponding alarm levels. 

Tables 10 and 11 summarize the sensor requirements and specifications for sensors around 

battery backup systems deployed in buildings and in CEVs, respectively. A relative ranking of 

each identified performance metric is presented and assigned a qualitative ranking of low, 

medium, or high importance, as indicated by the marker in the appropriate column. The position 

of the marker refines the assessment; a left justified mark indicates a lower importance than a 

right or middle alignment.

Table 10. Hydrogen Sensor Metric Rankings for Battery Backup Applications – 

Building Deployment 

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO

LDL X 0.4 vol% (10% of the LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X 0.1–4 vol% H2
50 

Parameters Importance Specifications and Notes Low Medium High

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 30 s

Recovery Time X 60 s

Repeatability X

Signal Drift X To avoid false positives and negatives for >5 years

Environmental Impacts

- T X 15°C to 30°C for indoor building applications

- P X Must be calibrated for deployment altitude

 - RH X 5 to 65% RH

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Highly unlikely to experience 100% H2 (or high H2) 

Deployment Parameters

Capital Cost X <$1,000

Installation Cost X <$1,000, should not be excessive

Physical Size X Moderate size restriction if deployed in ventilation system

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf

Alarm Thresholds

- Trace (1,000 ppmv) X Possible for operation in ventilation system (dilution effects)

- Low (low risk) X 0.4 vol% H2 (10% of the LFL) to track intermediate levels

- High (pending risk) X 25% of the LFL, audible alarm

Regulations and Codes X Local AHJs, possible update on IEEE/ASHRAE

Deployment Placement X Improved guidance needed

Operational Parameters

Lifetime 5 year minimum, 10 year desired

- Sensing Element X 5 years, 10 years desired

- Unit Replacement X 5 years

Consumables X None

Calibration Schedule X >once per year, lower calibration requirements the better

Maintenance X No routine maintenance other than calibrations

Sample Size X No critical restriction

Matrix Requirements X Normal air environment

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X Readout is useful

- Remote monitoring X Remote interrogation may be useful

Device Repeatability X Plug in replacement not required if recalibrated

Warm-Up Time X <1 hour for initial installation,15 minutes for all shutdowns

Alarm Interface X Must connect to control system

- Number of set points X 1: 10% LFL to activate ventilation; 2: 25% LFL alarm

- Audible X Activate alarm at lowest set point (25% of LFL)

- Ventilation X Activate alarm at lowest set point (10% of LFL)

- Shutdown X Charging system shutdown (50% LFL) until inspected

- Remote X To external alarms, possible to remote transmitters
51 

Parameters Importance Specifications and Notes Low Medium High

Mechanical Stability X Vibration tests, specification TBD

Power Requirements X Moderate power requirements preferred (<0.1 W)

Table 11. Hydrogen Sensor Metric Rankings for Battery Backup Applications – 

Telecommunications CEVs 

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Sulfur, silicone resistance, CO; methane detection useful

LDL X 0.4 vol% (10% of the LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X 0.1–4 vol% H2

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 30 s

Recovery Time X 60 s

Repeatability X

Signal Drift X To avoid false positives and negatives for >5 years

Environmental Impacts

- T X 10°C to 40°C for indoor building applications

- P X Must be calibrated for deployment altitude

- RH X 5 to 65% RH; some RH regulation exists at high end

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Highly unlikely to experience 100% H2 (or high H2) 

Deployment Parameters

Capital Cost X <$100; sensors are part of (commercial) control system

Installation Cost X As part of capital cost; sensors are part of control system

Physical Size X Moderate size restriction if deployed in ventilation system

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf, existing market

Alarm Thresholds X

- Trace (1,000 ppmv) X Probably not necessary, small background is likely

- Low (low risk) X 0.4 vol% H2 (10% of the LFL) to track intermediate levels

- High (pending risk) X 25% of the LFL, audible alarm

Regulations and Codes X Local AHJs, OSH Confined space entry, possibly lEEE/ASHRAE

Deployment Placement X Improved guidance needed

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years, 10 years desired

- Unit Replacement X 5 years

Consumables X None

Calibration Schedule X >once per year, lower calibration requirements the better

Maintenance X No routine maintenance other than calibrations

Sample Size X No critical restriction
52 

Parameters Importance Specifications and Notes Low Medium High

Matrix Requirements X Possible depressed oxygen level (confined space)

Signal Management

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X vol% H2 readout is useful but not critical

- Remote monitoring X Remote interrogation may be useful, but core with industry

Device Repeatability X Plug in replacement not required if recalibrated

Warm-Up Time X <1 hour for initial installation,15 minutes for all shutdowns

Alarm Interface X Must connect to control system

 - Number of set points X 1: 10% LFL to activate ventilation; 2: 25% LFL alarm

- Audible X Activate alarm at lowest set point (25% of LFL)

- Ventilation X Activate alarm at lowest set point (10% of LFL)

- Shutdown X Charging system shutdown possible, not system shutdown

- Remote X To external alarms, possible to remote transmitters

Mechanical Stability X Vibration tests, specification TBD

Power Requirements X Moderate power requirements preferred (<0.01 W)

The major gaps inhibiting sensor deployment can be summarized as:

• Major

o Maintenance and calibration are major shortcomings 

– Long-life sensors (>5 years)

o Calibrations no oftener than 1 year 

– Auto-Cal or an automated calibration system (acceptable target: 

<$100/sensor)

o All other maintenance requirements less often than once per year

o Deployment Guidance for Building applications 

– In ventilation system versus on upper walls or on the ceiling 

– Sensor placement density (number of sensors per area)

– Area monitors

• Secondary 

o Capital cost for appropriate monitoring systems 

o Listing requirements

2.6.3 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

One proposed strategy to facilitate sensor deployment is to simplify and economize the 

operational requirements. Robust sensors with long operational lifetimes would reduce 

replacement costs, and could in principle lower the cost of maintenance by increasing the time 

between calibrations. Robust sensors should eliminate most nonroutine maintenance 

requirements, although sensor calibrations would still be required. Many manufacturers require 

periodic calibration to ensure accuracy and functionality. Sensor performance can be affected by 
53 

a variety of stresses, including chemical (e.g., exposure to poisons), mechanical (e.g., thermal 

stresses and vibration), and electrical factors (e.g., power surges, shutdowns, and circuit failures 

– the electrical factors can be mitigated by powering from the highly stable, power-surge 

protected 48 VDC bus, typically available in telecommunications CEVs). 

Underlying codes may specifically require a minimal calibration cycle. This is usually at least

once per year, but could be more frequent based on manufacturer recommendations. Because of 

the broad range of factors that can affect sensor performance, it is unlikely that the sensor 

calibration requirement will be eliminated. Thus,automated calibration system would 

significantly lower operational costs associated with wide-scale sensor deployment for battery 

backup systems. Members of the group discussing the application Hydrogen Sensors for Battery 

Backup asserted that if such a system were to become available at a reasonable cost (<$100 per 

sensor), the resistance to the use of hydrogen sensors monitoring building battery systems would 

be appeased to the extent that IEEE/ASHRAE Guide for the Ventilation and Thermal 

Management of Batteries for Stationary Applications could strongly suggest their use. Currently, 

the sensor of choice for battery backup applications has been the CGS, although no specific 

technology has been endorsed. 

Sensor capital cost is likely to decrease with demand as the hydrogen infrastructure develops. 

However, certain platforms and designs will be better impacted with economies of scale, 

especially those that are amenable to high-scale manufacturing afforded by technology 

developed by the microelectronics industry. Miniaturized devices show particular potential.

Miniaturized sensors, mass-produced using methods developed for the electronics industry (e.g., 

microfabrication, MEMs, or thin-film deposition protocols), could provide low-cost, low-power 

devices, coupled with a rapid response; a 1-s response time is feasible. However, the sensor must 

also still meet the analytical requirements including range (e.g., multiple alarm levels) and 

robustness requirements (insensitive to poisons, interferences, and drift). 

Guidance on sensor deployment (e.g., ceiling, walls, or in ventilation systems) and the 

relationship to appropriate action levels is needed. Hydrogen will be produced and released from 

a localized source or, in the case of a bank of batteries, a series of sources. The sensor will be 

placed remote from the source. Thus, the detected hydrogen gas concentration could be 

significantly attenuated. This is especially true for sensors deployed in ventilation systems, 

which may be required for recirculation ventilation designs. Guidance on the means to best 

position the sensor to capture a hydrogen generation event is needed. CFD modeling of various 

generation rates (low and high) and sensor deployment should be performed to specifically 

provide such guidance on position and appropriate alarm levels.

2.7 Storage

• Application 

o Hydrogen sensors installed near on-site bulk compressed or liquid storage systems 

• Applicable standards 

o NFPA 2 Hydrogen Technologies Code

o Local AHJs invoking modified fire code 

o Listing of sensor and sensor components 
54 

o UL 60079-15 or equivalent listing requirements for use in Class I, Division 2 

hazardous locations

o UL 2075 or CSA C22.2, No. 152 or equivalent listing for sensor performance 

o FM Global Class Number 6310 and 6320 

o DOT transport of flammable material

 2.7.1 Background

The hydrogen infrastructure requires several support systems and operations, including delivery 

of hydrogen or on-site production, on-site storage, 

transfer hardware, and hydrogen dispensing systems. 

Two storage-related applications covered in the 

workshop were indoor and outdoor storage (see 

Table 12 and Table 13). Also, large-scale storage is 

associated with site operations and small-scale 

storage with on-board vehicle storage. The pertinent 

sensor requirements associated with on-board 

storage were discussed in Sections 2.2 and 2.5. 

This section discusses issues pertaining to sensor requirements for on-site bulk hydrogen storage. 

Two approaches are compressed hydrogen gas and cryogenic liquid hydrogen. Bulk storage 

capacity can vary significantly, ranging from less than 100 kg to thousands of kilograms for large 

facilities. For example, 9000 gallons (2400 kg) of hydrogen is stored at the Defense Distribution 

Depot in Susquehanna, Pennsylvania, to power its forklift fleet [2]. Storage vessels containing 

this amount of hydrogen are routinely transported by truck and railroad to the deployment site. 

Bulk storage thus requires a transfer of large quantities of hydrogen or changeout operations in 

which the depleted storage vessel is replaced. Such activities increase the possibility of hydrogen 

releases and leaks.

A large quantity of hydrogen in a single location presents obvious hazards, and many systems are 

in place to keep the area safe, including outdoor deployment, setback distances, and safety 

systems. Although the equipment used to store compressed gas and liquid hydrogen are quite 

different, the sensor requirements are comparable. Unique to liquid storage facilities, however, is 

the controlled venting of “evaporated” hydrogen, so sensor deployment must take into account 

the presence of ambient hydrogen via controlled releases. Although flow regulators, shutoff 

valves, and bypass valves will be incorporated into the fuel system safety design to minimize 

released hydrogen volumes, the use of hydrogen sensors in areas around the storage vessel will 

increase safety. The sensors must respond to leaks before the release becomes dangerous.

Safety systems around large storage facilities typically include flame detectors designed to alarm 

in the event of hydrogen combustion. Flame detectors are beyond the scope of this report and 

were not explicitly included in the sensor workshop. Furthermore, they respond only during a 

combustion event and do not provide early warning capability. 

Chemical sensors near storage areas are typically installed outdoors and need to be able to handle 

temperature and humidity fluctuations. Humidity will reach condensing levels, so the sensor 

must be able to handle water condensate. Water condensate can be handled by intrinsic sensor 

design, by adding a cover that will keep water away from the sensor, or other deployment 

DLA, Susquehanna Image, Used with permission
55 

designs. Temperature extremes will vary by location, but will range from below freezing to very 

hot in direct sunlight. Pressure will vary by location, so the sensor should be calibrated to the 

specific pressure when installed. The sensor signal should not vary significantly with local 

pressure fluctuations. The storage area may be near loading docks and parking areas, so the 

sensor needs to be selective against exhaust gases and other flammable gases.

Being outdoors, the hydrogen will be able to dissipate quickly, but design features of the storage 

facility and surrounding structures may provide entrapment zones. Sensor placement is an 

important consideration in this situation. More studies need to be done in this area, but in 

general, the sensor should be placed above the storage apparatus and away from any piping that 

normally vents hydrogen. Alarm levels should be at 10% of the LFL and safety action should 

take place at 25% of the LFL. The sensor should respond within 30 s of reaching these 

conditions. Indoor storage of hydrogen may include the use of approved cylinders stored in a 

safe area. Special requirements for indoor storage may include explosion proof or intrinsically 

safe electrical certification, added ventilation or the use of ventilated enclosures, and possibly 

added calibration requirements. The Idaho National Laboratory has developed guidance for the 

indoor storage of hydrogen and oxygen [17]. The guide includes recommendations for sensor 

placement based on the square footage of laboratory space. It suggests that these guidelines 

define an area of 400–900 ft2 for each installed sensor.

2.7.2 Critical Gaps and Deficiencies

There is some debate about the optimal type and deployment of hydrogen sensor technology. As 

an outdoor facility, hydrogen released from storage facilities will dissipate into the atmosphere 

and could be transitory. However, sensors and flame detectors are often deployed to ensure site 

safety. Sensor placement remains a major gap for outdoor storage areas. An improperly placed 

sensor could result in not detecting a dangerous situation. Alternatively, placement of a sensor 

too close to a normal ventilation pipe, such as that required for liquid storage tanks, may cause 

spurious false alarms. Many of the issues with outdoor sensor placement and rapid dispersion of 

released hydrogen could be alleviated using wide area monitors [19] with rapid real-time 

response (<1 s), but such technology is not currently readily available commercially and 

significant development is still required for cost-effective, reliable wide area monitoring. Thus 

point sensors distributed at or around the storage facility are still necessary to detect hydrogen 

releases prior to adverse events.

When designing a hydrogen safety system for outdoor storage, one of the first things to consider 

is pressure holds and flow continuity checks to determine if the system is pressure tight. When 

employing sensors as part of a safety system, there are special considerations that should be 

considered. Boots can be added to valves and fittings that will trap hydrogen leaks and improve 

the capability of detecting external leaks. Other considerations include using hydrogen sensitive 

paint, wireless communication or odorants. Each safety system should be engineered to provide 

the level of safety required for the specific application.

The “operational requirements” for sensors is a major limiting factor for extensive sensor 

deployment. Acceptance is severely affected by cost of maintenance, especially calibration 

requirements. Small drifts in the sensor background oftentimes leads to the need for on-site 

calibration. Capital costs for the sensors are also a factor impacting acceptance. Since this factor 

is typically a one-time upfront expense, it does not impact as strongly as the ongoing operational 
56 

costs. Nevertheless, there is always a need for lower cost systems, providing the sensor reliability 

is not compromised. 

• Major

o Sensor placement requires much more research

o Maintenance and calibration are major shortcomings 

– Long-life sensors (>5 years)

o Manual calibrations no more than once per year (>1 year or no calibration 

requirement is preferable)

o All other maintenance requirements >1year 

o Robustness against poisons and dust 

o Unacceptable sensor drift (necessitating recalibration or system shutdowns). 

• Secondary 

o Capital cost for appropriate monitoring systems 

o Acceptable target: <$500 per sensor

2.7.3 Alleviating Gaps and Deficiencies To Facilitate Sensor Utilization

One proposed strategy to facilitate sensor deployment would be to simplify and economize the 

operational requirements. Robust sensors with long operational lifetimes would cut down 

replacement cost, and could in principle lower the cost of maintenance by increasing the time 

between calibrations. Robust sensors should eliminate most non-routine maintenance 

requirements, although sensor calibrations would still be required. Currently, many if not all 

manufacturers require periodic calibration to assure both sensor accuracy and functionality.

Sensor performance can be affected by a variety of stresses, including chemical (e.g., exposure to 

poisons), mechanical (e.g., thermal stresses and vibration), and electrical (e.g., power surges, 

shutdowns, and circuit failures) factors. Underlying codes may specifically require a minimal 

calibration cycle. Usually calibrations are to be performed a minimum of once per year, but 

could be more frequent based upon manufacturer recommendations. Because of the broad range 

of factors that can affect sensor performance, it is unlikely that the sensor calibration requirement 

will be eliminated. Thus, the development of “automated calibration system” would significantly 

lower operation costs associated with wide-scale sensor deployment. Acceptance of such a 

system would be facilitated if it were available at a reasonable cost (<$100 per sensor).

Currently, the sensor of choice for deployment around storage tanks has been the catalytic bead 

sensors (also called combustible gas sensors, CGS), but no specific technology has been 

explicitly endorsed. Guidance on sensor deployment and the relationship to appropriate action 

levels is needed. The number of sensors placed around storage facilities will be kept to a 

minimum, mostly likely one or two. The deployed sensors must be able to respond to releases 

that are dispersing quickly.

Sensor capital cost is likely to decrease with demand as the hydrogen infrastructure develops.

However, certain platforms and designs will be better impacted by economy of scale, especially 

those platforms which are amenable to high scale manufacturing afforded by technology 

developed by the microelectronics industry. Miniaturized, thin-film devices show particular 
57 

potential. Miniaturized devices show particular potential. Miniaturized sensors, mass-produced 

using methods developed for the electronics industry (e.g., microfabrication, MEMs, or thin-film 

deposition protocols), could provide low-cost, low-power devices, coupled with a rapid response, 

with a 1-s response time being very feasible. However, the sensor must also still meet the 

analytical requirements including range (e.g., multiple alarm levels) and robustness requirements 

(insensitive to poisons, interferences, and drift). Commercial hydrogen sensors based on thinfilm and other microfabrication platforms are emerging. While these show a fast response to 

hydrogen, there is a tendency for many of these devices to saturate at low concentrations and to 

exhibit a poor dynamic range. In addition, the long-term stability of these devices and robustness 

to stresses (e.g., chemical, thermal, and moisture) should be more thoroughly studied. 

Table 12. Hydrogen Sensor Metric Rankings for Hydrogen Storage – Outdoor

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Exhaust gases

LDL X 0.4 vol% (10% of LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X To LFL (0.1–4 vol% H2)

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 30 s

Recovery Time X 60 s

Repeatability X Not critical

Signal Drift X To avoid false positives and negatives at all alarm levels

Environmental Impacts X T and RH are more relevant than P

- T X -40°C to +40°C

- P X Must be calibrated for deployment altitude

- RH X 15%–100% RH – Must be able to handle condensing

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Relevant for sensors mounted near tank

Deployment Parameters

Capital Cost X <$500

Installation Cost X

Physical Size X Size not a significant issue

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf

Alarm Thresholds X Must indicate adverse condition

- Trace (1,000 ppmv) X Not required for outdoor

- Low (low risk) X 10% of the LFL, indicate adverse for single alarm level

- High (pending risk) X 25% of the LFL, shutdown system for multiple alarm levels

Regulations and Codes X Possible local AHJs

Deployment Placement X Must be above system

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years
58 

Parameters Importance Specifications and Notes Low Medium High

- Unit Replacement X 5 years

Consumables X None

Calibration Schedule X >once per year; longer time between calibrations is desired

Maintenance X Maintenance free except calibration

Sample Size X No critical restriction

Matrix Requirements X Outdoor air environment

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% of LFL

 - Displays X Readout is not necessary

- Remote Monitoring X Remote interrogation is not necessary

Device Repeatability X Plug in replacement is useful

Warm-Up Time X <1 hour for initial installation,15 minutes for all shutdowns

Alarm Interface

- Number of set points X Low-level mandatory for alarm, a second useful to shutdown

- Audible X Activate alarm at lowest set point

- Ventilation X Activate at 1st set point, if feasible

- Shutdown X Activate at 2nd set point (25% of LFL)

- Remote X Not necessary

Mechanical Stability X Vibration tests, specification TBD

Power Requirements X Moderate power requirements preferred (<0.5 W)

Table 13. Hydrogen Sensor Metric Rankings for Hydrogen Storage – Indoor 

Parameters Importance Specifications and Notes Low Medium High

Analytical Parameters

Selectivity X Exhaust gases

LDL X 0.4 vol% (10% of LFL)

Analytical Resolution X Differentiation is not important other than alarm set points

Linear Range/Dynamic X To LFL (0.1–4 vol% H2)

Accuracy X Within ±20% of reading for all working (T,P, RH) conditions

Response Time X 30 s

Recovery Time X 60 s

Repeatability X Not critical

Signal Drift X To avoid false positives and negatives at all alarm levels

Environmental Impacts X T and RH are more relevant than P

- Temperature X -40°C to +40°C

- Pressure X Must be calibrated for deployment altitude

- Relative Humidity X 15%–100% RH – Must be able to handle condensing

Reversibility X Postexposure recovery should be >95%

Limits of Quantization X Quantization of trace hydrogen is not as critical as LDL

Saturation Stability X Relevant for sensors mounted near tank

Deployment Parameters

Capital Cost X <$500

Installation Cost X

Physical Size X Size not a significant issue

Control Circuitry X Compatible with commercial control systems

Electronic Interface X Output in engineering units (e.g., % H2) or easily converted
59 

Parameters Importance Specifications and Notes Low Medium High

Pneumatic Connections X Passive (no power) sampling system 

Shelf Life X >5 years

Commercial Maturity X Must be off-the-shelf

Alarm Thresholds X Must indicate adverse condition

- Trace (1,000 ppmv) X Not required but useful in ventilation system or extractive

- Low (low risk) X 10% of the LFL, indicate adverse for single alarm level

- High (pending risk) X 25% of the LFL, shutdown system for multiple alarm levels

Regulations and Codes X Possible local AHJs

Deployment Placement X Must be above system

Operational Parameters

Lifetime X 5 year minimum, 10 year desired

- Sensing Element X 5 years

- Unit Replacement X 5 years

Consumables X None

Calibration Schedule X once per year; longer time between calibrations is desired

Maintenance X Maintenance free except calibration

Sample Size X No critical restriction

Matrix Requirements X Indoor air environment

Signal Management X

- Alarm (audible, lights) X Alarm set at 10% of LFL

- Displays X Readout is not necessary

- Remote Monitoring X Remote interrogation is not necessary

Device Repeatability X Plug in replacement is useful

Warm-Up Time X <1 hour for initial installation,15 minutes for all shutdowns

Alarm Interface

- Number of Set Points X One mandatory, two useful shutdown

- Audible X Activate alarm at lowest set point

- Ventilation X Activate at 1st set point, if feasible

- Shutdown X Activate at 2nd set point (25% of LFL)

- Remote X Not necessary 

Mechanical Stability X Vibration tests, specification TBD 

Power Requirements X Moderate power requirements preferred (<0.5 W)

3 Summary

The 2007 DOE/NREL Hydrogen Sensor Workshop resulted in a short list of technical targets 

(primarily Analytical Performance Metrics) for hydrogen sensors. These targets are presented in 

Table 1. The metrics listed in Table 1 were identified as critical sensor performance parameters 

and specifications, and as such, were viewed as necessary targets to meet 2012 requirements as 

envisioned in 2007. The 2011 workshop, however, approached the task of assigning metrics 

differently. Rather than developing generalized sensor technology performance requirements,

specific applications were identified and the corresponding sensor requirements were thoroughly 

reviewed. It was recognized that different applications can have significantly different 

requirements. For example, the residential consumer market requires low-cost technology. In this 

application the target price to the end user (the consumer) would be low (e.g., $100). This 

contrasts to deployment of hydrogen dispensers in multimillion dollar warehouse facilities. 
60 

Facility protection is highly critical, such that capital cost concerns are less restrictive and a 

$500–$1000 capital cost is not prohibitive. Of course, industrial or commercial applications 

would have higher expectations and higher cost systems would have additional features such as 

certification to safety and performance standards, multiple (and adjustable) alarm levels, 

interface capability to control the system, remote interrogation, and hopefully, high reliability. In 

commercial applications, hydrogen levels are often continually monitored, and a small drift in 

the sensor background results in either a false alarm or a recalibration of the sensor, or both. 

Trained personnel calibrate sensors in the field, which is quite costly. For residential 

applications, drift in the sensor response would ultimately lead to a false positive (for a positive 

drift) or a possibility of a false negative (for a negative drift).

In the 2011 sensor workshop, breakout groups were organized by a variety of applications. Three 

sequential breakout sessions were held to allow attendees to participate in up to three topical 

areas. The preceding sections were based on the findings of these breakout groups, and 

comprehensive sensor requirements were developed for each specific application. Clearly, each 

application had its unique needs. Table 14 through Table 24 contain short lists of applicationspecific critical sensor performance metrics.
61 

Table 14. Critical Sensor Requirements for Industrial Truck Applications – On-Board Deployment

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point of 1%) between calibrations; no 

false positive or negative

Temperature Stability No on-board thermal regulation, except ventilation, 0° to +50°

Relative Humidity Minimal on-board humidity regulation; 0 to 90% RH at prevailing T; RH drift should be 

<0.25 vol% H2

Selectivity Inert to silicone compounds, dust, no cross sensitivity to CO, H2S, other application 

specific chemicals

Saturation Stability No adverse effect or quick (<5 min) recovery

Response Time 1 s at 1 vol% H2

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Physical Size Must fit on-board in enclosure around tank

Regulation and Codes Possible UL 2075 or equivalent

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm

Table 15. Critical Sensor Requirements for Indoor Fueling – Facility Monitoring

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10 vol% may be useful (and maybe required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point of 1%) between calibrations; no 

false positive or negative

Temperature Stability Depends on location; 0° to 40°C for most, but -25° to +40°C for refrigerated facilities

Relative Humidity 25 to 90% RH at prevailing T in unregulated facility; RH drift should be <0.25 vol% H2 

equivalent response

Selectivity Inert to silicone compounds, no cross sensitivity to CO, H2S, or other application specific 

chemicals

Response Time 30 s at 1 vol% H2 (or the alarm level)

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Regulation and Codes NFPA 2, Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent

Deployment Placement Required by NFPA2 but no guidance on placement is given 

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm
62 

Table 16. Critical Sensor Requirements for Indoor Fueling – In-Dispenser Deployment

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and maybe required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point of 1%) between calibrations; no 

false positive or negative

Temperature Stability No on-board thermal regulation, except ventilation, 0° to +50°

Relative Humidity Minimal on-board humidity regulation; 0 to 90% RH at prevailing T; RH drift should be 

<0.25 vol% H2

Saturation Stability No adverse effect or quick (<5 min) recovery

Response Time 1 s at 1 vol% H2

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Physical Size Must fit on-board in enclosure around tank

Regulation and Codes Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent 

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm

Table 17. Critical Sensor Requirements for Residential Applications 

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and maybe required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point of 1%) between calibrations; no 

false positive or negative

Temperature Stability Little thermal regulation likely (garages) -40° to +40°C 

Relative Humidity Little RH regulations; 25%–95% at prevailing T, RH drift should be <0.25 vol% H2 

equivalent response

Selectivity Inert to silicone compounds, no cross sensitivity to CO, H2S, or other application specific 

chemicals

Response Time 30 s at 1 vol% H2 (or the alarm level)

Deployment Parameters

Commercial Maturity Does not currently exist

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Regulation and Codes NFPA 2, Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent

Deployment Placement Plug and go type of installation; no electrical interfaces other than power

Capital cost <$100

Operational Parameters

Operational Lifetime >5 years

Calibration No calibration requirements during deployment lifetime,

Maintenance No routine maintenance other than periodic battery (backup power system) check

Audible Alarm Minimal requirement, must be heard external to garage
63 

Table 18. Critical Sensor Requirements for Production – Facility Monitoring

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and maybe required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point of 1%) between calibrations; no 

false positive or negative

Temperature Stability Depends on location; 0° to 40°C for most, but -25° to +40°C for refrigerated facilities

Relative Humidity 25 to 90% RH at prevailing T in unregulated facility; RH drift should be <0.25% H2 

equivalent response

Selectivity Inert to silicone compounds, no cross sensitivity to CO, H2S, or other application specific 

chemicals

Response Time 30 s at 1 vol% H2 (or the alarm level)

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Regulation and Codes NFPA 2, Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent

Deployment Placement Required by NFPA2 but no guidance on placement is given 

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm

Table 19. Critical Sensor Requirements for Production – In-Dispenser Deployment 

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and mayb e required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point o 1%) between calibrations; no 

false positive or negative

Temperature Stability No on-board thermal regulation, except ventilation, 0° to +50°

Relative Humidity Minimal on-board humidity regulation; 0 to 90% RH at prevailing T; RH drift should be 

<0.25 vol% H2

Saturation Stability No adverse effect or quick (<5 min) recovery

Response Time 1 s at 1 vol% H2

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Physical Size Must fit on-board in enclosure around tank

Regulation and Codes Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent 

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm
64 

Table 20.Critical Sensor Requirements for On-Board Light-Duty Road Vehicles 

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and may be required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point o 1%) between calibrations; no 

false positive or negative

Temperature Stability No on-board thermal regulation, except ventilation, 0° to +50°

Relative Humidity Minimal on-board humidity regulation; 0 to 90% RH at prevailing T; RH drift should be 

<0.25 vol% H2

Saturation Stability No adverse effect or quick (<5 min) recovery

Response Time 1 s at 1 vol% H2

Deployment Parameters

Commercial Maturity Must be off-the-shelf, technology exists, but needs advancement (cost and reliability)

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Physical Size Must fit on-board in enclosures in vehicle (passenger and trunk compartments

Cost (to manufacturer) <$15, including basic electronics (output signals in engineering units of vol% H2, alarm 

set points)

Regulation and Codes SAE, ASIL regulations

Operational Parameters

Operational Lifetime >5 years

Calibration No calibration during lifetime

Maintenance No routine maintenance 

Audible Alarm Minimal (and current) requirements call for an audible alarm

Table 21. Critical Sensor Requirements for Battery Backup – Dedicated CEVs

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and maybe required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point o 1%) between calibrations; no 

false positive or negative

Temperature Stability Minimal facility thermal regulation, -10° to +50°

Relative Humidity Minimal humidity regulation; 65 to 90% RH at prevailing T; RH drift should be <0.25

vol% H2

Response Time 30 s at 1 vol% H2

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Regulation and Codes Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent 

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm; connects to remote 

monitoring system
65 

Table 22. Critical Sensor Requirements for Battery Backup – Multiple-Use Buildings 

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and may be required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point o 1%) between calibrations; no 

false positive or negative

Temperature Stability Depends on location; 0° to 40°C for most, but -25° to +40°C for refrigerated facilities

Selectivity Inert to silicone compounds, no cross sensitivity to CO, H2S, or other application specific 

chemicals

Response Time 30 s at 1 vol% H2 (or the alarm level)

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Regulation and Codes NFPA 2, Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent

Deployment Placement Required by NFPA2 but no guidance on placement is given 

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm

Table 23. Critical Sensor Requirements for Hydrogen Storage – Outdoor 

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and maybe required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point o 1%) between calibrations; no 

false positive or negative

Temperature Stability Outdoor environment temperature extremes

Relative Humidity Outdoor environment humidity extremes

Response Time 30 s at 1 vol% H2

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Regulation and Codes Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent 

Deployment Placement Required by NFPA2 but no guidance on placement is given 

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm; connects to remote 

monitoring system
66 

Table 24. Critical Sensor Requirements for Hydrogen Storage – Indoor 

Parameters Comments and Specifications

Analytical Parameters

Linear Range 0–4 vol% H2, but extended to 10% may be useful (and may be required by local AHJ)

Signal Drift <10% of alarm (<0.1 vol% H2 drift for a alarm set point o 1%) between calibrations; no 

false positive or negative

Temperature Stability Depends on location; 0° to 40°C for most, but -25° to +40°C for refrigerated facilities

Relative Humidity 25 to 90% RH at prevailing T in unregulated facility; RH drift should be <0.25 vol% H2 

equivalent response

Selectivity Inert to silicone compounds, no cross sensitivity to CO, H2S, or other application specific 

chemicals

Response Time 30 s at 1 vol% H2 (or the alarm level)

Deployment Parameters

Commercial Maturity Must be off-the-shelf

Alarm Set Points 25% of LFL (or 10% depending upon local AHJ); multiple set points may be required

Regulation and Codes NFPA 2, Class I Division 2 (UL 60079 or equivalent); possible UL 2075 or equivalent

Deployment Placement Required by NFPA2 but no guidance on placement is given 

Operational Parameters

Operational Lifetime >5 years

Calibration Minimal manual calibration, no more than annual

Maintenance No routine maintenance other than calibrations

Audible Alarm Minimal (and current) requirements call for an audible alarm

3.1 Review of Gaps and Deficiencies

Several sensor performance gaps and deficiencies cut across multiple applications. These 

include: 

• Analytical performance gaps

o Response time (and to a lesser extent, recovery time)

o Cross sensitivity/poisons 

o Signal drift

• Operational gaps

o Calibration frequency 

o Cost of maintenance and calibrations

• Deployment gaps 

o Code requirements 

o Placement 

o Distributed point sensors versus wide area monitoring

In terms of analytical performance, avoidance of false positives (e.g., a positive drift in the 

sensor output misinterpreted as hydrogen present at or above the alarm level) remains a 

consistent concern. False positives, perceived or real, cause stakeholders to lose confidence. 

False positives can occur by a variety of mechanisms, including impact of fluctuations in 
67 

environmental parameters, aging of the sensor, and chemical environment (e.g., exposure to 

interferents or poisons). An interferent induces a reversible response that could be incorrectly 

interpreted as having hydrogen present, whereas a poison permanently affects the response of the 

sensor. A poison will usually decrease the sensitivity of the sensor to hydrogen (e.g., sensor 

electrical response divided by concentration); however, a sensitivity increase is possible. Thus, 

robustness to interferents and poisons is critical. Common interferents include CO, methane, and 

other combustible gases; however, not all platform types are equally susceptible to these gases.

Common poisons include sulfur and silicone compounds, but again some platforms are more 

susceptible than other. There is also the potential of application-specific interferents (e.g., in a 

warehouse storing large quantities of a product-specific industrial or residential chemical, such 

as a specific solvent).

Environmental factors can also induce false positives. Temperature and humidity effects on 

sensor responses are common. Some sensing element platforms are particularly sensitive to 

temperature changes, but methodologies to compensate for the temperature dependence have 

been developed and incorporated into the sensor control circuitry. It is important that sensor 

stability be validated throughout the temperature and humidity ranges that are likely to be 

encountered, and such tests are incorporated into national (e.g., UL 2075 and CSA C22.2, No. 

152) and international (e.g., ISO 26142) sensor performance standards. A third factor affecting 

false positives is sensor drift caused by aging, which affects the active sensing element. Overall 

sensor drift is typically addressed through periodic calibrations, which are critical for most

sensor deployments. There are, however, reports from end users of unacceptable drift in sensor 

response in excess of manufacturer specifications, including the magnitude of the drift and the 

time frame in which it occurred. 

Currently, most commercial sensors have response times on the order of 10–60 s. This is 

adequate for sensors deployed as area monitors, such as those mounted on the wall or ceiling 

around the dispenser, garages, or other facilities. Point sensors deployed as area monitors will 

typically operate remotely from the system; thus, the buildup of hydrogen to the alarm level will 

typically dominate the detection process. For these applications, a 30-s response time is adequate 

and readily accomplished by many commercial devices. However, sensor response time remains 

an important issue for applications where high concentrations of hydrogen may rapidly form in 

the event of small leaks, specifically leaks that occur in a contained environment or enclosure, 

such as within the dispenser or monitoring for on-board forklifts to monitor for leaks around an 

enclosed tank. Hydrogen concentrations can rapidly build up in a contained environment, so this 

application requires fast-responding sensors, preferably 1 second. 

Even slow leaks, which may not be readily detectable by other means (e.g., pressure transducers 

installed in the pneumatic lines), can rapidly lead to dangerous situations. For example, <0.75 g 

of hydrogen uniformly distributed within an 8-ft3 enclosure would exceed the LFL (a miniscule 

leak rate of 0.21 mg/s could reach this level within 1 h). Since the 2007 workshop, sensor 

response times have improved. Sensing elements (e.g., MOXs and CGSs) developed around 

microfabrication designs have shown improved response times relative to their conventional 

counterparts. Typically, however, the response times of these sensors are still around 5 s, and the 

1-s response time remains a gap. Further, many thin-film devices are more adversely affected by 

environmental, chemical, and aging effects than are their conventional counterparts. 
68 

Gaps in deployment metrics also cut across the range of applications. Although NFPA 2 requires 

the use of hydrogen sensors around indoor operations, no guidance about sensor placement and 

recommended number of sensors for a facility is provided. This placement guidance has to be 

developed using CFD modeling with empirical validation, and should include assessments within

a range of likely scenarios (e.g., relationship among leak rates, volumes versus facility designs). 

There is also the need to have sensor technologies listed to national standards. Current building 

codes may require that the sensor be certified for combustible environments (e.g., Class I, 

Division 2). It is common practice in the petroleum industry to require Class I, Division 2 listing 

(or more rigorous certifications), so Class I, Division 2 certified hydrogen sensors are 

commercially available, albeit the number of platform types is limited. The certification 

requirement for hydrogen sensors is increasing, and some jurisdictions already require that 

sensors be listed to performance standards (e.g., UL 2075) for indoor operations. However, as of 

the publication of this report no hydrogen sensors are currently listed as certified for UL 2075. 

The lack of certified sensors slows the permitting process.

Capital costs associated with sensor purchases may be envisioned as a third deployment concern. 

However, current capital cost of sensors was not a major issue among the stakeholders attending 

the DOE/NREL workshop. Of course, lower costs are always desired, but certainly not if they 

come about through the loss of performance. Cost does remain an issue for the emerging 

applications of residential hydrogen sensors, including sensors deployed on-board light-duty 

road vehicles. Currently no technology is available that meets the cost requirements for the 

consumer market. 

By far the most critical gap associated with sensor technologies pertains to maintenance. Every 

identified application requires more robust sensor technologies. In all indoor/enclosed 

applications (buildings, CEVs), the costs associated with routine calibrations are the major 

expense. Sensor deployment will be facilitated by either a fully reliable sensor with minimal 

calibration requirements or a means for autocalibration, providing the automated calibration 

system would be sufficiently low cost, simple to use, reliable, and accepted by AHJs.

3.2 Sensor Specification and Gap Level by Application

The relationship between a sensor specification for the various performance metrics and its 

relevance to each identified application is represented in Table 25. Rankings are given as high 

(■) for a critical metric, medium (■) for an important metric, low (■) for a metric that may be 

useful but not critical for the application, and not applicable (■). A comparable ranking of high, 

medium, or low provides an assessment of current commercially available technology to meet 

the indicated specification. The table provides a visualization of the relationship among sensor 

requirements for the various applications. Rankings typically vary with applications, but there 

are some trends. Area monitor applications, such as sensors deployed in residential garages, 

warehouses, battery backup rooms, have similar requirements. Sensors deployed within 

contained environments, such as in-dispenser or in-production unit (e.g., within an enclosure 

surround a reformer or electrolysis unit) also have similar requirements.
69 

Table 25. Sensor Metric Rankings by Application and Sensor Gap Level

Parameters

Industrial 

Trucks

Indoor Fueling 

(Area Monitor) 

Indoor Fueling 

(In-Dispenser) 

Residential 

Production 

(Area Monitor) 

Production (InUnit) 

Road Vehicles 

Battery Backup 

(Building)

Battery Backup 

(CEV)

Storage 

(outdoor)

Storage 

(indoor) Performance

Analytical Parameters Gap Level

Metric Spec.

Selectivity Std. Mix ........... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Medium

Poisons .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

LDL <0.1% .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

0.4%–1% .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Analytical Resolution <0.2% H2 .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Linear Range 0–0.1% .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

0–>4% .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Response Time 1 s .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

10–30 s .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Recovery Time 1–10 s .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Medium

30–60 s .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Repeatability ±20% .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Signal Drift <0.2% H2/cal .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Medium-High

<1% H2 5 years .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

Temperature -40° to +40°C .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Medium

0° to +40°C .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Medium

Pressure (fixed 0.6–1.0 Bar .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Pressure (var. 0.6–1.0 Bar .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Medium

Relative Humidity 0%–95% .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Reversibility >95% in 5 min .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Limits of Quantization .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Saturation Stability <1–5 min .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... unknown

Deployment Parameters

Metric Spec. Gap Level

Capital Cost <$100 .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

<$1,000– .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Physical Size .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Medium

Level of Maturity COTS .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Medium

Codes and Standards compliance .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

Alarm Set Points <0/1% .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

10%–25% LFL .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Low

Deployment Placement Guidance .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

Operational Parameters

Metric Spec. Gap Level

Lifetime (deployment) >5 years .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Unknown

Calibration Cycle >1 year .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

>5 year .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

Interchangeable .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... High

Mechanical Stability Vibration/Shock .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... .......... Unknown

Legend

High importance ..........

Medium importance ..........

Low importance ..........

Not applicable ..........

A “high” importance coupled with a high gap rating represents a significant gap between 

application requirements and sensor capability and identifies specific areas for improvement.

Examples include selectivity to poisons (indoor fueling area monitors and on-board sensors), 1-s 

response time (for contained environments including the on-board sensor and internal to 
70 

production units), signal drift (all, except backup battery systems), saturation stability, regulation 

codes and standards, deployment guidance, and especially calibration requirements. These 

represent specific areas in sensor technology in which additional research and development is 

necessary.
71 

References 

1. Energy Efficiency & Renewable Energy, Alternative Fuels & Advanced Vehicles Data 

Center www.afdc.energy.gov/afdc/, accessed September 27, 2011. Codes and Standards 

Requirements for Deployment of Emerging Fuel Cell Technologies, R. Burgess, W. 

Buttner, and C. Rivkin, NREL/TP-5600-52641 (December 2011) 

www.nrel.gov/docs/fy12osti/52641.pdf) 

2. “Codes and Standards Requirements for Deployment of Emerging Fuel Cell 

Technologies”, R. Burgess, W. Buttner, and C. Rivkin, NREL/TP-5600-52641, 

(December, 2011) (www.nrel.gov/docs/fy12osti/52641.pdf)

3. “2011 Toyota FCV-R Concept Unveiled, Previews Fuel-Cell Production Car in 2015” 

www.autoevolution.com/news/2011-toyota-fcv-r-concept-unveiled-previews-fuel-cellproduction-car-in-2015–40402.html, accessed December 1, 2011.

4. The Department of Energy Hydrogen and Fuel Cells Program Plan – An Integrated 

Strategic Plan for the Research, Development, and Demonstration of Hydrogen and Fuel 

Cell Technologies (September 2011) see https://www1.eere.energy.gov or 

http://search.nrel.gov/query.html?qp=&style=eere&qs=&qc=eren&ws=0&qm=0&st=1&

nh=10&lk=1&rf=0&oq=&col=eren&qt=2010_market_report.pdf&x=8&y=12) 

5. NREL Safety Codes & Standards Web Page: 

www.nrel.gov/hydrogen/proj_safety_codes_standards.html 

6. H2 Incident Reporting and Lessons Learned Database (see www.h2incidents.org/) 

7. International Association for Hydrogen Safety (see www.hysafe.org/IAHySafe) 

8. Agreement concerning the establishing of global technical regulations for wheeled 

vehicles, equipment, and parts which can be fitted and/or be used on wheeled vehicles, 

Draft GTR, Hydrogen Fueled Vehicle, (ECE/TRANS/132 and Corr.1); draft document 

(September 19, 2011) 

9. Hydrogen Sensor Workshop, Department of Energy, April 4, 2007. See 

www.lanl.gov/orgs/mpa/mpa11/sensors.html, accessed October 13, 2011.

10. See http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/safety.pdf, accessed 

October 13, 2011.

11. “Hydrogen Sensors – A review,” T. Hübert, L. Boon-Brett, G. Black, U. Banach, Sensors 

and Actuators (B) 157 (2011) 329–352. 

12. “Identifying performance gaps in hydrogen safety sensor technology for automotive and 

stationary applications,” L. Boon-Bret, J. Bousek, G. Black, P. Moretto, P. Castello, T. 

Hübert, U. Banach, International Journal of Hydrogen Energy 35 (2010) 373–384. 

13. “An overview of hydrogen safety sensors and requirements”, W. J. Buttner, M. B. Post, 

R. Burgess, C. Rivkin, International Journal of Hydrogen Energy 36 (2011) 2462-2470. 

www.eiseverywhere.com/ehome/2011DOEHydrogenSensorWorkshop/presentations 
72 

14. https://www.eiseverywhere.com/ehome/2011DOEHydrogenSensorWorkshop/presentations, 

accessed December 2, 2011.

15. The NREL Sensor Testing Laboratory home page: 

www.nrel.gov/hydrogen/facilities_hsl.html, accessed April 1, 2012. 

16. “Hydrogen Leak from Onboard Storage System of a Fuel Cell-Powered Lift Truck” in 

http://www.h2incidents.org/incident.asp?inc=298, accessed October 13, 2011. 

17. “Hydrogen and Oxygen Gas Monitoring System Design and Operation Safety and 

Technology of Nuclear Hydrogen Production, Control, and Management,” L.E. 

Cadwallader, K.G. Dewall, J.S. Herring, Idaho National Laboratory INL/CON-07-12079, 

June 2007. www.inl.gov/technicalpublications/Documents/3772047.pdf. 

18. “Wide Area and Distributed Hydrogen Sensors,” R. Zalosh; N. Barilo, 3rd International 

Conference on Hydrogen Safety, Paper 186, September 2009. 

http://conference.ing.unipi.it/ichs2009/images/stories/papers/186.pdf

19. UL 2034 “Single and Multiple Station Carbon Monoxide Alarms,” 3rd Edition 2008. 

20. OSHA minimal oxygen requirement 

www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=9801&p_table=STANDAR

DS 

21. “A Numerical Study of the Release and Dispersion of a Bouyant Ggas in Partially 

Confined Spaces,” K. Prasad, W.M. Pitts, M. J.C. Yang, , International Journal of 

Hydrogen Energy 36(8) (2011) 5200.5210.

22. “Numerical Analysis of Accidental Hydrogen Release in a Laboratory,” M. Heitsch, D. 

Baraldi, P. Moretto, International Journal of Hydrogen Energy 35(9) (2010) 4409–4419.
73 

Appendix A Definitions

Analytical, Deployment, and Operational Parameters

Analytical Parameters

Accuracy The relationship between the sensor indication and the nominal (accepted) 

concentration of the analyte, typically expressed as a percentage 

(Sensor Readout/Actual*100).

Analytical Resolution The minimum change of concentration that a sensor can accurately resolve. It 

has been defined as the slope of a linear calibration curve divided by the 

standard deviation of the measurement, which includes the noise contributions 

from the baseline and the response due to analyte exposure (also called 

analytical sensitivity).

Cross Sensitivity See selectivity

Drift Slow change in the response of the sensor due to various changes in electrical 

components, temperature, and aging. Drift can be defined as an apparent 

change in concentration per unit time for a fixed input concentration that can 

be due to a change in baseline or degree of response to the desired analyte.

Dynamic Range The range of analyte concentration above which a change in concentration gives 

a change in detector response, but not necessarily a linear change.

Environmental Effect The effect of changes in Pressure (barometric pressure), Temperature, and 

Relative Humidity / Moisture to induce a change in sensor electrical response. If 

uncompensated, such changes may be mis-read as a change in analyte 

concentration.

Interferent A chemical which produces a reversible response on the analytical detector. The 

impact of the interferent may be an apparent positive response, a negative 

response, or an attenuation of the response of the detector to the target 

analyte. Reversible implies that the detector will return to its normal behavior 

upon removal of the interferent; this recovery should be on the order of the 

stated response time of the detector. (see Poison) 

Limit of Quantization The smallest signal above background noise an instrument can reliably assign 

quantitative information; frequently calculated as 10 times the standard 

deviation of the background signal

Linear Range The concentration range above which the change in detector (sensor) response 

is proportional to the change in analyte concentration. The output can be 

represented by the equation: Concentration = Signalout × Gain. 
74 

Lower Detection Limit The smallest signal above background noise an instrument can reliably detect; 

frequently calculated as 3 times (or 2 times) the standard deviation of the 

background signal

Poison: A chemical which produces a permanent or slowly recovering response on the 

analytical detector. The impact of the interferent may be an apparent positive 

response, a negative response, or an attenuation of the response of the 

detector to the target analyte. (see Interferent) 

Recovery Time The time for the sensor to recover to baseline following exposure to vapor.

Response Time The time for a sensor signal to adjust during exposure to the analyte. For 

sensors that reach well-defined steady-state signals it is calculated as the time 

to reach a certain fraction of the response. Herein, the sensor response time can 

be defined as the time to reach 90% of the final indication that is achieved when 

the sensor is exposed to a constant flux of hydrogen at 10% of the LFL; 10% of 

the hydrogen LFL corresponds to the typical required alarm set point. T, P, RH, 

test gas flow rates and other parameters may be set by the manufacturer, but 

they must be consistent with the likely deployment conditions.

 

Repeatability The degree of consistency in response time and signal range between sensor 

responses induced by identical sequential vapor exposures.

Recovery See Reversibility. 

Reversibility The ability of the sensor to recover to baseline following an analyte exposure.

(also called “Recovery”)

Saturation Stability The ability of the sensor to provide quantitative information regarding the 

analyte within the specified concentration range of the device following a brief 

exposure to the analyte of concentrations in excess of the specified range 

including pure analyte exposure. The Saturation Stability can be defined as the 

time required for a sensor to be able to measure, within the instrument’s 

specified accuracy, hydrogen at the LFL or user-defined alarm threshold 

following a brief exposure to pure hydrogen.

Selectivity A measure of the relative response of the sensor to two different analytes. For 

example to measure 100 ppmv CO2 in nearly 100% oxygen, the CO2 detector 

must have a minimal CO2/O2 selectivity factor of at least 104

. See Interferent and 

Poison.

Sensitivity The electronic response of a sensor or instrument to a given amount of an 

analyte and has units of Concentration/Electronic-signal. It is set by the 

electronic gain of the sensor and associated electronic circuitry. In a linear 

system the sensitivity is a constant equal to the slope of the calibration curve.

Frequently used, improperly, synonymously for LDL. 
75 

Deployment Parameters

Capital Costs The actual cost of the sensor and required hardware (control circuitry) for 

general applications

Control Circuitry Required electronic circuitry to control and obtain the electronic output signal 

from a sensing element. Control circuitry can be trivial (a resistor for a 

chemiresistor), simple (a fixed potential potentiostat with I-V converter for EC 

sensor), or complex (frequency analyzers).

Electronic Interface Requirements to electrically connect the sensor to Instrument system and 

includes both power input requirements and signal output connections

Installation Costs The costs associated with installation associated with the specific instrument, 

and includes custom pneumatic interfaces, mounts, fixtures, etc.

Maturity Availability of the sensor technology for current deployment, including ability to 

meet market requirements.

Placement Location selection of the deployed sensor 

Physical Size Basic dimensions of the sensor and of the required circuit board

Power Requirement Voltage and current requirements to operate sensor and control circuitry

Pneumatic design Requirements to make the gas-sensor interface

Regulations Codes, standards and other government regulations which may mandate use of 

or properties of the sensor

Shelf life Effective time that a sensor can be stored prior to installation.

Operational Parameters

Alarm Set Points Analyte concentration(s) that activates an audible alarm or relay connected to 

an alarm system or other systems (e.g., ventilation system, shutdown system, 

remote monitoring systems). Although somewhat arbitrary, alarm levels for a 

hydrogen sensor can be described as trace (e.g., 1% of the LFL), low (10% of the 

LFL), medium (25% of the LFL) or high (LFL or higher).

Calibration (Cal) Protocol and frequency for calibrating the sensor to assure accuracy 

and/or compliance to regulations.

Consumables (Con) Required and depletable supplemental material requirements. For 

example, hydrogen in an FID; calibration and validation gases
76 

Device Repeatability Similarity of response characteristics between multiple samples of the same 

sensor type or model.

Maintenance (Mtc) Required procedures to assure accuracy, includes replacement

Matrix requirements (Mtx) Effect of background gas on sensor response. For example MOX sensors 

require oxygen for reversibility

Operational Lifetime (Lif) Expected useful life of the sensor under operating conditions

Orientation Effect Change in sensor response to a test gas as the sensor is rotated through various 

orientations (side, upside down, etc.)

Sample size (Sam) Volume of gas sample required by sensor to perform accurate 

measurements

Signal Management (SM) Interfacing the sensor signal to the DAQ system.

Warm-up Time The time required for a sensor to produce analytically useful outputs following 

powering up. 
77 

Appendix B The Hydrogen Sensor Workshop

Rosemont, Illinois (June 8, 2011)
78 
79 
80 

Appendix C Hydrogen Sensor Codes and Standards

(Partial List)

ANSI/ISA–12.13.01 – Performance Requirements for Combustible Gas Detectors

www.isa.org/Template.cfm?Section=Standards8&Template=/Ecommerce/ProductDispla

y.cfm&ProductID=6740 

CSA C22.2, No 152 – Performance of Combustible Gas Detection Instruments

www.csa-international.org/product_areas/hazloc_equip/special_services/ 

performance_of_combustable_gas_detection_instruments/

DOT Transport of flammable material

FM Global Class Number 6310 and 6320 – Approval Standard for Combustible Gas Detectors

www.fmglobal.com/assets/pdf/fmapprovals/6310.pdf 

IEC 61508 – Functional Safety

 www.iec.ch/functionalsafety/ 

IEEE/ASHRAE – Guide for the Ventilation and Thermal Management of Batteries for Stationary 

Applications 

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5399268&abstractAccess=no&us

erType=inst 

ISO 26142:2010 – Hydrogen detection apparatus – Stationary applications

ISO 26262 – Road Vehicles – Functional Safety 

www.iso.org/iso/catalogue_detail.htm?csnumber=43464 

NFPA 2 – Hydrogen Technologies Code

www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=2 

NFPA 52 – Vehicular Gaseous Fuel Systems Code

www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=52&cookie_test=1 

OSHA 1910.146 – General Environmental Controls – Permit Required Confined Spaces 

www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=9797 

SAE J2579 – Technical Information Report for Fuel Systems in Fuel Cell and Other Hydrogen 

Vehicles 

http://standards.sae.org/j2579_200901/ 

UL 2075 – Gas and Vapor Detectors and Sensors

http://ulstandardsinfonet.ul.com/scopes/2075.html

UL 2267 – Fuel Cell Power Systems for Installation in Industrial Electric Trucks

http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=2267.html 

UL 60079-15 – Electrical Apparatus for Explosive Gas Atmospheres

http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=60079-15.html