A national laboratory of the U.S. Department of Energy

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National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Test Protocol Document, 

Hydrogen Safety Sensor Testing 

Phase I: Non-Flammable Mixtures 

R. Burgess, C. Blake, and C.E. Tracy 

Technical Report 

NREL/TP-560-42666 

September 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Test Protocol Document, 

Hydrogen Safety Sensor Testing 

Phase I: Non-Flammable Mixtures 

R. Burgess, C. Blake, and C.E. Tracy 

Prepared under Task No. H274.7110 

Technical Report 

NREL/TP-560-42666 

September 2008 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

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Contract No. DE-AC36-99-GO10337
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Table of Contents 

1 Objectives .................................................................................................................................. 1

2 Scope ......................................................................................................................................... 1

3 Overview of Sensor Technology ................................................................................................ 2

4 Test Hardware and Data Acquisition ......................................................................................... 4

5 Standard Conditions ................................................................................................................... 6

6 Standard Protocol ...................................................................................................................... 8

7 Baseline Testing ......................................................................................................................... 9

8 Life Testing ................................................................................................................................. 9

9 Plans for Future Work .............................................................................................................. 10

10 References ............................................................................................................................... 10

iii 
1 Objectives 

The objective of the testing is to evaluate the performance of hydrogen safety sensors in 

terms of the U.S. Department of Energy (DOE) technical targets listed in the 2007 

technical plan for hydrogen safety (see Table 1) and other codes and standards emerging 

for hydrogen sensors (e.g., UL 2075, ISO). Testing will provide quantitative assessment 

on a variety of critical sensor performance specifications, including linear range, lower 

detection limit, cross reactivity, response/recover time, and others. This test protocol 

document includes an overview of sensor technologies, test hardware requirements, and 

an outline of potential testing. A more definitive standard operating procedure (SOP) will 

be prepared prior to the start of test lab operation. 

2 Scope 

The DOE has assigned the National Renewable Energy Laboratory (NREL) to provide 

technical support for hydrogen codes and standards R&D. The success of the DOE’s 

Hydrogen Program is critically dependent upon the timely development of standards to 

facilitate the emergence of a viable hydrogen infrastructure. In this role, NREL is 

supporting research and development (R&D) needed to establish the technical basis for 

requirements that can be incorporated into standards and model codes. One of the key 

areas where requirements need to be further defined is related to hydrogen sensors. 

A critical area of work identified by experts in codes and standards is the refinement of 

existing and the development of new standardized analytical test and qualification 

procedures for hydrogen sensors. In anticipation of the increased use of hydrogen, and to 

support existing markets, members of the chemical sensor industry are developing and 

marketing hydrogen sensors. It is critical that hydrogen sensors meet DOE targets. 

Accordingly, test methods need to be established to validate hydrogen sensors 

performance. This requires the design and construction of a test apparatus and 

implementation of a specified set of standards and testing criteria. Researchers and 

engineers generally accept that the ubiquitous deployment of hydrogen energy 

technologies will not take place without an accurate, durable, and reliable hydrogen 

sensor. 

Phase I testing will consist of test situations involving non-flammable, non-explosive 

mixtures of hydrogen. The lower flammable limit (LFL) of hydrogen in air is 4% by 

volume. All Phase I testing will be conducted with hydrogen concentrations below the 

LFL or with limited oxygen content. Future testing, designated as Phase II testing, will 

expand the test operation envelope to include mixtures above the LFL. Phase II test plans 

and scope will be covered in future documents. 

Test methods need further standardization in both the national and international areas. 

NREL has initiated collaborations with other organizations to facilitate the harmonization 

of test methods; examples include organizations such as UL (Underwriters Laboratory), 

IIT (Illinois Institute of Technology), and the JRC (Joint Research Centre) lab in Europe. 

1 
NREL researchers will conduct R&D of emerging sensor technologies in conjunction 

with testing commercially available sensors to validate and compare the various 

technologies. Special emphasis will be placed on identifying Technology Gaps in which 

critical needs are not met by existing or emerging hydrogen sensors. 

Table 1. DOE Targets for Hydrogen Safety Sensor R&D [1] 

3 Overview of Sensor Technology 

The need for robust, affordable, and compact hydrogen safety sensors is driving the 

development of new sensor technology. Current production sensors can provide 

repeatable and accurate hydrogen sensing; however these sensors are costly and often not 

ideally selective to hydrogen. Preproduction sensors which show promise for meeting 

the DOE targets are also a focus of this effort. From an application standpoint, the 

placement of hydrogen sensors to detect a hydrogen leak is also a major challenge. 

3.1 Sensor Placement 

Sensor placement depends on many environmental factors that affect the gas cloud 

dispersion, and it is important to prepare for all potential failure scenarios. For this 

reason, it would be beneficial to have an array of hydrogen sensors to insure that a leak is 

detected quickly and that an effective shutdown occurs. Requiring multiple sensors to be 

placed in a single application further drives the need for accurate, reliable, low-cost 

sensors. 

3.2 Sensor Reliability 

There are sensors in use today that have shown a history of faulty signals (e.g., false 

positives or false negatives) especially over extended period of deployment. Short 

operational lifetime is a concern among stakeholders. The root cause of the faulty signals 

varies, but includes environmental factors such as dirt/dust, vibration, fluctuating 

temperature and humidity, and contaminant substances. Testing in a controlled laboratory 

environment can be used to identify causative factors that induce faulty sensor excursions 

and their effect. Additionally, testing conditions can be controlled to simulate sensor 

2 
operation in many varied environments and applications. A sensor that works well in one 

application may not be ideal for another. 

3.3 Hydrogen Sensor Types 

Included here is a general listing of sensor types with a brief description of the 

technology. These sensor technologies are in various stages of development, from initial 

proof of concept stages to full production. Note: information in this section was presented 

at the 2007 Hydrogen Sensor Workshop by Dr. William Buttner [2]. 

Some overall conclusions are that cost reductions are needed for all the sensors. The 

metal oxide (MOX) sensors typically are lower cost than other sensors but have 

limitations in response time, selectivity, and drift. All sensor types include commercially 

available products; however, there are also many products that are in the process of 

development or not in full production. 

Electrochemical

This general class of sensors includes amperometric, potentiometric, and solid/liquid 

electrolyte type sensors. These sensors are linear and repeatable over a broad range but 

have selectivity and response time limitations. They are generally commercially 

available. 

Palladium and Palladium Alloy Film

There are several technologies that employ palladium and palladium alloys for hydrogen 

sensing, including electronic (resistance, capacitance), thermoelectric, evanescence wave, 

and mechanical (cantilever) and SAW (Surface Acoustic Wave). The palladium film is 

generally more expensive and not yet as readily available commercially as other types of 

sensors. There is potential for palladium film-based sensors due to the high number of 

designs being explored. 

Metal Oxide (MOX)

Heated MOX sensors are a mature commercial technology that is available for use at an 

affordable cost. There are issues such as high power requirements, non-linear response, 

and poor selectivity that limit the widespread acceptability for all applications. 

Pellistor

A pellistor bead sensor uses a catalyst of platinum or palladium to produce a resistivity 

change. There are issues relative to the selectivity, response time, and power 

requirements that need further improvement. 

Thermal Conductivity

The thermal conductivity of hydrogen gas is almost seven times higher than the thermal 

conductivity of air. Thermal conductivity sensors measure the thermal conductivity of the 

ambient gas, which is proportional to the hydrogen content. Most other gases of interest 

3 
also have thermal conductivities much closer to air. See table 2 below for a summary of 

thermal conductivities for several gases of interest. Note that helium gas has a thermal 

conductivity closest to hydrogen. 

Table 2. Thermal Conductivity of Gases 

Gas Thermal Conductivity 

K (mW/m) 

K/KAIR

Air 33.3 1.00 

Helium, He 190.6 5.72 

Hydrogen, H2 230.4 6.92 

Methane CH4 49.1 1.47 

Propane C3H8 30.6 0.92 

Water Vapor H2O 27.1 0.81 

Nitrogen, N2 32.3 0.97 

Oxygen, O2 33.7 1.01 

Thermal conductivity sensors are commercially available. They have good response time 

but tend to be sensitive to environmental drift and have selectivity issues. 

Optical/Acoustic Devices

This general category contains devices such as colorimetric and indicator dyes, 

evanescent wave, fiber optic hydrogen absorption, fiber optic with thin palladium film, 

speed of sound, and tuning fork technologies. Several of these technologies are in the 

early stages of development, but commercial prototypes may soon be available. One 

advantage is that they are intrinsically safe with no electrically powered device in contact 

with the hydrogen. The relative merits of several technology types need to be measured 

and analyzed. 

4 Test Hardware and Data Acquisition 

4.1 Test Station Definition 

In order to perform the desired tests, we will equip the sensor test laboratory with test 

apparatus capable of exposing sensors to controlled environmental conditions. The test 

station schematic is presented in . Gases are provided in pressurized cylinders, 

and certified for purity and composition. These gases are metered using mass flow 

controllers. Bypass lines around the mass flow controllers are provided for dilution and 

purging purposes. Additionally, water vapor can be added to the gas mix. Potentially, 

other vapors can be added to this configuration. Gases can either be premixed as supplied 

from the gas vendor or can be mixed in-situ to the required concentration in the mixing 

Figure 1

4 
chamber. From here gas/vapor mixtures can flow to the test chamber where sensors are 

mounted. 

The test hardware’s response time is minimized by minimizing the gas volume between 

the mixing chamber downstream valve and the test chamber. The test chamber design 

includes the capability for controlling the temperature and pressure environment. 

Pressure is to be regulated by using a combination of a compressed gas supply, vacuum 

pump venting, and pressure regulators. Gas sampling from various test points and 

analysis by GC is planned to validate mixtures in the test chamber. We expect that the 

accuracy of data obtained by gas analysis will provide more precise test conditions and 

increase sensor measurement accuracy. All valves are controlled by the lab-based 

computer and automated test operation will allow for extended unattended life testing. 

Additional interlock shutdowns will be used to insure proper isolation of hydrogen gas in 

the event of an upset condition. 

Residual Gas 

Environmental Analyzer

Control 

Chamber

Metering Flow 

Controllers

Turbo Molecular Dry Pump Pump

P,T,RH

P,T,RH

Dry Pump 

Exhaust 

Exhaust 

N2 

Air 

+ 

CO

Air

H2O Bubbler

Mixer

Valve 

Butterfly Valve 

Sensor Test Module 

LABview Based 

Computer Control 

and Data Acquisition

Notes: 

 System in Exhaust Hood

 Chamber Design for Multiple Sensor Sizes 

 Metal Fittings 

Automatic Valve 

Controller

H2

Figure 1. Hydrogen Safety Sensor Test Schematic 

5 
4.2 Data Acquisition 

A LabView based computer data acquisition system will provide automated controls, 

actuation for all test functions, and data logging into an electronic file of all sensor 

responses. Controls will include appropriate interlocks for safe shutdown of the apparatus 

in the event of a system upset or failure. Data sampling rates will be sufficient to verify 

response times of less than one second, but will be adjustable so as to allow lifetime 

testing. A response time of less than one second is targeted for hydrogen safety sensors 

based on published DOE safety plans specifications[1]. Data acquisition and controls 

will be capable of autonomous operation of test parameters. This will allow for extended 

life testing. Sensitive data will be stored in a secured location to protect against 

unintended disclosure to non-essential personnel and to protect proprietary information. 

5 Standard Conditions 

Input parameters that may have an effect on the output of the sensor are listed in the 

following outline. 

Hydrogen Safety Sensor Performance Matrix 

1 Analytical 

1.1. Environmental 

1.1.1. Temperature -30°C to 80°C 

1.1.2. Pressure 0.8 atm to 1.2 atm 

1.1.3. UV exposure 

1.1.4. RF Exposure 

1.1.5. Velocity Effects 

1.1.6. Non-homogeneous Effects 

1.1.7. Hydrogen Consumption 

1.1.8. Permeation/Soak Back 

1.1.9. Strain 

1.1.10. Dust 

1.1.11. Static Discharge 

1.1.12. Vibration 

1.1.13. Orientation 

1.1.14. Selectivity/ Contamination 

1.1.14.1. Gas Contamination 

1.1.14.2. Vapor Contamination 

1.1.14.3. Multiple Contaminant Effects 

1.1.14.4. Reversibility/Irreversibility 

1.1.14.5. Poisoning Effects 

1.2. Accuracy/Capability 

1.2.1. Signal Drift 

1.2.2. Baseline Drift (Zero Drift) 

1.2.3. Analytical Resolution 

1.2.4. Linear Range 

1.2.5. Dynamic Range 

1.2.6. Lower Detection Limit 

6 
1.2.7. Gauge R&R 

1.2.7.1. Reproducibility 

1.2.7.2. Repeatability 

1.3. Sampling Size /Time Scale 

1.3.1. Response Time 

1.3.2. Recovery Time 

1.3.3. Saturation (flooding) 

1.3.4. Statistical Sampling 

1.3.4.1. Test to Test Variation 

1.3.4.2. Part to Part Variation 

1.3.4.3. Design of Experiment 

2 Logistical 

2.1. Deployment 

2.1.1. Capital Cost 

2.1.2. Installation Cost 

2.1.3. Physical Size 

2.1.4. Control Circuitry 

2.1.5. Power Requirement 

2.1.6. Electronic Interface 

2.1.7. Pneumatic Connections 

2.1.8. Shelf Life 

2.1.9. Maturity/Availability 

2.1.10. Manufacturability 

2.1.11. Grounding 

2.1.12. Alarm/Audibility 

2.1.13. Government Regulation 

2.1.14. Certification (UL, IEC...) 

2.2. Operational 

2.2.1. Operational Lifetime 

2.2.2. Consumables 

2.2.3. Calibration & Maintenance (frequency and complexity) 

2.2.4. Sample Size 

2.2.5. Matrix Requirements 

2.2.6. Signal Management 

2.2.6.1. Data acquisition 

2.2.6.2. Control Protocol 

2.2.6.3. Fail Safe 

2.2.6.4. Redundancy 

Sensor output can be defined as the device’s ability to accurately measure the hydrogen 

concentration. The overall purpose of the testing is to determine the input parameters that 

have the maximum effect on the output and to quantify those effects. Use of “design of 

experiment” methods can be used to help identify the primary input parameters and 

determine effects on output. An added advantage of the design of experiment technique is 

7 
that is can be used to define confounding effects, where multiple input parameters have a 

combined effect on the output. 

6 Standard Protocol 

Prior to performing tests on hydrogen safety sensors, a number of standard tests will be 

performed on the test apparatus to determine the capabilities of the system and insure the 

accuracy of the test methodologies. 

6.1 Leak Integrity Test 

The test apparatus will be checked for leakage prior to testing. The leak check will be 

repeated after hardware changes, on a regularly scheduled timetable, and anytime there is 

question about the integrity of the system. Leak check will consist of pressurizing the 

system to a known pressure above atmosphere and verifying that the system is able to 

maintain pressure over time. To isolate and repair faulty fittings or components, the 

system can be pressurized with a known service gas such as helium. Helium leak 

detectors can be employed to search for the location of the leak. 

6.2 Contamination/Cleanliness Test 

All test apparatus components will be cleaned and inspected prior to any required 

assembly or reassembly. Care will be taken to cover openings to prevent contaminants 

from entering. The assembled system will be purged of impurities by inert gas. Gas 

samples will be analyzed to insure that impurity levels are below detectable limits. If 

needed, a bake-out process can be used to accelerate the purging process. A steady state 

hold, with regular gas sampling, will be used to determine that impurities are not 

permeating into the test gas. 

6.3 System Response Time Test 

One of the key parameters for hydrogen safety sensor testing is response time. The DOE 

target of less than one second response time will be a difficult target to reach for many of 

the sensor technologies in current use. It will help if the test apparatus response time is 

much faster than the sensor response time being measured. In order to determine the test 

apparatus system response time, a heated gas test can be used. The heated gas at 

equilibrium conditions in the mixing chamber can be introduced to the test chamber. A 

fast-acting thermocouple can be used to determine the time between opening the valve 

and measuring the elevated temperature. Fast acting thermocouples with small wire 

diameter can have response times on the order of 0.003 seconds. 

6.4 Sensor Calibration 

Sensor calibration procedures will vary with each sensor. Sensors that are to be tested 

will be calibrated per the manufacturer’s recommendations. Calibration data will be 

recorded at regular intervals or as required by the test sequence. 

8 
7 Baseline Testing 

All test data and product specific information will be maintained in secure storage. Test 

data of existing commercial products will be reviewed with the manufacturer prior to 

publishing the data for general use. Test data of prototype sensors will be confidential. 

Testing defined in this section represents typical tests. Specific testing to recognized 

codes and standards and /or SOPs will be defined in a later document. 

7.1 Linear Range (dynamic range)/Accuracy/Zero Drift 

Basic sensor capability will be measured by this series of tests. The linear range will 

define the lower detection limit and the point where saturation is reached. Hold points at 

zero concentration will be used to determine zero drift. Output readings will be compared 

with gas analysis to determine the accuracy of the readings. 

7.2 Response/Recovery Time 

The sensors will be exposed to a cyclical test, varying the concentration of hydrogen 

from zero to a fixed fraction of the LFL (LFL = 4%). Response time and recovery time 

will be measured for each cycle and compared over multiple cycles. 

7.3 Environmental Testing (Temperature, Pressure, Relative Humidity) 

Effects of pressure, temperature, and relative humidity will be measured under controlled 

conditions in accordance with the DOE target specifications (Table 1). Temperature will 

be varied from -30°C to 80°C; pressure will be varied from 0.8 to 1.2 atm. Relative 

humidity will be varied from 10% to 98%. 

7.4 Contamination Testing 

Contaminants that have been identified for potential testing are hydrogen sulfide, carbon 

monoxide, carbon dioxide, nitrogen oxides, specific hydrocarbons, ammonia, and select 

alcohols. Testing for interference effects of these contaminants will require an extensive 

testing program. Priorities will be based on selecting contaminants with known effects on 

specific sensor technologies. As an example, thermal conductivity sensors will respond to 

helium due to its high thermal conductivity. Helium would not be expected to 

contaminate other sensor types. 

8 Life Testing 

Extended life testing will be conducted to determine the ability of sensors to meet the 

DOE goal of 10 years of operational life. The lifetime of a sensor is a critical parameter 

that needs to be addressed by further R&D testing. Environmental factors that need to be 

considered when conducting life testing are repeated exposure, temperature cycling, 

pressure cycling, humidity cycling, dust exposure, and UV exposure. The automated test 

apparatus will be capable of running continuous testing with variable conditions, 

including accelerated life type tests. 

9 
10 

9 Plans for Future Work 

Future plans are to expand the testing capability to include flammable and explosive 

operating regimes, denoted as Phase II testing. Most of the interest in safety sensor 

performance is to detect hydrogen before reaching flammable concentrations. For this 

reason, we have limited the Phase I testing to non-flammable (below 4%) conditions. The 

driving force for testing in flammable/explosive atmospheres is to verify that sensors are 

capable of operation under higher concentrations and to examine whether the sensors 

themselves (many electrically-based) are potentially capable of being an ignition source. 

Test hardware requirements for Phase II testing will require designing the test housing for 

explosion containment. This is intended to provide an added safety measure in the remote 

chance of an uncontrolled thermal excursion induced by unintended hydrogen 

combustion. Hydrogen forms a flammable mixture over a wide range of concentrations in 

air and requires a minimum ignition source; only one-tenth of the energy required for 

gasoline vapors. When using flammable mixtures, it is possible that an unintended 

ignition could occur. Additionally, the high flame speed of a hydrogen deflagration will 

create a pressure peak within a few milliseconds. Burst disk technology would not be 

capable of relieving the pressure peak due to the inherently slow burst disk response time 

of 5 to 10 milliseconds. National Fire Protection Association (NFPA) 69 provides design 

requirements for explosive containment pressure vessels [3]. 

In addition, we plan to test sensors under simulated/controlled leak scenarios. This testing 

would be used to anchor computational fluid dynamics (CFD) model codes. CFD codes 

are currently being used to predict hydrogen cloud dispersion under various leak 

scenarios. Modeling results are then used to determine the most effective placement 

locations for safety sensors. We envision that testing would place sensors in a simulated 

garage space environment while providing a known hydrogen gas leak. 

10 References 

1. DOE Office of Energy Efficiency and Renewable Energy, Hydrogen Safety Plan, 

http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/safety.pdf 

2. “Today’s Commercial Hydrogen Sensors” Presentation at the Hydrogen Sensor 

Workshop, 04 Apr 2007, by Dr. William Buttner. 

http://www.lanl.gov/orgs/mpa/mpa11/TechBriefing-Buttner-Sensors.pdf 

3. NFPA 69 “Standard on Explosions Prevention Systems” 
F1147-E(09/2007) 

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R. Burgess, C. Blake, and C.E. Tracy 

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This test protocol document includes an overview of hydrogen sensor technologies, test hardware requrements, and 

an outline of potential testing. 

15. SUBJECT TERMS

hydrogen sensor; hydrogen sensors; hydrogen testing; hydrogen safety; sensor; test protocol 

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