NIST Special Publication 1201

Measurement Challenges and 

Metrology for Monitoring CO2

Emissions from Smokestacks –

Workshop Summary

Aaron N. Johnson

Rodney A. Bryant

Tamae Maeda Wong

James R. Whetstone

Toralf Dietz

Donald Giel

David Elam Jr.

Scott Swiggard

Woong Kang

Iosif Shinder

Eric Harman

Hsin-Hung Lee

Liang Zhang

Keith Gillis

Richard Grot

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.1201
NIST Special Publication 1201

Measurement Challenges and Metrology 

for Monitoring CO2 Emissions from 

Smokestacks – Workshop Summary

Aaron N. Johnson 

Sensor Science Division 

Physical Measurement 

Laboratory

Rodney A. Bryant 

Fire Research Division 

Engineering Laboratory

Tamae Maeda Wong 

James R. Whetstone 

Special Programs Office

Toralf Dietz

Sick Engineering Gmbh

Donald Giel

Teledyne Monitor Labs

David Elam Jr.

TRC Environmental Corp.

Scott Swiggard

Golden Specialty Inc.

Woong Kang

Korea Research Institute of 

Standards and Science (KRISS)

Iosif Shinder

Sensor Science Division 

Physical Measurement 

Laboratory

Eric Harman

Colorado Engineering

Experiment Station (CEESI)

Hsin-Hung Lee

Industrial Technology

Research Institute (Taiwan)

Liang Zhang

National Institute of 

Metrology (China)

Keith Gillis

Sensor Science Division 

Physical Measurement 

Laboratory

Richard Grot

LAGUS Applied 

Technology Inc.

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.1201

December 2015

U.S. Department of Commerce

Penny Pritzker, Secretary

National Institute of Standards and Technology

Willie May, Under Secretary of Commerce for Standards and Technology and Director
Disclaimer

NIST Special Publication 1201 is intended to capture external perspectives related

to NIST standards, measurement, and testing-related efforts. These external perspectives

can come from industry, academia, government, and other organizations. This report was 

prepared as an account of a workshop; it is intended to document external perspectives;

and does not represent official NIST positions.

Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by NIST,

nor is it intended to imply that the entities, materials, or equipment are necessarily

the best available for the purpose.

National Institute of Standards and Technology Special Publication 1201

Natl. Inst. Stand. Technol. Spec. Publ. 1201, 17 pages (December 2015) 

CODEN: NSPUE2

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.1201
ii

Preface

NIST traceable standards, reference materials and certifications have provided advances in 

measurement sciences and calibrations needed for industry to compete in the global economy. 

Emissions monitoring for electric power generation is a vital sector that merits investments for 

industrial competiveness and regulatory effectiveness. Responding to the needs of the public, 

private industries, and regulatory agencies, NIST has supported scientific research and 

technology development to provide the best measurement standards and innovation in 

instrumentation. Electric power is the engine for U.S. industry and commercial growth. 

Therefore effective action is needed to meet regulatory requirements and improve greenhouse 

gas measurements. 

Coal and natural gas are important domestic resources in the U.S. Both provide over 65% of 

the nation’s electricity and serve the economic demand for stable, reliable, and cost effective 

electric power. Accurate and reliable emissions monitoring from smokestacks is thus vital for

complying with regulations and minimizing environmental and health effects. NIST’s role in 

supporting this industry is to improve the accuracy in emissions measurements and to minimize 

the uncertainty in operating conditions, testing, and analysis.

Developing standards and establishing common measurements has benefits beyond ensuring 

optimal emissions monitoring operations and meeting regulatory compliance. It offers the 

opportunity to make improvements and develop procedures that can form industry standards. 

Best practices can be developed and dialog can begin with other emissions monitoring

operations, local and state regulators, and more importantly the community in which the 

source resides. Standards and reference materials are not only to fulfill management demands 

or regulatory oversight; they offer a baseline from which improved systems can develop. Thus, 

NIST’s role is not only to provide a service but to catalyze innovation by empowering experts in 

the field.

The opportunities to share best practices between measurement experts (e.g., National 

Metrology Institutes - NMIs) and emissions monitoring specialists are limited. As such, NIST 

organized a forum to bring together all interested parties including NIST research staff, research 

staff from other NMIs, industrial professionals and researchers, regulators, and equipment 

suppliers. The venue is intended to generate ideas for broad improvements in the field by 

sharing solutions to common emissions monitoring problems. 

Opportunities that empower experts and simulate improvements can achieve far reaching 

impact by involving the international community. Understanding the challenges and factors 

that can provide improvements should be shared not only in the U.S. but with experts in other 
iii

countries. While clean coal is making improvements in many U.S. power plants, basic 

monitoring systems are only beginning to be introduced in other countries where smog is a 

serious health risk. In China, for example, it is reported that a new coal fueled power plant is 

built each month. New technologies and engineering expertise are valuable assets that merit 

support by any nation. To realize Greenhouse Gas (GHG) reductions globally, lessons learned 

and best practices must be widely disseminated. 

Addressing climate change will require accurate measurements for greenhouse gases, and 

stable long term reference standards to measure small variations over long periods of time. 

Such measurements will contribute to better reporting values, and in turn build confidence for 

the industry. To achieve clean energy systems, commitment to sound engineering practices 

built upon scientific standards that are accepted by users, regulators, and consumers is a first 

step. Participants of this workshop are actively engaged in that process.

James R. Whetstone

Special Assistant to the Director for Greenhouse Measurements
iv

Acknowledgements

The success of any workshop is dependent on the hard work of multiple individuals. We wish to thank 

all participants for their contributions and engaging discussions. We are sincerely grateful for the candid 

exchange of knowledge, experiences, and best practices. We especially want to thank the presenters for 

providing valuable information that stimulated important discussions and served to open new 

opportunities to exchange insights. Your contribution was an important catalyst for the workshop’s 

success. In addition we wish to express our sincere thanks to Gina Kline of the Fluid Metrology Group 

for her expert assistance, logistics arrangements, and workshop tours. A sincere thanks is also extended 

to Iosif Shinder for hosting a tour of the NIST Wind Tunnel.

Aaron Johnson, Rodney Bryant, Tamae Wong, and James Whetstone
v

Table of Contents 

Preface ...............................................................................................................ii

Acknowledgements ...........................................................................................iv

1. Introduction..................................................................................................1

1.1 Overview of the NIST Greenhouse Gas and Smokestack Projects......................................................2

2. CEMS and RATA Measurements....................................................................4

3. Pitot Probe Characterization and Calibration ................................................9

4. NIST Facilities and Flow Measurement Research.........................................10

5. Closing Remarks..........................................................................................16

6. References..................................................................................................17

7. Appendices.................................................................................................19

7.1 Appendix A: Workshop Agenda ........................................................................................................19

7.2 Appendix B: List of Attendees...........................................................................................................21

7.3 Appendix C: Workshop Presentations..............................................................................................23

7.3.1 NIST Greenhouse Gas and Climate Science Measurements Program...................................23

7.3.2 Improving the Accuracy of CEMS by Means of Multipath Ultrasonic Flowmeter................26

7.3.3 Practical Experience with CEMS Measurements ...................................................................34

7.3.4 Overview of ASTM D7036: A Quality Management Standard for Emission Testing ...........40

7.3.5 Volumetric Flow Measurements of Stationary Sources: Common Mistakes, Corrective 

Measures.................................................................................................................................45

7.3.6 Stack/Duct Flow Measurements – Tracer Gas Method ASTM E2029 ...................................56

7.3.7 3D Pitot Tube Measurements and Calibration in the Wind Tunnel ......................................60

7.3.8 Experimental and Numerical Investigations of the Factors Affecting the S-type Pitot Tube

Coefficients in GHG Emission Monitoring ..............................................................................65

7.3.9 Greenhouse Gas Emissions – A Case Study on the Calibration of an L-pitot Static Tube.....75

7.3.10 NIST’s New 3D Airspeed Calibration Rig, Turbulent Flow Measurement Challenges ..........83

7.3.11 Scale-Model Smokestack Simulator (SMSS) – A Facility to Study the Uncertainty of CEMS 

and RATA Flow Measurements..............................................................................................91

7.3.12 Performance Evaluation of Ultrasonic Flow Meters in NIST’s Smokestack ..........................98

7.3.13 Using the National Fire Research Laboratory (NFRL) as a Test Bed for Traceable CO2

Measurements......................................................................................................................107

7.3.14 Is a Long-Wavelength Acoustic Flowmeter Feasible for Smokestacks? .............................113
vi

7.4 Appendix D: Stationary Source Sampling & Analysis for Air Pollutants (SSSAAP) Conference ....121
1

1. Introduction

Reducing greenhouse gas emissions requires accurate and reliable measurements to evaluate 

the effectiveness of the mitigation efforts. To address these measurement needs NIST hosted

its first workshop on Measurement Challenges and Metrology for Monitoring CO2 Emissions 

from Smokestacks. The workshop was held on April 20 and 21, 2015 at NIST’s Gaithersburg 

Maryland campus. The purpose of the workshop was to exchange experiences, best practices, 

and ideas related to current and emerging issues concerning the accuracy of CO2 emissions 

measurements from smokestacks. The workshop brought together diverse stakeholders who

shared perspectives, and discussed strengths and weakness of current CO2 emissions

measurement protocols. The workshop provided a forum for NIST to share its progress toward 

improving the accuracy of smokestack CO2 emissions measurements and establishing a 

traceability chain that clearly ties these measurements to internationally recognized standards. 

The workshop participants included regulators, power plant continuous emissions 

measurement systems (CEMS) operators, CEMS manufacturers, relative accuracy testing audit 

(RATA) companies, U.S. wind speed calibration laboratories, NIST1staff, and researchers from 

other National Metrology Institutes (NMIs) with expertise in wind speed and flow 

measurements. Speakers gave presentations on the following topics:

 RATA2and CEMS3 measurements, 

 characterization of different pitot probe types and methods of calibration, and

 newly constructed NIST research facilities focused on providing traceability and 

improved accuracy of CO2 emissions measurements. 

Although the workshop discussed both concentration and flow measurements, the greater 

emphasis was placed on improving smokestack flow measurements. This report is organized

according to the three discussion topics detailed above. The key points from the presentations 

and participant discussions are summarized for each topic. For convenience, the appendices 

include the workshop agenda (Appendix A), list of attendees (Appendix B), and workshop 

presentations (Appendix C). In addition, we have summarized previous discussions between 

NIST and members of the stack testing community that served as a precursor to this workshop.

(Appendix D).

 

1

The National Institute of Standards and Technology (NIST) is the U.S. National Metrology Institute.

2

The Relative Accuracy Test Audit (RATA) is a procedure developed by the EPA, and used by certified testers to measure the 

flow velocity and concentration in a smokestack. RATA testers temporarily install their measurement equipment in a 

smokestack to calibrate CEMS equipment that is permanently installed in smokestacks. 

3

The Continuous Emissions Monitoring Equipment (CEMS) consist of instrumentation permanently installed in smokestack and 

used to continuously measure both the flow and concentration of regulated pollutants. 
2

1.1 Overview of the NIST Greenhouse Gas and Smokestack Projects

NIST is a non-regulatory agency of the U.S. Department of Commerce and the country’s 

National Metrology Institute (NMI). The mission of NIST is

to promote U.S. innovation and industrial competiveness by advancing 

measurement science, standards, and technology in ways that enhance economic 

security and improve our quality of life.

NIST executes this mission by performing measurement science research with the goal of 

improving accuracy and ensuring international recognition of measurement methodologies and 

standards. NIST approaches these tasks in part through cooperative activities with the 

interested and impacted communities. Hence NIST is working collaboratively with industry, 

other Federal agencies, and the states to enhance the international acceptance of U.S. 

measurement standards and methods as a foundation for quantitative, science-based 

greenhouse gas emission inventories and offsets. 

The main program objectives are to:

 improve the current measurement and standards infrastructure to improve the accuracy 

of greenhouse gas measurements in the U.S.;

 promote these measurements and standards internationally;

 transfer improved measurement technologies and best practices to other government

agencies and the private sector;

 support the measurement standards as needed.

Motivation for addressing CO2 emissions monitoring from the smokestacks of stationary 

sources is generated by the significant contribution of CO2 emissions from the electric power 

sector. For 2013 the U.S. Environmental Protection Agency (EPA) estimated that electricity 

generation accounts for 31% of total CO2 equivalent emissions, Figure 1. [1]
3

Figure 1 Total U.S. Greenhouse Gas Emissions by Economic Sector in 2013. Total = 6673 Million Metric Tons of CO2 equivalent. 

(http://www.epa.gov/climatechange/ghgemissions/sources.html)

Carbon dioxide emissions determination from electrical generation plants may be based on 

either fuel calculation or continuous emissions monitoring methodologies. Comparisons of 

these two methods have been published in the scientific literature [2, 3], primarily for coal-fired 

plants, and indicate a significant disparity in emission values. Independent NIST investigations 

of similar data also indicate such a disparity. Both methodologies are recognized by the 

International Panel on Climate Change as acceptable methodologies for reporting greenhouse 

gas emissions inventory information. Assessment of measurement challenges presented by 

both methodologies indicates that focusing on continuous emissions monitoring technologies 

as a means to improve the accuracy of emissions data in the electrical generation sector has the 

highest likelihood to improve such data. In addition, CEMS are recognized as being in the top 

tier of determination methodologies supporting inventory reporting. Since total flow rate of 

stack gases is an essential part of the CEMS measurement, NIST has embarked upon a 

measurement science research program for directly improving the accuracy of stack flow rate 

measurements. 

Should greenhouse gas mitigation efforts begin in the U.S., they will likely be complemented by 

similar efforts in other countries. Greenhouse gas inventory reports will likely become the 

metric by which nations gauge their compliance or contribution to worldwide mitigation 

objectives. Understanding the challenges and sharing technological advances among nations to 
4

improve emissions data from the electric power sector can have positive impact on efforts to 

reduce global emissions.

2. CEMS and RATA Measurements

The session was moderated by Rodney Bryant, NIST, and presentations were provided by the 

following workshop participants: 

Toralf Dietz, Sick Engineering GMbh

Donald Giel, Teledyne Monitor Labs

David Elam Jr., TRC and STAC Environmental Corp. and STAC

Scott Swiggard, Golden Specialty Inc.

The amount of CO2 emitted from a stationary source such as an electric power plant is 

measured by a continuous emissions monitoring systems (CEMS). CEMS are permanently 

installed at the smokestack and measure pollutant concentration (e.g., CO2), and the bulk or 

volumetric flow of gas through the smokestack. The rate of CO2 emissions equals the product 

of the measurements of volume flow rate and CO2 concentration. The EPA requires periodic 

(annual) calibration of a smokestack’s CEMS using a test procedure called a relative accuracy 

test audit (RATA). Presentations for this session provided a general overview of the flow 

components of the CEMS and the flow RATA procedure, as well as presenting some of the 

technical challenges to consider. 

The majority of CEMS flow measurement devices are ultrasonic meters (USM), Figure 2. The 

device measures the time of flight of an acoustic signal along a given path to infer gas velocity. 

USMs are very accurate for ideal flow profiles; however, stack flows are complex and present 

challenges to achieve high accuracy. Two flow related factors that reduce USM accuracy are

cross flow or swirl (i.e., flow not parallel to the axis of the stack), and a non-uniform axial 

velocity profile. Both phenomena are typical in smokestacks and caused by blowers that are 

used to move flue gas toward the smokestack and by bends and obstructions in the conduit 

used to transport the flow into the smokestack. Other important sources of flow uncertainty 

are dimensional uncertainty components and any bias errors introduced by the RATA. 

Dimensional uncertainty sources include measurements of the stack diameter as well as the 

length and angle of the USM acoustic path.
5

Figure 2 Schematic of stack mounted CEMS with a single path ultrasonic meter installed. The flow velocity is determined by 

correlating the times of flight (T1 and T2) of the ultrasonic signals propagating with and against the flow over a measured 

distance (L).

Technical strategies are available to help improve USM measurement errors related to the 

complexity of the flow field and dimensional uncertainty components. For example, a USM can 

be equipped with multiple measurement paths to account for profile skew and/or swirl. Dual 

crossing path USMs (i.e., x-pattern) are already in use and provide partial compensation for 

swirl effects. Empirical and computational evidence shows that additional paths would likely 

improve USM measurement accuracy. In addition, precision dimensional measurements can be 

applied to reduce the uncertainty of acoustic path length and duct diameter. The frequency 

and the path angle of the acoustic transducers can be optimized for the application. A 

promising technique is the use of computational fluid dynamics (CFD) to optimize installations 

and perform a virtual calibration of the USM for the anticipated flow distribution.

L 

T1

T2

Bulk flow

Pollutant

Concentration

USM

CO2 Emissions
6

Figure 3 Schematic of flow RATA traverse points along 2 diametric chords in the cross section of a smokestack. Flow velocities 

are measured at each point with a pitot probe.

All CEMS flow measurement devices must be calibrated with a flow RATA. A flow RATA is a 

detailed measurement of the average gas velocity and hence the volume flow rate in the 

smokestack. The flow RATA uses pitot probes to measure the velocity of the stack gas at 

discrete points across the cross section of the smokestack, Figure 3. The principle of operation 

of the pitot probe is: differential pressure across the pitot ports is correlated to the fluid 

velocity via a calibration factor. Of the three types of pitots, standard L-type, S-type, and threedimensional, the S-type is the most widely used for flow RATAs. The pitot probes are traversed 

along two orthogonal chords and the velocity measurements are used to generate a 

representative sample of the flow distribution. Volumetric flow is determined by averaging the 

point velocity measurements and multiplying by the cross sectional area. The relative accuracy 

is the percent difference between the volumetric flow determined by the RATA with that 

determined by the CEMS.

The flow RATA is the reference standard for the flow measurement; hence the CEMS 

measurement is only as good as the flow RATA. Current flow RATA measurements are not 

rigorously traceable to NIST primary flow standards. As such, the flow RATA cannot determine 

the absolute uncertainty of a CEMS measurement, but instead only provides relative accuracy

between the EPA Method and the CEMS flow monitor. Even if this difference is small, it does 

not guarantee a low uncertainty flow measurement. A CEMS flow monitor that is recurrently 

calibrated against the same type of probe in the same flow conditions could yield consistent 

Equal Area Traverse Points

d1

d2

d3

d....

d....

dN/2-1

dN/2

dN/2+1

dN/2+2

dN

dN-1

d....

d....

Probe Access Port 

Chord 2, θ2 = 135°

Probe Access Port 

Chord 1, θ1 = 45°

θ = 0o

θ = 270o

θ = 90o

θ = 180o

θ
7

good agreement with RATA results, and yet have a significant uncertainty. To quantify the 

absolute uncertainty of CO2 emissions measurements requires establishing rigorous 

metrological traceability.4That is, CO2 emissions must 1) be tied back to NIST primary standards 

via an unbroken chain of measurements; 2) have known and documented uncertainty for each 

step in the chain; and 3) have maintained all of the measurement equipment used in the CEMS 

and the RATA in a quality system that ensures the fidelity of the measurement uncertainties. 

The calibration hierarchy or traceability chain, shown in Figure 4, portrays how CO2 emissions 

measurements must be tied back to NIST primary standards. 

A subtle and often overlooked aspect of satisfying condition 2 includes accounting for 

uncertainties related to differences between the flow conditions during calibration versus 

application. For example, S-probes are generally calibrated in swirl-free conditions, but are 

used in swirling flow conditions. The swirling flow impacts the device's measuring 

performance, but is not accounted for in the uncertainty budget. In these cases metrological 

traceability is not established.

Figure 4 CO2 emissions measured by CEMS (A)

5

, and the corresponding metrological traceability chain to NIST standards (B).

Conducting a flow RATA is a manual process and therefore subject to errors and mistakes. 

Examples of common mistakes that can significantly reduce the accuracy of flow RATAs are: 

improper or infrequent measurements of stack diameter; using damaged probes; not 

accounting for cyclonic flow in the measurement; conducting test during transient or non-

 

4

http://www.nist.gov/pml/mercury_traceability.cfm

5

Image used with permission of the Utah Geological Survey
8

steady conditions; and transcribing the wrong values. Some Air Emission Testing Bodies 

(AETBs), companies that conduct the flow RATAs, are adopting quality management standards 

such as ASTM D7036 [4] in order to reduce these common mistakes and increase the quality of 

their product. Figure 5 provides an overview of ASTM D7036. Steps such as adopting a quality 

management program, introducing redundant measurements for real-time quality checks, or 

introducing automated procedures and data collection have demonstrated improved 

measurement precision and in some cases improvements in customer (i.e. power plant)

performance. However these steps are only reflective of the AETBs that invest in quality 

management accreditation and careful measurements.

Figure 5 Overview of ASTM D7036 - Standard Practice for Competence of Air Emission Testing Bodies

An alternative stack flow measurement technique is the tracer gas dilution method. The 

method is based on the conservation of mass. A known concentration of tracer gas is injected 

in a duct or stack and the airflow is determine by measuring the tracer concentration 

downstream from the injection site to determine the dilution ratio. In situations where 

traditional flow measurements cannot be conducted due to hazards or obstructions, this 

method provides an effective alternative. The tracer dilution method is independent of flow 

conditions such as angle, swirl, or turbulence but is subject to uncertainty due to incomplete 

mixing of the tracer in the stack flow. The measurement does not require a measurement of 

stack diameter, which is typically a significant source of error. 

An Overview of ASTM D7036

Purpose Enables Air Emission Testing Bodies (AETBs) to deliver data of 

defined and documented quality

Applicability For use by firms making emission testing measurements.

Required by USEPA for AETBs performing Part 75 tests.

History First published in 2004. Modeled after ISO 17025.

Current Version 2012, but nomaterial change from 2004 version

Accreditations ≈ 5 Accredited AETBs, 18 Interim Accredited AETBS

Benefits Ability to perform Part 75 work.

Excellent platform for AETBs involved in other ISO 17025-based 

standards.

Consistent, predictable delivery of services.

A management standard without scope limitations.

Criticisms Adoptionexpense, accreditation expense, “won’t change data 

quality,” just a “paper exercise.”
9

3. Pitot Probe Characterization and Calibration

This session was moderated by Aaron Johnson, NIST, and presentations were provided by: 

Woong Kang, Korea Research Institute of Standards and Science (KRISS)

6

Iosif Shinder, NIST.

Eric Harman, Colorado Engineering Experiment Station Inc. (CEESI)

7

and 

Hsin-Hung (Kyle) Lee, Industrial Technology Research Institute (ITRT)

8

Richard Grot, Lagus Applied Technology Inc.

The flow RATA provides the basis of accuracy and traceability of smokestack flows. Accurate 

flow RATA rely on how well a pitot probe can measure the flue gas axial velocity at discrete 

points in the cross section of a smokestack. Currently, most flow RATA are performed without 

using a calibrated S-type probe. Instead the calibration factor is assumed to be 0.84 as long as 

the geometric requirements of the probe are met. However, the accuracy of this assumed 

value is unknown, and stack testers have reported values ranging from 0.72 to 0.84 (Appendix 

D). To what degree does the calibration of an S-type probe depend on velocity? What is the 

pitch and yaw response of various types of pitot probes? Does turbulence affect the 

performance of pitot probes? Research programs have been established to answer these 

questions. Researchers from NIST, KRISS, ITRT, and CEESI identified above described their work, 

their wind speed facilities, and showed calibration data for different types of pitot probes.

NIST’s wind tunnel has a 2 m long test section with a rectangular cross sectional area of 1.2 m 

by 1.5 m, Figure 6. Wind speed is measured using either a calibrated L-type probe installed in 

the tunnel or with a Laser Doppler Velocimetry system. The facility has an uncertainty of 

0.42 % for wind speeds ranging from 6 m/s to 26 m/s (20 ft/s to 85 ft/s). The wind tunnel can 

control turbulence intensity levels from 0.1 % to 20 % by installing a turbulence generator 

upstream of the test section. The NIST wind tunnel has an automated traversing system to 

change the pitch and yaw angles of pitot probes during a calibration. [5] The NIST research 

program includes 

 Understanding the effect of turbulence on pitot probes

 Characterizing the pitch and yaw angle response of pitot probes

 

6

The Korea Research Institute of Standards and Science (KRISS) is the Korean National Metrology Institute.

7

The Colorado Engineering Experimental Station Inc. (CEESI) is a U.S. flow calibration laboratory.

8

The Industrial Technology Research Institute (ITRI) is Taiwan’s National Metrology Institute.
10

 Characterizing the velocity dependence of S-probe and multi-hole pitot probe calibrations

 Probe alignment effects, and

 Intercomparisons with other National Metrology Institutes to demonstrate proficiency and 

support uncertainty claims

Figure 6 NIST wind tunnel used for calibration of S-type, L-type, and 3D pitot probes.

NIST presented preliminary data showing 1) the velocity dependence of the S-probe calibration 

factor differing from the assumed value of 0.84, 2) pitch and yaw response of an S-probe, and 

3) the effect of turbulence on a five hole three-dimensional pitot probe. [5, 6] Presentations 

made by KRISS showed similar results for S-probe velocity dependence and pitch and yaw 

response. [7] ITRI showed progress toward development of a fully automated wind tunnel to 

calibrate pitot probes as a function of pitch, yaw, and velocity. [8] Finally, CEESI presented an 

alternate methodology (other than using a wind tunnel) to calibrate pitot probes. [9]

4. NIST Facilities and Flow Measurement Research

Research advances at the NIST facilities were presented by the following researchers during this 

session which was moderated by James Whetstone, NIST. 

Aaron Johnson, NIST Physical Measurements Laboratory

Liang Zhang, National Institute of Metrology, China

Rodney Bryant, NIST Engineering Laboratory

Keith Gillis. NIST Physical Measurements Laboratory 
11

Current flow measurement accuracy is based on the RATA, which is not necessarily traceable to 

the derived SI9unit for flow through NIST standards. NIST is working toward establishing 

internationally recognized and highly accurate measurement standards to provide an SI 

traceable basis to quantify the accuracy of CO2 emissions measurements. These standards will 

enable the owners of stationary sources and the regulatory agencies for these sources to 

improve the accuracy of CO2 emissions. Better measurements are needed so that wellcharacterized scientific data can be used as the basis to determine if local, regional, and global 

emission targets are being met. NIST programs are focused on quantifying CO2 emissions from 

fossil-fuel-burning power plants due to their significant carbon footprint.

Figure 7 . Schematic of NIST Scale-Model Smokestack Simulator (SMSS). A calibrated flow meter in the reference section is used 

to assess both the EPA RATA and a CEMS flow meter installed in the 1.2 m diameter test section. The sharp corner introduces 

swirling, asymmetric flow in the test section that is similar to the complex flow conditions in an industrial-scale smokestack.

(http://www.nist.gov/pml/div685/grp02/scale-model-smokestack.cfm)

NIST has designed and built two independent reference facilities, the Scale-Model Smokestack 

Simulator (SMSS), Figure 7, and the National Fire Research Laboratory (NFRL), Figure 9. The 

SMSS will be used to quantify the flow uncertainty of the EPA RATA and CEMS measurements; 

and the NFRL will be used as a near industrial-scale test bed to evaluate the overall 

performance of CEMS and RATA methods. The SMSS has the capability to independently vary 

many of the parameters (e.g., number of diametric chords, number of points on each chord, 

levels of swirl in the velocity field, the axial velocity magnitude and profile) affecting the 

accuracy of the flow RATA and flow CEMS such as ultrasonic flow meters. [10] Recently

completed computational fluid dynamics modeling indicates that such meters may be able to 

provide low-uncertainty flow measurements. 

 

9

SI is an abbreviation for the international System of Units
12

Figure 8 SMSS air intake unit, reference section, test section, and air exhaust

The SMSS is essentially a 1/10th scale horizontal smokestack that uses air as a surrogate for flue 

gas. The test section and reference section of the SMSS facility are housed indoors while the air 

intake and air exhaust sections are outside. Ambient air is drawn into the air intake unit by 2 

fans at the facility exit depicted in the Figure 8. The air enters the facility at relatively low 

speeds (2 m/s to 8.5 m/s) and is accelerated to air speeds ranging from 11 m/s to 46 m/s in the 

0.9 m diameter reference section, and subsequently to speeds of 6 m/s to 26 m/s in the 1.2 m 

diameter test section.

The SMSS is designed to produce nearly ideal flow conditions (i.e., a symmetric velocity profile 

with negligible swirl) in the reference section. This well-conditioned flow is measured using an 

8 path ultrasonic flow meter (USM) with an expanded uncertainty of 0.5 % at a 95 % confidence 

level. Flow distortions are introduced in the Test Section by the sharp 90 degree corner. This 

geometry is typical of many industrial smokestacks and results in swirling, asymmetric flow in 

the SMSS test section. Other velocity profiles can be generated at the test section by using flow 

“de-conditioning” plates upstream. If there are no leaks in the connecting volume between the 

reference section and the test section and the flow is steady, the mass flow measured in the 
13

reference section can be used to assess the performance of different flow meter technologies 

(e.g., CEMS and RATA) installed in the test section.

Figure 9 The National Fire Research Laboratory (NFRL). CO2 emissions are predicted using fuel 

consumption measurements for natural gas. The predicted CO2 emissions are used to quantify 

the accuracy of CEMS and RATA measurements in the rooftop exhaust ducts.

(http://www.nist.gov/el/fire_research/nfrl.cfm) 

The NFRL is used for the study of full-scale fires in buildings. During the routine fire 

experiments conducted in the facility, the flow and concentration of effluents in the exhaust 

duct are measured, much like CEMS measurements at the smoke stack of a stationary source. 

Therefore the NFRL will be used to simulate some of the operating conditions of a fossil-fuel 

burning power plant. Research results from the SMSS for improving flow RATA accuracy will be 

applied in the NFRL and therefore under more realistic conditions. 

The NFRL has the capability of deriving CO2 emissions from fuel consumption measurements 

while simultaneously measuring CO2 emissions using its exhaust duct CEMS. It has two natural 

gas burners that can operate at heat release rates up to 8 MW and 20 MW. Measurements of 

volume flow rate, pressure, temperature, and gas composition are made in the natural gas 

delivery system just upstream of the burners, making it possible to compute the amount of CO2

generated by the fire, Figure 10. Large canopy exhaust hoods capture the combustion products 

from the natural gas fires and direct the flow into the exhaust ducts that run along the roof of 

the facility. The maximum exhaust flow capacity is approximately 100 kg/s of air. The exhaust 

ducts are instrumented to measure gas temperature, gas velocity, and gas volume fraction of 

selected combustion products, including CO2. From these measurements the direct emissions 

of CO2 are derived as in a CEMS measurement, Figure 10. These two measurements, fuel 
14

calculation and flue gas CEMS, are independent and therefore provide a method to check 

measurement accuracy. [11, 12] 

Figure 10 Schematic of the process comparing the fuel consumption and CEMS measurements for accuracy.

These NIST facilities will be used to study new methods of accurately measuring smokestack 

flows. Efforts will focus on assessing the accuracy of multipath ultrasonic flow meters and 

researching the potential of long wavelength acoustic flow meters (LWAMs). [13] Single path 

and dual crossing path ultrasonic flow meter designs are already widely used as CEMS flow 

meters. However, these designs do not correct for profile effects and offer only limited 

compensation for swirl. In contrast, multi-path designs would effectively compensate for both 

swirl and profile effects. Preliminary CFD models predict that multi-path ultrasonic flow meters 

would significantly reduce errors over the currently used single and dual path designs. [14]

Predicted CO2 = Direct CO2

(Fuel Consumption = CEMS) 
15

Both facilities will be used to assess the accuracy of multipath ultrasonic flow meter designs in 

asymmetric swirling smokestack flows.

Figure 11 NIST 1/100th scale Long Wavelength Acoustic Flow Meter (LWAM). Results show good agreement between LWAF 

and a NIST traceable flow standard (better than 1 %) over flows ranging from 1 to 24 m/s for a variety of non-ideal flow 

conditions.

Preliminary research is underway to assess the effectiveness of a long wavelength acoustic flow 

meter (LWAF) as an alternative method of measuring the flue gas flow rate. Existing single path 

and dual path ultrasonic flow meters are subject to installation effects that only measure the 

velocity over a small fraction of the smokestack cross section. In particular, the short millimeter 

wavelengths used in ultrasonic flow meter technologies measure the average velocity along 

narrow beams across the smokestack cross section. If the velocity measured along the acoustic 

path or length of the sound beam is not indicative of the overall flow field, then results will 

include sampling errors. Increased sampling via multi-path ultrasonic flow meter designs is one 

method to reduce such sampling errors. The LWAF offers an alternative approach. In this case, 

the frequency of the sound wave is such that the wavelength is larger than the duct diameter. 

As a result, wave fronts distorted by the complex velocity field approach a plane wave that 

averages over the flow distortions. In this way, a LWAF inherently accounts for the flow 

distortions present in smokestack flows. Preliminary results for a 1/100th scale LWAF show 

accuracy better than 1 % in highly asymmetric and swirling flow fields, Figure 11. [13, 15] To 

verify the scalability of the method, LWAFs will be tested in NIST’s 1/10th SMSS.
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5. Closing Remarks 

The workshop convened experts at a time when attention to atmospheric conditions and 

demand for accurate measurement testing were on the rise. It anticipated the timely need to 

foster collaborative intervention by involving the broad stack testing community in dialog, to 

identify critical issues, explore differing perspectives, and assess best practices. Additionally, it 

provided a unique opportunity for NIST to share its research interests and progress with a 

diverse group of stakeholders in the electric power industry. Conversely, feedback from 

workshop participants enabled better understanding of current CO2 emission measurement

practices and sources of uncertainty. Finally, the workshop enabled NIST to make connections 

with both U.S. and international stakeholders in the electric power industry. These connections 

will be useful for continued feedback of NIST research programs and in realizing NIST long-term 

objectives which include 1) improving GHG measurement standards and the accuracy of GHG 

measurements, 2) promoting these measurements and consensus standards internationally, 

3) transferring measurement technologies and best practices to other government agencies, 

industrial organizations, and to the private sector.
17

6. References 

[1] EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013, (2015). 

[2] K. V. Ackerman and E. T. Sundquist, Comparison of two US power-plant carbon dioxide 

emissions data sets, Environmental Science & Technology. 42 (15), 5688-5693 (2008). 

[3] J. C. Quick, Carbon dioxide emission tallies for 210 U.S. coal-fired power plants: A 

comparison of two accounting methods, Journal of the Air & Waste Management Association. 

64 (1), 73-79 (2014). 

[4] A. International, Standard Practice for Competence of Air Emission Testing Bodies, Ambient 

Atmospheres and Source Emissions. (D7036-12), (2012). 

[5] I. I. Shinder, V. B. Chromchenko and M. R. Moldover, NIST’s New 3D Airspeed Calibration 

Rig, Addresses Turbulent Flow Measurement Challenges, 9th International Symposium on Fluid 

Flow Measurement, Arlington, VA, 2015.

[6] C. Crowley, Shinder, II and M. R. Moldover, The effect of turbulence on a multi-hole Pitot 

calibration, Flow Measurement and Instrumentation. 33 106-109 (2013). 

10.1016/j.flowmeasinst.2013.05.007

[7] W. Kang, Y. M. Choi, N. D. Trang and H. S. Jang, Uncertainty Analysis of Stack Gas Flowrate 

Measurement with S-Type Pitot Tube for Estimating Greenhouse Gases Emission, 9th 

International Symposium on Fluid Flow Measurement, Arlington, VA, 2015.

[8] H. H. Lee and J. Y. Chen, Automated 3D Traverse System Design and Pitot Tubes Calibration 

in the Wind Tunnel, 9th International Symposium on Fluid Flow Measurement, Arlington, VA, 

2015.

[9] E. Harman and W. Johansen, Alternate Calibration Methodology for Point-Velocity Devices 

(Pitot-Tubes, Anemometers, Hot-Wire Probes), 9th International Symposium on Fluid Flow 

Measurement, Arlington, VA, 2015.

[10] A. N. Johnson, J. T. Boyd, E. Harman, M. Khalil, J. Ricker, C. Crowley, R. Bryant and I. I. 

Shinder, Design and Capabilities of NIST's Scale-Model Smokestack Simulator (SMSS), 9th 

International Symposium on Fluid Flow Measurement, Arlington, VA, 2015.

[11] R. Bryant, M. Bundy and R. W. Zong, Evaluating measurements of carbon dioxide emissions 

using a precision sourceA natural gas burner, Journal of the Air & Waste Management 

Association. 65 (7), 863-870 (2015). 10.1080/10962247.2015.1031294

[12] R. Bryant, O. Sanni, E. Moore, M. Bundy and A. Johnson, An uncertainty analysis of mean 

flow velocity measurements used to quantify emissions from stationary sources, Journal of the 

Air & Waste Management Association. 64 (6), 679-689 (2014). 
18

[13] L. J. Gorny, K. A. Gillis and M. R. Moldover, Testing Long-Wavelength Acoustic Flowmeter 

Concepts for Flue Gas Flows, 8th International Symposium on Fluid Flow Measurement, 

Colorado Springs, CO, 2012.

[14] L. Zhang, Performance Evaluation of Ultrasonic Flow Meters in NISTs Smokestack 

Simulator, 9th International Symposium on Fluid Flow Measurement, Arlington, VA, 2015.

[15] L. J. Gorny, K. A. Gillis and M. R. Moldover, Calibration of a Long-Wavelength Acoustic 

Flowmeter Using a Lumped Impedance Acoustic Model, NOISE-CON 2013, Denver, CO, 2013.
19

7. Appendices 

7.1 Appendix A: Workshop Agenda 

Measurement Challenges and Metrology for Monitoring CO2 Emissions from Smokestacks

April 20 – 21, 2015

NIST, Gaithersburg, Maryland, Building 215/C103

April 20, 2015

8:45am Registration

9:00am Welcome / Introductions

9:15am James Whetstone (NIST) Overview of the NIST Green House Gas and 

Climate Science Measurements Program

CEMS and RATA Measurements – Rodney Bryant, Session Moderator

9:45am Toralf Dietz (Sick Engineering) Improving the Accuracy of CEMS by Means of 

Multipath Ultrasonic Flowmeter

10:15am Break

10:30am Donald Giel (Teledyne) Practical Experience with CEMS Measurements

11:00am David Elam Jr. (TRC and STAC) Overview of ASTM D7036: A Quality 

Management Standard for Emission Testing

11:30am Scott Swiggard (Golden Specialty, 

Inc.) 

Volumetric Flow Measurements of Stationary 

Sources: Common Mistakes, Corrective Measures

Pitot Probe Characterization and Calibration – Aaron Johnson, Session Moderator

1:45pm Woong Kang (KRISS) Experimental and Numerical Investigation of the 

Factors Affecting the S-type Pitot Tube 

Coefficients in GHGs Monitoring

2:15pm Iosif Shinder (NIST) NIST’s New 3D Airspeed Calibration Rig,

Turbulent Flow Measurement Challenges

2:45pm Break

3:00pm Eric Harman (CEESI) Alternate Pitot-Tube Calibration Methodology 

Using NIST Traceable Mass Flow Standards

3:30pm Hsin-Hung (Kyle) Lee (ITRI) 3D Pitot Tube Measurements and Calibration in 

the Wind Tunnel
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4:00pm Richard Grot (LAGUS) Stack Duct Flow Measurements – Tracer Gas 

Method, ASTM E2029

4:30pm Discussions

5:00pm Adjourn

April 21, 2015

NIST Facilities and Flow Measurement Research – James Whetstone, Session Moderator

9:00am Aaron Johnson (NIST) Scale-Model Smokestack Simulator (SMSS) – A 

Facility to Study the Uncertainty of CEMS and 

RATA Flow Measurements

9:30am Liang Zhang (NIM China) Performance Evaluation of Ultrasonic Flow 

Meters in NIST’s Smokestack Simulator

10:00am Rodney Bryant (NIST) Using the National Fire Research Laboratory 

(NFRL) as a Test Bed for Traceable CO2

Measurements

10:30am Break

10:45am Keith Gillis (NIST) Is a Long-Wavelength Acoustic Flow-meter 

Feasible for Smokestacks? 

11:15am Discussions

12:00pm Lunch

NIST Laboratory Tours

1:30 – 3:45pm Iosif Shinder NIST Wind Tunnel

Rodney Bryant National Fire Research Laboratory 

Aaron Johnson Scale-Model Smokestack Simulator 

4:00pm Break

4:15pm Discussions

5:00pm Adjourn
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7.2 Appendix B: List of Attendees 

Name Company Phone Email

Rodney Bryant NIST (301)-975-6487 rodney.bryant@nist.gov

Yong Moon Choi

Korea Research 

Institute Of Standards 

and Science

+82 42 868 5317 ymchoi@kriss.re.kr

Toralf Dietz Sick Engineering 

Gmbh +49 172 3514326 toralf.dietz@sick.de

Chris Dimopoulos National Physical 

Laboratory (UK) +44 20 8943 6127 Chris.dimopoulos@npl.co.uk

David L Elam Jr TRC Environmental 

Corporation (919) 622-1846 delam@trcsolutions.com

Robert Finken Montrose Air Quality 

Services, Inc. (714) 448-6150 bfinken@montrose-env.com

Gerald Fraser NIST (301) 975-3797 Gerald.fraser@nist.gov

Donald Giel Teledyne Monitor 

Labs, Inc. (412) 897-6457 dgiel@teledyne.com

Keith Gillis NIST (301) 975-2468 keith.gillis@nist.gov

Bobbi Greenwalt Minnesota Power (218) 313-4376 bgreenwalt@mnpower.com

Richard Grot Lagus Applied 

Technology (301) 529-3845 ragrot@tracergas.com

Eric Harman

Colorado Engineering 

Experiment Station, 

Inc.

(303) 638-1384 eharman@ceesi.com

Heming Hu National Institute Of 

Metrology, China +86 10 6452 5126 huheming@gmail.com

Jenny Jachim US EPA (202) 343-9590 Jachim.jenny@epa.gov

Aaron Johnson NIST (301)975-5954 aaron.johnson@nist.gov

Woong Kang

Korea Research 

Institute of Standards 

and Science

+82 42 8685319 woong.kang@kriss.re.kr

Gina Kline NIST (301) 975-4813 Gina.kline@nist.gov

Ching-Yi Kuo

Industrial Technology 

Research Institute

(Taiwan)

+886 3 5741206 ellykuo@itri.org.tw

Jordan Laster Alliance Source 

Testing, LLC. (610) 500-3615 jordan.laster@stacktest.com

Hsin-Hung (Kyle) Lee

Industrial Technology 

Research Institute

(Taiwan)

+886 3 5741221 kylelee@itri.org.tw

Thomas Martz Electric Power 

Research Institute (949) 395-9647 tmartz@epri.com

Michael Moldover NIST (301) 975-2459 michael.moldover@nist.gov

David Nuckols Dominion Generation (804) 796-6021 david.nuckols@dom.com

Jeremy Schreifels US EPA (202) 343-9127 Schreifels.jeremy@epa.gov

David Seidl

Colorado Engineering 

Experiment Station, 

Inc.

(970) 897-2711 dseidl@ceesi.com
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Iosif Shinder NIST (301) 975-5943 iosif.shinder@nist.gov

Joe Simeonsson Environmental Supply 

Company, Inc. (800) 782-2575 joesimeonsson@gmail.com

Hratch Semerjian NIST (301) 975-8110 Hratch.smerjian@nist.gov

Lauren Smith

American Association 

for Laboratory 

Accreditation

(240) 575-7482 lsmith@a2la.org

Tate Strickler Montrose Air Quality 

Services, Inc. tstrickler@montrose-env.com

Chun-Min Su

Industrial Technology 

Research Institute

(Taiwan)

+886-3-5741205 CMSu@itri.org.tw

Lei Sui General Electric (978) 437-7576 lei.sui@ge.com

Scott Swiggard Golden Specialty, Inc. (713) 306-2208 sswiggard@goldenspecialty.com

James Whetstone NIST (301) 975-2738 james.whetstone@nist.gov

Tamae Wong NIST (301) 975-5415 tamae.wong@nist.gov

Liang Zhang National Institute Of 

Metrology China +86 1064525126 zhangliang@nim.ac.cn
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7.3Appendix C: Workshop Presentations 

7.3.1 NIST Greenhouse Gas and Climate Science Measurements Program

James Whetstone (NIST)

The NIST Greenhouse Gas and Climate Science Measurements Program was initiated to improve the current measurement and 

standards infrastructure for accurate measurement of greenhouse gases. Motivation for addressing CO2 emissions monitoring from 

smokestacks was generated by the significant contribution of CO2 emissions from stationary sources such as electric power plants. 

Continuous emissions monitoring systems (CEMS) are recognized as the top tier methodology for determining and supporting the 

CO2 emissions inventory. Since total flow rate of stack gases is an essential part of the CEMS measurement, NIST has embarked 

upon a measurement science research program for directly improving the accuracy of stack flow rate measurements.
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7.3.2 Improving the Accuracy of CEMS by Means of Multipath Ultrasonic Flowmeter

Toralf Dietz (Sick Engineering Gmbh)

Session: CEMS and RATA Measurements

This talk presents the application of multipath ultrasonic flowmeters at a coal fired power plant. Specific topics discussed are: the 

uncertainty analysis of the flow meter installation; the use of Computational Fluid Dynamics (CFD) as basis for an optimized 

alignment of the measuring paths; improved installation and calibration procedures; and verification of results.
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7.3.3 Practical Experience with CEMS Measurements 

Donald Giel (Teledyne)

Session: CEMS and RATA Measurements

An overview of the practical application of ultrasonic flow meters for CEMS flow measurements is presented. Knowledge from 

practical experience on the challenges associated with making reliable, believable, and continuous flow measurements utilizing 

ultrasonic flow technology in large utility smokestack environments will be shared. Discussions will address instrument design 

features to maximize system uptime while minimizing the effects of the process conditions on the hardware.
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7.3.4 Overview of ASTM D7036: A Quality Management Standard for Emission Testing

David Elam (TRC Environmental Corporation and Stack Testing Accreditation Council)

Session: CEMS and RATA Measurements

Emission testing remains one of the most challenging environmental measurement disciplines. A successful emission test program 

requires a solid understanding of test program objectives and the proper combination of test methodology, expertise, process 

operations, and often, regulatory agency coordination. Because there are so many variables and potential sources of error, many 

emission testing programs are not properly completed or are completed at significant - and often unanticipated - expense. The good 

news is that the challenges associated with emission testing can be effectively managed with proper program design, planning,

coordination, and implementation.

ASTM D7036, Standard Practice for Competence of Air Emission Testing Bodies, is a quality management standard specifically 

designed for emission testing. ASTM D7036 provides the structure under which an Air Emission Testing Body (AETB) can deploy a 

process-based management system that addresses the challenges of emission testing. This presentation provides background 

information on process-based management systems, introduces ASTM D7036 with an emphasis on its application to process-based 

management, and highlight the relevance of ASTM D7036 to greenhouse gas measurement programs.
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7.3.5 Volumetric Flow Measurements of Stationary Sources: Common Mistakes, Corrective Measures

Scott Swiggard (Golden Specialty)

Session: CEMS and RATA Measurements

In stationary source testing accurate flow measurement is of very high importance. Inaccurate flow measurements lead to under or 

over reporting emissions, both bad for different reasons. In addition, performance of CEMS and CERMS (Continuous Emission Rate 

Monitoring System) is impacted by performance when either the Reference Method (Stack Tester) or the CERMS is reporting 

inaccurate data. The discussion will cover some common flow measurement mistakes, corrective actions, and other technical 

hurdles encountered.
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7.3.6 Stack/Duct Flow Measurements – Tracer Gas Method ASTM E2029 

Richard Grot (Lagus Applied Technology, Inc.)

Session: CEMS and RATA Measurements

The presentation discusses an alternative stack flow measurement technique, namely the tracer gas dilution method, which can be 

used to measure the mass flow in stacks and pipes. The underlying concept of the method is to release a well-characterized tracer 

gas in a duct or stack and to determine its airflow by measuring concentration across a cross section downstream from the injection 

site. In situations where traditional flow measurements cannot be conducted, due to hazards or obstructions, this method provides 

excellent measurements and serves as an effective compliance technique. In most situations it will provide a measurement with

greater accuracy than other methods. The method is based on first principles (conservation of mass) and significant advantages are 

gained by use of this method over the more traditional Pitot tube Transverse Method. The tracer dilution method is independent of 

flow conditions such as angle, swirl, or turbulence. The measurement does not require gas composition nor density of measured 

flow stream. Furthermore, at standard conditions, the method does not require temperature, pressure, humidity, nor stack 

diameter which is typically a significant sources of error.
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7.3.7 3D Pitot Tube Measurements and Calibration in the Wind Tunnel

Kyle Lee (ITRI, Taiwan)

Session: Pitot Probe Characterization and Calibration

Greenhouse gas emissions have been regarded as a global challenge and it is even more serious in the Asia Pacific region. 

Smokestack emissions are one of the main pollution sources and its flow measurements draw much attention due to the unstable 

flow conditions and complex gas composition. Pitot tubes have been widely used for flow measurements in the environmental 

analysis. However, the traditional pitot tubes (L type or S type) can only provide one-dimensional flow velocity and the 

measurement locations require considerable care. The U.S. EPA already announced that 3D pitot tubes (prism type, spherical type) 

can be used for three-dimensional swirl flow measurements in the smokestack and could provide more detailed flow information. 

Nevertheless, the calibration facility and procedures still need more studies in order to fulfill the standard traceability and 

uncertainty evaluation. Therefore, the Center for Measurement Standards started to design an automated 3D traverse system last 

year and installed it in the wind tunnel for pitot tubes calibration. After testing, the design has been proved to be feasible to operate 

in the test section of wind tunnel. The calibration can be performed with different angles in the air speed calibration system with 

relative expanded uncertainty of 0.5 %. The pitch and yaw angle range from -40 degrees to +40 degrees and -180 degrees to +180 

degrees, respectively.
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7.3.8 Experimental and Numerical Investigations of the Factors Affecting the S-type Pitot Tube Coefficients in GHG Emission 

Monitoring

Woong Kang (KRISS, Korea)

Session: Pitot Probe Characterization and Calibration

In greenhouse gas emission monitoring from industrial stacks, the most common device used to measure stack gas velocity is the Stype Pitot tube. Various factors such as the Reynolds number and misalignment of the installation angle can be additional error 

sources for the S-type Pitot tube coefficients due to harsh environments. Manufacturing quality of the S-type Pitot tube is also a 

factor affecting the measurement uncertainty of stack gas velocity. In the present study, wind tunnel experiments were conducted 

in the KRISS (Korea Research Institute of Standards and Science) standard air speed system to examine the effects of various factors 

on the S-type Pitot tube coefficients. Numerical simulations were also used to understand flow phenomena around the S-type Pitot 

tube in the presence of misalignment and distortion of the geometry.
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7.3.9 Greenhouse Gas Emissions – A Case Study on the Calibration of an L-pitot Static Tube

Eric Harman (CEESI)

Session: Pitot Probe Characterization and Calibration

An alternate methodology for calibrating Pitot tubes, Anemometers, Hot-Wire Probes, and other Point-Velocity Devices is described 

utilizing NIST Traceable Mass Flow Measurement Standards, velocity profile conditioning, velocity profile mapping and normalization 

techniques. This methodology was used to determine three Pitot-static flow coefficients. The resulting average of the three 

experimentally determined flow coefficients was within 0.4% of a theoretically calculated flow coefficient. An uncertainty analysis of 

the experimentally determined flow coefficients produced an estimated uncertainty of 0.62% at one sigma.
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7.3.10 NIST’s New 3D Airspeed Calibration Rig, Turbulent Flow Measurement Challenges

Iosif Shinder (NIST)

Session: Pitot Probe Characterization and Calibration

The presentation summarizes the NIST 3-D airspeed calibration rig and sensor calibrations in turbulent flow. Specific topics of 

discussion are: how to simulate high intensity turbulence in a wind tunnel; how to measure it; the effect of turbulence on 2-D and 3-

D airspeed differential sensors; and challenges and future research in 2-D and 3-D airspeed measurements.
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7.3.11 Scale-Model Smokestack Simulator (SMSS) – A Facility to Study the Uncertainty of CEMS and RATA Flow Measurements

Aaron Johnson (NIST)

Session: NIST Facilities and Flow Measurement Research

The amount of CO2 emitted from a coal-fired power plant (CFPP) is measured by continuous emissions monitoring systems (CEMS) 

permanently installed in the exhaust smokestack. Both the CO2 concentration and the bulk flow are continuously measured by 

CEMS, and the product of these measurements gives the CO2 flux. The EPA requires CEMS to be calibrated yearly using a test 

procedure called a relative accuracy test audit (RATA). This calibration procedure links the concentration measurement to the SI 

through reference gas standards. However, establishing flow traceability is more difficult because the CEMS flow meter and the 

flow meter used to perform the RATA can be adversely affected by the complex velocity fields (i.e., swirling flow with a skewed 

velocity profile) prevalent in smokestacks. As a result the RATA only provides “relative accuracy” instead of flow traceability to a 

primary standard. In order to quantify the uncertainty of smokestack flow measurements, and to establish a calibration platform 

with documented traceability to the derived SI unit of flow, NIST constructed a 1/10th scale model smokestack simulator (SMSS). The 

test section of the SMSS will have the same velocity range and similar flow distortions found in to a typical CFPP smokestack. 

However, the SMSS will provide reference flow measurements at expanded uncertainties of less than 1 %. This presentation

discusses the design and capabilities of the SMSS and presents Computational Fluid Dynamics (CFD) results of the expected velocity 

field in the SMSS facility.
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7.3.12 Performance Evaluation of Ultrasonic Flow Meters in NIST’s Smokestack

Liang Zhang (National Institute of Metrology China)

Session: NIST Facilities and Flow Measurement Research

Accurate flow measurements are necessary to quantify the level of hazardous emissions from the smokestacks of fossil fuel burning 

power plants. Typical smokestack flow measurements are made using an ultrasonic flow meter (USM) with either a single diametric 

path or a symmetrically oriented dual path configuration. Due to the size of smokestacks the flow performance of USMs has not 

been quantified at the industrial scale. NIST designed and built a Scale-Model Smoke-stack Simulator (SMSS) that is 1/10th the 

industrial size to use as a test bed for research purposes. In this study, we use CFD to simulate the flow field in the SMSS. The results 

show that the SMSS generates asymmetric, swirling flows typical of industrial smokestacks. The computed flow field is subsequently 

used to predict the performance and characterize the measurement error of a single path, dual path, and variety of multi-path USMs 

as a function of the installation position and orientation, the path configuration, and the integration method. Measurement errors 

are categorized into integration error, transverse flow error, and axial flow error. The computational results predict that even using 

the same number of path, mid-radius USM have better performance than the diametric path USM, an 8 path USM using the OWICS 

integration scheme can determine the flow to better than 1% when the distance between the flowmeter and the T corner is above

3D.
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7.3.13 Using the National Fire Research Laboratory (NFRL) as a Test Bed for Traceable CO2 Measurements

Rodney Bryant (NIST)

Session: NIST Facilities and Flow Measurement Research

The newly built National Fire Research Laboratory (NFRL) is developing capabilities to assess and reduce GHG emissions 

measurement uncertainty. The facility has two major capabilities: 1) precisely generating natural gas fires up to 20 megawatts and 

2) conducting accurate measurements of gas emissions from its exhaust system. The NFRL employs well-controlled and wellcharacterized natural gas burners as a quantitative source of CO2. These burners provide a tool for assessing CO2 emissions derived 

from fuel consumption measurements. The exhaust ducts of the NFRL are instrumented to continuously measure velocity, pressure, 

temperature, and composition of the flue gas, similar to a power plant continuous emissions monitoring system (CEMS). From these 

measurements CO2 emissions are derived. The NFRL also has the capability to conduct flow RATAs for its exhaust ducts. By having 

well-characterized CO2 emission sources, CEMS, and flow RATA measurements, the NFRL can perform cross checks of CO2 emissions 

measurements by confirming a CO2 mass balance for the facility. Therefore the facility is capable of evaluating current CEMS and 

RATA measurement methods under conditions similar to a real power plant as well as providing a test bed for developing and 

evaluating new methods.
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7.3.14 Is a Long-Wavelength Acoustic Flowmeter Feasible for Smokestacks?

Keith Gillis (NIST)

Session: NIST Facilities and Flow Measurement Research

Conventional gas flow measurements conducted in large ducts (such as the smokestack of a coal-burning power plant) have 

uncertainties of 5 % to 20 %. Consequently, the quantity of pollutants (sulfur dioxide, mercury, nitrogen oxides, and carbon dioxide) 

emitted by such ducts have equally large uncertainties. As part of NIST’s Greenhouse Gas and Climate Science Measurements 

Program, we are testing long-wavelength acoustic flow meters (LWAFs) to reduce this uncertainty. LWAFs measure the average 

volume flow of flue gases in smoke stacks. The measured volume flow can be combined with measurements of the flue gas’s 

pressure, temperature, and composition to determine the mass of pollutants emitted by the stack.

To test LWAFs, we constructed a 1:100 scale model (10 cm diameter) test facility equipped with a variable-speed fan. The model 

LWAF determined the speed of sound in ambient air with a standard uncertainty of 0.01 %. Within this uncertainty, the speed of 

sound agrees with the value calculated from NIST’s REFPROP database.

The same LWAF determined the average flow velocity. It agreed, within ±1 %, with the velocity determined from a NIST-calibrated 

flow standard upstream from the LWAF. This good agreement was maintained for flows up to 25 m/s, even after various swirlinducing bends were inserted between the flow standard and the LWAF. Similar uncertainties were obtained with highly distorted 

flows generated by placing obstructions upstream of the LWAF.

A preliminary test of the scalability of the technique using a 20 cm diameter LWAF with the same variable-speed fan and flow 

reference gave similar uncertainties in the measured flow up to 6 m/s (limited by the fan). To further verify the scalability of the 

method, we will test LWAFs in NIST’s 1:10 Scale-Model Smokestack Simulator (SMSS).
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7.4 Appendix D: Stationary Source Sampling & Analysis for Air Pollutants (SSSAAP) 

Conference 

38th Annual SSSAAP Conference (March 6, 2014, Point Clear, Alabama)

Stack Gas Velocity Measurements Session

Co Chairs: Rodney Bryant and Aaron Johnson, NIST

Abstract

Emission rates of regulated pollutants are determined by multiplying the measured pollutant 

concentration by the total measured flow in a stack. Large uncertainties in either concentration 

or flow measurements result in large uncertainties in reported emissions. Hence, accurate flow 

measurements are critical for quantifying pollutant emissions from the smokestacks of fossil 

fuel burning sources. The National Institute of Standards and Technology (NIST) is addressing

the need for accurate air pollutant measurements by constructing a new facility - the Scale 

Model Smokestack Simulator (SMSS), and by expanding an existing facility - the National Fire 

Research Laboratory (NFRL). The objectives of this breakout session include:

1) present capabilities and opportunities of the NIST facilities, 

2) discuss use of SMSS for establishing flow traceability and assessing the accuracy of pitot 

traverse methods with RATA testing, 

3) enable use of NFRL as a test bed for accurate determination of carbon dioxide emissions, 

4) identify issues surrounding velocity probe calibrations (standard pitot, S-probe, and 3D) and 

performance under different flow profile regimes, and 

5) facilitate discussion of industry’s perspective on flow measurement challenges and 

improvements needed to achieve higher levels of accuracy. 

Anticipated outcomes of this session were to establish a continuing dialogue regarding the real 

world issues of stack gas velocity measurements and flow traceability, and to identify 

continuing stakeholders’ concerns with improved stack gas velocity measurements.

Summary

Members of the stack testing industry, along with staff from the U.S. Environmental Protection 

Agency (EPA) and NIST gathered to discuss the topic of stack gas velocity and flow 

measurement methodologies. The session was well attended with 40 to 50 participants. NIST 

began the session by providing a brief overview of its Greenhouse Gas and Climate Science 
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Measurements Program and descriptions of its flow measurements research and instrument 

evaluation efforts. The long term objective of this program is to enhance measurement 

accuracy for greenhouse gases in the U.S. by:

 improving the accuracy of stack flow measurements,

 providing standards as needed by the industry, and 

 transferring improved measurement technologies to other government agencies and 

the private sector.

The rationale for this measurement science research is the need for higher accuracy 

measurement capabilities for improved greenhouse gas mitigation efforts in the U.S. and with 

other countries.

NIST provided descriptions of three major facilities, the Scale Model Smokestack Simulator 

(SMSS), the Wind Tunnel Facility, and the National Fire Research Laboratory (NFRL). SMSS is a 

facility capable of evaluating the performance of flow RATA measurement instruments and 

methods, and flow CEMS devices. The SMSS can simulate flow conditions potentially impacting 

stack gas velocity measurement accuracy and quantify the impact of these conditions on 

measurement performance. Next, they presented an overview of NIST’s low turbulence wind 

tunnel facilities, with capabilities to calibrate air speed measurement devices. They provided 

recent results from an investigation of the effect of yaw angle on S-probe calibration accuracy. 

The NFRL is a reduced-scale analog of a stationary emissions source, and its capabilities to 

precisely generate and measure carbon dioxide were explained. As such, it is a test bed for 

accurate greenhouse gas measurements. Further details on these facilities are available from 

<www.nist.gov>.

Stack-testing industry representatives raised issues impacting their field, and provided real 

world perspectives on why stack gas velocity measurements are challenging. These difficulties

covered a range of issues, some inherent to the equipment and testing methodologies that 

should be addressed. Industry feedback pointed to the following measurement issues:

 S-probe calibration coefficients variability– anywhere from 0.72 to 0.84;

 accurate determination of stack cross sectional area;

 site conditions such as weather, vibrations, particulates, etc.;

 operator skill. 

Industry feedback also generated the following suggestions for NIST. They called for the need 

to consider the impact of environmental and operational conditions, such as vibrations, 

sampling locations, weather, particulates, and others, on measurement accuracy. They raised 

the need to reduce blockage effects for probe calibrations, and to account for turbulence and 

high temperature (viscosity) representative of real stack flows during probe calibrations.

Participants noted the importance of continued discussions. The EPA, in particular, welcomed 

NIST’s involvement to address important measurement challenges. They expressed interest in 

working with NIST and representatives at the session to set agendas for future workshops. 
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Session chairs expressed intent to bring back to NIST, the status of issues most important to the 

stack testing community. The session was an important step forward in identifying issues, 

convening representatives with diverse perspectives, and seeking common ground to seek 

solutions.