NIST Standard Reference Photometer - MARAMA

Ozone Measurement Traceability with the 

NIST Standard Reference Photometer 

James E. Norris 

Maryland Department of the Environment 

May 18, 2011 

NIST Analytical Chemistry Division 

1

• NIST 

Outline 

• Measurement Standards 

• Ozone Standards 

• NIST Standard Reference Photometer 

• U.S. Ozone Measurement Traceability 

• BIPM/NIST Collaboration 

• International Ozone Measurement Comparability 

• Summary 


2

National Institute of Standards and 

Technology 

is a 

non-regulatory agency 

in the 

Technology Administration 

of the 

US Department of Commerce. 

Mission … to strengthen the U.S. economy and improve the quality 

of life by working with industry to develop and apply technology, 

measurements and standards. 


3

U.S. Federal Role in Metrology 

The Constitution of the United States 

Article 1, Section 8: The Congress shall have the power … to 

coin money, regulate the value thereof, and of foreign coin 

… and fix the standard of weights and measures … (1788) 

“Foreign traders had begun to voice concern that goods 

might not be assigned a proper quantitative value at 

American custom-houses and that, as a result, assessed 

duties might be unfair and uneven from port to port.” 

John Quincy Adams (1817) 


4

NBS Organic Act of 1901 

“It is therefore the unanimous opinion of 

your committee that no more essential aid 

could be given to 

• manufacturing 

• commerce 

• the makers of scientific apparatus 

• the scientific work of Government 

• schools, colleges, and universities 

than by the establishment of the 

institution proposed in this bill.” 

• Custody of the standards; 

• Comparison of the standards used in scientific investigations, engineering, manufacturing, commerce, 

and educational institutions with the standards adopted by or recognized by the Government; 

• Construction of standards, their multiples and subdivisions; 

• Testing and calibration of standard measuring apparatus; 

• Solution of standards problems; and, 

• Determination of physical constants and the properties of materials, 

when such measurements, standards and data are of great importance and are not to be 

obtained of sufficient accuracy elsewhere. 


5

Early Driver for Standards 

1904 

• Out-of-town fire 

companies arriving at a 

Baltimore fire cannot 

couple their hoses to 

the hydrants. 1526 

buildings razed. 

1905 

• National Fire Protection 

Association adopted 

NBS-developed 

national hose coupling 

standard 


6

Birth of the “Standard Samples” Program 

1905 

Standard samples program begins 

with “standardized irons” in 

collaboration with the American 

Foundrymans Association 

1906 

At the request of the Association of 

American Steel Manufacturers, NBS 

began work on certification of 17 

types of steel 

• By 1951, there were 502 Standard 

Samples, 98 of these were steels 

• Today there are more than 1400 

different SRMs 

• ~ 32,000 units sold annually 


7

Train Derailments 

… Poor Quality Control 

1912 

A report by the Interstate Commerce Commission revealed alarming statistics for 

railroad accidents; almost 42,000 over the past decade. NBS began a program to 

examine cracked rails and other failed parts through chemical, microscopic, and 

mechanical tests. 


8

• Technology Innovation Program 

• Manufacturing Extension Partnership 

• Baldrige National Quality Award 

• NIST Laboratories 

– ~1,400 technical staff including ~700 PhD 

scientists and engineers 

– 3 Nobel Laureates since 1997 

– MacArthur Fellowship winner in 2003 

– National Medal of Science winner in 1998 

– 14 National Academy Members 

– ~120 National Society Fellows and 

recipients of ~60 National or International 

Awards per year 

• One of three Federal Agencies singled out for 

significant budget growth under the American 

Competitiveness Initiative. 

NIST Today 

Chemists 

12% 

Engineers 

26% 

Physicists 

21% 

Comp.Sci./Prog. 

21% 

Other 

16% 

Math 

4% 


9

NIST still fulfills the NBS traditional function: 

the development & dissemination of primary standards 

pendulum clock 

1 s in 3 years 

(1904) 

silver voltameter 

current standard 

(1910) 

physical artifact 

(1889) 

second 

ampere 

kilogram 

NIST F1 

atomic clock 

1 s in 30 million 

years 

(1999) 

single 

electron 

counter 

(20xx) 

electronic 

kilogram 

(20xx) 

10 

NIST Analytical Chemistry Division

NIST Measurement Standards 

•Physical 

• Mass 

• Force 

• Pressure 

• Temperature 

• Humidity 

• Chemical 

• Environmental 

• Food and Drug 

• Biological 

• Metals and Alloys 

11 


NIST Standards for Chemical Measurements 

Chemical standards constitute over 2/3 of ~1,400 NIST SRM types 

• High Purity Neat Chemicals 

• Organic Solution Standards 

• Inorganic Solution Standards 

• Gas Mixture Standards 

Complex Matrix Standards 

• Advanced Materials 

• Biological Fluids/Tissues 

• Foods/Botanicals 

• Geologicals 

• Metals and Metal Alloys 

• Petroleum/Fossil Fuels 

• Sediments/Soils/Particulates 

• Cements 

• Molecular Spectrometry Standards 

• Electrolytic Conductivity Standards 

• pH / Ion Activity Standards 

12 


Hierarchy of Gas Standards 

Primary Standards 

Standard Reference Materials 

NIST Traceable Reference Material 

Research Gas Mixtures 

EPA Protocol Gases 

13 


EPA Protocol Gas Audits 

• Compliance to EPA Traceability Protocol for Assay 

and Certification of Gaseous Calibration Standards 

(EPA-600/R-97/121), September 1997. 

– Certificate of Analysis with vendor certified value. 

– Cylinder Labels. 

– Cylinder contents analyzed by NIST. 

– Vendor value within 2 % of NIST value to pass audit. 

14 


Ozone Measurement Instruments 

• Wet Chemistry Titration 

• neutral buffered potassium iodide (NBKI) 

• boric acid potassium iodide (BAKI) 

• Chemiluminescence 

• reaction of O 3 with ethylene (C 2H 4) 

• Gas Phase Titration 

• reaction of O 3 with NO 

• UV Photometric 

• absorption at 253.7 nm 

15 

NIST Analytical Chemistry Division Division

C 

= 

UV Photometric Instruments 

Derivation of Beer-Lambert Law equation used 

for calculating ozone mole fractions 

− 

lnT 

α L 

× 

⎛ 

⎜⎜ 

⎝ 

T ( sample) 

T ( std) 

C = ozone concentration 

T = transmittance (I/I 0 ) 

α = ozone absorption coefficient (at STP) 

L = optical path-length 

T (sample) = sample gas temperature 

T (std) = standard temperature 

P (std) = standard pressure 

P (sample) = sample gas pressure 

× 

P( 

std) 

P( 

sample) 

⎞ 

⎟⎟ 

⎠ 

16 


NIST Standard Reference Photometer (SRP) 

U.S. EPA and NIST Collaboration (beginning 1981). 

UV Photometry (Absorption of 253.7 nm Hg Line). 

Custom Designed and Built Instrument. 

Measurement Range: 0 to 1000 nmol/mol (ppbv). 

Estimated Uncertainty: ± 1 ppbv (0 - 100 ppbv). 

± 1% (100 - 1000 ppbv). 

NIST Ozone Reference Standard - 1982 to Present. 

Ozone Reference Standard for many other Countries. 

BIPM Ozone Reference Standard. 

17 


Arnold Bass and Jim Norris with SRP 2, circa 1984 

18 


NIST SRP 47 

19 


Schematic of SRP System 

20 


Component (y) 

Optical Path 

Lopt 

Source Distribution 

Uncertainty u(y) 

Standard 

Uncertainty 

Measurement 

Scale 

Rect. 0.0006 cm 

Repeatability Normal 0.01 cm 

Bias Rect 0.52 cm 

Pressure gauge Rect. 0.029 kPa 

Pressure P Difference 

between cells 

Rect. 0.017 kPa 

Temperature T 

Temperature 

probe 

Rect. 0.03 K 

Residual bias Rect. 0.058 K 

Scalers 

resolution 

Rect. 8×10 -6 

Ratio of 

intensities D 

Repeatability Triang. 1.1×10 -5 

Absorption 

Cross section σ 

Uncertainty Estimation for SRP 

Conventional 

value 

1.22×10 -19 

cm²/molecule 

Combined 

standard 

uncertainty 

u(y) 

0.52 cm 

Sensitivity 

coefficient 

∂x 

ci = 

∂y 

opt 

x 

0.034 kPa 

− 

P 

x 

0.07 K T 

1.4×10 -5 

2 −2 

2 

u ( x) 

= ( 0. 

28) 

+ ( 1. 

1⋅10 

x) 

nmol / mol 

2 −3 

2 

u ( x) 

= ( 0. 

28) 

+ ( 2. 

92⋅10 

x) 

nmol / mol 

contribution 

to u(x) 

ci ⋅ u( 

y) 

nmol/mol 

x 

− 2.89×10 

L 

-3 x 

x 

D ln(D) 

3.37×10 -4 x 

2.29×10 -4 x 

0.28 

1.22×10 -19 x 

− 1.06×10 

cm²/molecule σ 

-2 x 

summarised in one equation describing the uncertainty as a function of ozone mole fraction: 

without the absorption cross-section uncertainty (for UV photometer comparisons): 

21 


NIST SRPs 

(United States Network) 

SRP # Agency Location 

0 NIST (Traveling Standard) Gaithersburg, MD 

1 EPA Office of Research and Development Research Triangle Park, NC 

2 NIST (National Standard) Gaithersburg, MD 

3 EPA Region 2 Laboratory Edison, NJ 

4 California Air Resources Board Sacramento, CA 

5 EPA Region 6 Laboratory Houston, TX 

6 EPA Region 5 Laboratory Chicago, IL 

7 EPA Office of Air Quality and Planning Stds. Research Triangle Park, NC 

8 EPA Region 8 Laboratory Golden, CO 

9 EPA Region 1 Laboratory North Chelmsford, MA 

10 EPA Region 4 Laboratory Athens, GA 

13 EPA Region 7 Laboratory Kansas City, KS 

36 EPA Region 9 Laboratory Richmond, CA 

22 


International NIST SRPs 

SRP # Location Organization 

11 Stockholm, Sweden ITM 

12 Toronto, Canada Ontario MOE 

14,18 Bern, Switzerland METAS 

15,23 Dubendorf, Switzerland EMPA 

16 Ottawa, Canada Environment Canada 

17 Prague, Czech Republic CHMI 

19 Braunschweig, Germany PTB 

20 London, United Kingdom NPL 

21 Sydney, Australia NSW-EPA 

22, 44 Madrid, Spain ISCIII 

24, 40 Paris, France LNE 

25 Lisbon, Portugal IA 

26 Vienna, Austria UBA-Austria 

27,28,31,32,33 Sevres, France BIPM 

29 Langen, Germany UBA-Germany 

30 Kaohsiung, Taiwan, R.O.C. Taiwan EPA 

34 Hong Kong, S.A.R., China Hong Kong EPD 

35 Tsukuba, Japan NIES 

37 Helsinki, Finland FMI 

38 St. Petersburg, Russia VNIIM 

39 Mexico City, Mexico INE-CENICA 

41 Beijing, China NIM 

42 Ispra, Italy EC-JRC 

43 New Delhi, India NPLI 

45 Santiago, Chile CENMA 

46 Singapore NMC 

23 


NIST SRP Network 

(Worldwide) 

24 


SRP Original Comparison vs. SRP 2 

(Slopes) 

Slope 

1.010 

1.008 

1.006 

1.004 

1.002 

1.000 

0.998 

0.996 

0.994 

0.992 

0.990 

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 

SRP Number 

Average Slope: 0.99890 

Standard Deviation: 0.00167 

Delta (Max – Min): 0.72% 

25 


SRP Original Comparison vs. SRP 2 

(Intercepts) 

Intercept (nmol/mol) 

1.6 

1.2 

0.8 

0.4 

0.0 

-0.4 

-0.8 

-1.2 

-1.6 

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 

SRP Number 

Average Intercept: - 0.079 


Delta (Max – Min): 1.715 nmol/mol 

26 


U.S. Ozone Monitoring Sites 

Courtesy: USEPA Website 

27 


Ozone Traceability in the United States 

NIST 

SRP 2, 0 

U.S. EPA OAQPS 

SRP 7 

U.S. EPA Regional Laboratories 

SRP 1,3,4,5,6,8,9,10,13,36 

State and Local Air Monitoring Laboratories 

Commercial Ozone Instrumentation 

Ambient Air Monitoring Stations 

Commercial Ozone Instrumentation 

28 


U.S. EPA - SRP Network Traceability 

US-EPA 

NIST 

Annually 

by NIST 

US-EPA 

Annually 

by EPA 

SRP 1 

SRP 3 

SRP 4 

SRP 5 

SRP 2 or SRP 0 SRP 7 SRP 6 

SRP 8 

SRP 9 

Periodic Upgrades 

SRP 10 

SRP 13 

SRP 36 

Agreement within 1% (Slope), 1 ppbv (Intercept) required! 

29 


NIST SRP Electronics Upgrade Project 

(1999-2000) 

• SRP 0-22 built with original electronics. 

• all have been upgraded. 

• Driven by obsolete electronic components. 

• Several circuit chips were no longer produced. 

• Consisted of completed electronics system redesign. 

• new Electronics module and Detector module. 

• Improved V/f conversion rate (increased signal). 

• Increased from 1V in-10KHz out to 1V in-30KHz out. 

• Included new dual external manifold. 

• Balanced pressure difference; improved intercepts. 

• Included new version 4.0 control software. 

• Windows based VB code. 

30 


Average Slope 

1.03 

1.02 

1.01 

1.00 

0.99 

0.98 

0.97 

U.S. EPA SRPs vs. NIST SRP 2 

before and after electronics upgrade 

0 2 4 6 8 10 12 14 

Before Upgrade 

After Upgrade 

SRP Number 

31 


Average Intercept 

2.00 

1.50 

1.00 

0.50 

0.00 

-0.50 

-1.00 

-1.50 

-2.00 

U.S. EPA SRPs vs. NIST SRP 2 

before and after electronics upgrade 

0 2 4 6 8 10 12 14 

Before Upgrade 

After Upgrade 

SRP Number 

32 


International SRP Network Traceability 

as of 2002 

Canada 

SRP 12 (MOE) 

SRP 16 (EC) 

NIST SRP 2 or SRP 0 

SRP 21 (NSW-EPA) 

SRP 30 (TEPA) 

1 - 2 years 

2 - ? years 

Asia- Pacific 

2 - ? years 

Europe 

SRP 11 (ITM - Sweden) 

SRP 14, 18 (METAS - Switzerland) 

SRP 15, 23 (EMPA - Switzerland) 

SRP 17 (CHMI - Czech Republic) 

SRP 19 (PTB - Germany) 

SRP 20 (NPL - United Kingdom) 

SRP 22 (ISCIII - Spain) 

SRP 24 (LNE – France) 

SRP 25 (IA – Portugal) 

SRP 26 (UBA – Austria) 

SRP 29 (UBA-Germany) 

33 


Bureau International des Poids et Mesures 

(International Bureau of Weights and Measures) 

The task of the BIPM is to ensure world-wide uniformity of 

measurements and their traceability to the International System 

of Units (SI). 

It does this with the authority of the Convention of the Metre, a diplomatic 

treaty between fifty-one nations, and it operates through a series of 

Consultative Committees, whose members are the national metrology 

laboratories of the Member States of the Convention, and through its own 

Laboratory work. 

The BIPM carries out measurement-related research. It takes part in, and 

organizes, international comparisons of national measurement standards, 

and it carries out calibrations for Member States. 

Source – BIPM website 

34 


Consultative Committees under the International 

Committee for Weights and Measures (CIPM) 

CCAUV: Consultative Committee for Acoustics, Ultrasound and Vibration 

CCEM: Consultative Committee for Electricity and Magnetism 

CCL: Consultative Committee for Length 

CCM: Consultative Committee for Mass and Related Quantities 

CCPR: Consultative Committee for Photometry and Radiometry 

CCQM: Consultative Committee for Amount of Substance 

(Metrology in Chemistry) 

CCRI: Consultative Committee for Ionizing Radiation 

CCT: Consultative Committee for Thermometry 

CCTF: Consultative Committee for Time and Frequency 

CCU: Consultative Committee for Units 


35 


Consultative Committee for Amount of Substance 

(CCQM) 

• Established by the CIPM in 1993 

• 35 member and observer organizations 

• Yearly meetings of CCQM plenary, attended by some 60 representatives 

• 7 working groups, meeting twice a year and attended by some 150 experts 

from NMI’s and other expert institutes 

Working Groups 

– Gas Analysis; chair: Dr. M. Milton (NPL, UK) 

– Organic Analysis; chair: Dr. W. May (NIST, USA) 

– Inorganic Analysis; chair: Dr M. Sargent (LGC, UK) 

– Electrochemical Analysis; chair: Dr. M. Máriássy (SMU, SK) 

– Bioanalysis; chair: Mrs H. Parkes, (LGC, UK) 

– Surface Analysis; chair; Dr. W. Unger BAM, DE) 

– Key Comparisons and CMC quality review; chair: Dr L. Mackay (NMIA, AU) 

Working Groups are responsible for selecting and overseeing the operation of 

key comparisons that address chemical measurement-related issues important 

for international trade, environmental health, and safety-related decision 

making. 

36 


CCQM Key Comparison Database 

Example (gases) 

CCQM-K1.a 

Carbon monoxide (CO) in Nitrogen (N2) 

1994 - 1995, Approved for equivalence 

Results available 

CCQM-K1.b 

Carbon dioxide (CO2) in Nitrogen (N2) 



CCQM-K1.c 

Nitrogen monoxide (NO) in Nitrogen (N2) 



CCQM-K1.d 

Sulfur dioxide (SO2) in Nitrogen (N2) 



CCQM-K1.e 

Natural gas type I 



CCQM-K1.f 

Natural gas type II 



CCQM-K1.g 

Natural gas type III 



CCQM-K3 

Automotive emission gases 




37 


Automotive Emission Gases 

38 


BIPM – NIST Collaboration 

• Delivery of SRP 27 and SRP 28. 

• Detailed Training during assembly of SRP 31 and SRP 32. 

• SRP 33 assembled at BIPM using BIPM machined parts. 

• Detailed Review of SRP – publication available (Metrologia). 

• Temperature Measurement Bias: - 0.4% estimate. 

• Optical Path-length Bias: + 0.5% estimate. 

• SRP Bias Upgrade Project – publication in progress. 

• Develop International Ozone Traceability System through CCQM. 

• CCQM – P28 - Ozone Ambient Level Pilot Study – publication available. 

• BIPM.QM-K1, CCQM Key Comparison 2007-2008, 2009-2010, etc. 

39 


NIST SRP Bias Upgrade 

(2008-2010) 

• Temperature and Optical Path-length Biases. 

• Determined by extensive SRP review by BIPM/NIST. 

• Development of new light source block design. 

• Thermal Isolation of Heated Source Block. 

• Improved Collimation of Light Beam. 

• Development of new absorption cells. 

• Optically sealed windows at 3° angle – fixed length. 

• Fused Quartz Inlet/Outlet Ports – No Teflon or O-rings. 

• Path-length measurement by Coordinate Measurement Machine. 

• Measurement Quantification from each Bias Correction. 

• Independent Measurements after each Bias Upgrade. 

• Manuscript in review process. 

40 


Derivation of Beer-Lambert Law equation used 

for calculating ozone mole fractions with the SRP 

C 

= 

− 

lnT 

α L 

× 

⎛ 

⎜⎜ 

⎝ 

T ( sample) 

T ( std) 

× 

P( 

std) 

P( 

sample) 

Optical Path-length Bias 

· Multiple internal reflections in absorption cells from cell windows 

and optical filter. 

· Path-length (L) is underestimated, C is overestimated. 

Temperature Gradient Bias 

· Sample and reference gases heated thru thermal transfer of 

heat from source lamp block. 

· Sample Temperature [T(sample)] is underestimated, C is 

underestimated. 

⎞ 

⎟⎟ 

⎠ 

41 


NIST SRP Bias Upgrade Project 

SRP Bias Upgrade (performed in steps). 

1. Initial measurements. 

2. New cells installed – set of measurements. 

3. New source/optics block & shutter cover installed – final set of measurements. 

• 22 of 38 SRPs upgraded so far, including SRP 2. 

• 7 directly vs. SRP 2 at NIST. 

• 10 using SRP 0 as transfer. 

• SRP 32 vs. SRP 27 (data not corrected to SRP 0 or SRP 2). 

• 4 SRPs upgraded by BIPM. 

• data not officially available. 

• 10 new SRPs built new with bias upgrade included. 

• Average shift in slope from new cells (optical bias): -0.67%. 

• Average shift in slope from new source (temp. grad. bias): +0.37%. 

• Average net change in slope from bias upgrade: -0.30%. 

• No noticeable change in the intercepts. 

• Overall agreement in all SRPs before bias upgrade: 0.72%. 

• Overall agreement in all bias upgraded SRPs: 0.33%. 

42 


Most Recent Comparison vs. SRP 2 

(Slopes) 

Slope 

1.010 

1.008 

1.006 

1.004 

1.002 

1.000 

0.998 

0.996 

0.994 

0.992 

0.990 

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 

Direct vs. SRP 2 

SRP 0 as Transfer 

SRP Number 

Average Slope: 0.99850 


Delta (Max - Min): 0.33% 

Original Comparisons 

43 


Most Recent Comparison vs. SRP 2 

(Intercepts) 

Intercept (nmol/mol) 

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

-0.2 

-0.4 

-0.6 

-0.8 

Original Comparisons 

-1.0 

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 

Direct vs. SRP 2 

SRP 0 as Transfer 

SRP Number 

Average Intercept: - 0.006 


Delta (Max - Min): 0.282 nmol/mol 

44 


CCQM – P28 - Ozone Ambient Level Pilot Study 

(slopes) 

45 


CCQM – P28 - Ozone Ambient Level Pilot Study 

(intercepts) 

46 


BIPM.QM-K1, ozone at ambient level results 

47 


BIPM.QM-K1, ozone at ambient level results 

48 


Regional Metrology Organizations 

Planned GULFMET and AFRIMETS sub-region NEWMET 

49 


NORAMET 

• Canada 

• Mexico 

• United States of America 

CAMET 

• Belize 

• Costa Rica 

• El Salvador 

• Guatemala 

• Honduras 

• Nicaragua 

• Panama 

SISTEMA INTERAMERICANO DE METROLOGÍA (SIM) 

(INTERAMERICAN METROLOGY SYSTEM) 

ANDIMET 

• Bolivia 

• Colombia 

• Ecuador 

• Peru 

• Venezuela 

CARIMET 

• Antigua & Barbuda 

• Bahamas 

• Barbados 

• Dominica 

• Dominican Republic 

• Grenada 

• Guyana 

• Haiti 

• Jamaica 

• Saint Kitts and Nevis 

• Saint Lucia 

• St. Vincent and the 

Grenadines 

• Suriname 

• Trinidad and Tobago 

SURAMET 

• Argentina 

• Brazil 

• Chile 

• Paraguay 

• Uruguay 

50 


Ozone Instrument Comparisons 

• NIST Calibrations 

• NASA 

• NOAA 

• Universities 

• Commercial Organizations 

• World Meteorological Organization (WMO) 

• Global Atmospheric Watch (GAW) 

• Euromet Programs 

• Intercomparison of National Ozone Primary Standards, 

project 414 

51 


WMO-Global Atmospheric Watch (GAW) 

NIST is WMO-GAW Central Calibration Laboratory (CCL) 

52 


Acknowledgements 

NIST - Franklin Guenther 

Willie May 

Dana Strawbridge 

Jeff Anderson 

Jack Fuller 

Bob Zarr 

BIPM - Robert Wielgosz 

Joele Viallon 

Phillipe Moussay 

53 


Summary 

• NIST Standard Reference Photometer 

– Stable, Accurate Ozone Reference Standard for 27 Years. 

– Network agreement maintained within 1%. 

– Maintained in 22 Countries, designated National Standard in 13 Countries. 

– Total of 48 instruments Worldwide (currently). 

• SRP Bias Upgrade: 

– Significantly reduced measurement biases 

– Improved overall SRP measurement agreement. 

– Manuscript in review process. 

• BIPM – NIST Collaboration 

– Well Established. 

– Developed International Traceability through CCQM. 

– Research with GPT, UV Laser, Improved Optical Systems. 

• CCQM Ozone Key Comparisons 

– P28 Pilot Study Completed – Results Published. 

– BIPM.QM-K1 Official Key Comparison (ongoing). 

54

NIST Standard Reference Photometer - MARAMA

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