progressing optical gas standard concepts

Progressing Optical Gas Standard concepts

- from environmental measurements to industrial process control and AMC monitoring

Zhechao Qu, Javis Nwaboh, Thomas Benoy,

Olav Werhahn, Volker Ebert

2020-11-05 MetAMC2 Workshop on Advanced Optical Spectroscopy for Gas Detection

2020-11-05 2 MetAMC2 Workshop on Advanced Optical Spectroscopy for Gas Detection

▪ National Metrology Institute of Germany, under the

authority of the Federal Ministry for Economic Affairs

and Energy (BMWi)

▪ approx. 1900 staff members,

700 scientific papers per year,

185 Mio. € annual budget

Braunschweig

Berlin

About PTB

Braunschweig

Optics

Ionizing

Radiation

Electricity

Administrative

Services

Mechanics

and Acoustics

Chemical Physics

and Explosion

Protection

Precision

Engineering

Cross-Sectional

Services

Legal and Inter-

national Metrology

QUEST

Institute at PTB

Fundamental Physics

for Metrology

MetAMC2 Workshop on Advanced Optical Spectroscopy for Gas Detection32020-11-05

Berlin

Temperature and

Synchrotron Radiation

Medical Physics and Metrological

Information Technology

4 MetAMC2 Workshop on Advanced Optical Spectroscopy for Gas Detection2020-11-05


About usChemical Physics

and

Explosion Protection

Analytical chemistry of the

gas phase

Spectrometric gas analysis

— Capabilities in 3.42 —

Laser spectroscopy

• Direct traceable methods for amount fraction measurements

→ the TILSAM method

• Direct laser absorption spectroscopy (TDLAS/QCLAS) –

development of spectrometers

• Field TDLAS instruments

(ground based, balloon, airplanes)

• Cavity-enhanced (CRDS/CEAS) and comb-assisted

techniques

• Optical isotope ratio spectroscopy (OIRS)

▪ Thermo (δ13C-CO2 and δ18O-CO2)

▪ Picarro (δ13C-CO2 and δ13C-CH4 )

▪ Los Gatos (triple water analyser)


Current running projects

16ENV08-IMPRESS2

16ENG05-BioMethane

17IND09-MetAMC2

EURAMET 1498 - pilot studybilateral comparison

100 µmol/mol HCl in N2

CCQM-Kxxkey comparison

30 µmol/mol HCl in N2

Laser spectroscopy

19ENG03-MefHySto

16ENV06-SIRS

19ENV05-STELLAR

HCl related

CO2 isotopes

CH4 /CO2 isotopesH2O in H2

19ENV09-MetroPEMS

NO2 emission

DFG SPP1924-HALO

H2O in atmosphere

2020-11-05 7

Technique – dTDLAS

𝑥HCl =𝑘B ∙ 𝐴 ∙ 𝑇

𝑆 ∙ 𝑝 ∙ 𝐿

MetAMC2 Workshop on Advanced Optical Spectroscopy for Gas Detection

measured parameters

constants

molecular line data

Drive electronicData

acquisition

DetectorDiode laser

Gas cell

Measure spectral

absorbance (via

laser intensity)

Line area, A

Traceable inputs:

p, T, L, S, kB

Calculate traceable

amount fraction, xHCl

Laser

tuning

dTDLAS: direct tunable diode laser absorption spectroscopy

*Traceable Infrared Laser-Spectrometric Amount fraction Measurement (TILSAM)| https://www.euramet.org/Media/docs/projects/934_METCHEM_Interim_Report.pdf

*

https://www.euramet.org/Media/docs/projects/934_METCHEM_Interim_Report.pdf


Advantages of dTDLAS-TILSAM:

robust, simple, in situ, linear, calibration-free

A(area)

wavenumber/cm−1

ln𝐼0𝐼

Amount fraction (concentration):

Quantities:

kB : Boltzmann constant

A : integrated absorbance (area), u ~ 1 %

T : gas temperature, u < 0.1 %

p : gas pressure, u < 0.2 %

L : optical path length, u ~ 0.1- 0.4 %

ST : line strength of the probed

molecular transition at T, uHITRAN ~ 2-20 %, uPTB < 1-3 %

dTDLAS uncertainty


𝑆 ∙ 𝑝 ∙ 𝐿

Via Etalon measurement

9

An optical gas standard is a laser spectrometer that can provide amount of substance fraction

(concentration) results that are directly traceable to the SI

→ TILSAM*

Optical gas standard (OGS)


Boltzmann const.

line strength (S)

pressure (p)

temperature (T)

path length (L)

laser tuningAnalyzer

(dTDLAS)

Optical Gas Standard (OGS)

No calibration gas needed

Field calibration gas

Analyzer

Gas standard approach

Tertiary, secondary, primary ref. gases

*Traceable Infrared Laser-Spectrometric Amount fraction Measurement (TILSAM)| https://www.euramet.org/Media/docs/projects/934_METCHEM_Interim_Report.pdf

https://www.euramet.org/Media/docs/projects/934_METCHEM_Interim_Report.pdf

2020-11-05 10

Optical gas standardCurrent realisation:

• Based on direct tunable diode laser absorption spectroscopy (dTDLAS)

→ accurate and reliable amount fraction measurements

• Calibration-free (no gaseous standards needed > low maintenance cost)

→ no need for calibration procedures … just validation

• dTDLAS-based amount fraction measurement instrument can be entirely

described by a first principle physical model – TILSAM compliant

→ all input parameters are directly traceable to the SI >> OGS!

• Especially for sticky and reactive gases which cannot be provided in static

gas cylinders (Certified Reference Materials)

→ to complement calibration gases


2020-11-05 11

Current and future HCl - traceability ?

➢ Current HCl reference method

• Biomethane : Reference method for HCl measurements is unavailable

• Combustion: emissions from stacks (EN1911)> indirect measurements via wet-chemistry

o extractive gas sample / drying / filtration > systematic effects

o stable gas standards for calibratoin (none for flue gas)

• Semiconductor: HCl in nmol/mol (ppb) to pmol/mol (ppt)

o no gaseous reference materials for instrument calibration

➢ HCl metrology

• no HCl CMC for amount fractions below 10 µmol/mol

• existing HCl CMCs

o NPL (UK): 10 – 1000 µmol/mol HCl in N2

o VNIIM (Russia): 20 – 1000 µmol/mol HCl in N2



HCl - OGS instrumentsdTDLAS: direct tunable diode laser absorption spectroscopy

Light source:- ICL laser at 3.6 µm - Swept at 140 Hz

Gas cell:- single pass 82 cm (instr. A)

77 cm (instr. B)

Detector/Sensors:- T sensor: PT100b -P sensor: MKS baratron- Vigo Mid-IR detector

Instr. A

2751.95 2752.00 2752.05 2752.10

0.00

0.01

0.02

Absorb

ance

Wavenumber / cm-1

10 ppm HCl

2.5 % CO2

97 % CH4

p = 50 hPa

L = 0.82 m

T = 296 K

2752.0 2752.1

0.0000

0.0005

0.0010

0.0015

2772 2774 2776 27780.00

0.04

0.08

0.12

0.16

0.20

Ab

so

rba

nce

Wavenumber / cm-1

H2O: 20%

CO2: 10%

HCl: 50 ppm

1 atm 0.77 m 400 K

0.000

0.002

0.004

0.006

0.008

Instr. B


BioMethane: HCl - in CH4 OGSTypical HCl dTDLAS signal in CH4

Relative uncertainty of HCl dTDLAS results: 4.6 %, k = 2

HCl/CH4: 50-500 µmol/mol

Slope:1.038±0.071

Intercept: (-2.41±5.57) µmol/mol

J. Nwaboh, Z. Qu, B. Buchholz, O. Werhahn and V. Ebert, OSA 2020 Optical Sensors and Sensing Congress

The detection limit of this HCl-OGS system is 29 nmol/mol at Δt= 54 s

14

IMPRESS2: HCl - in flue gas OGS


Typical HCl dTDLAS signals in CO2

0 1000 2000 3000 4000

50

60

70

80

90

100

110

120

130

1 10 100 1000

0.1

1

Co

ncen

tation / p

pm

Time / s

First filling

Second filling

115.9±0.5 ppm

(a) (b)

dT

DLA

S X

HC

l / p

pm

Time / s

Allan Deviation

0.17 ppm

p=954 hPa

T=295 K

L=77 cm

Instrument detection limit: 0.02 µmol/mol @1 s, 1 m

< EU emission limit 1.5 µmol/mol

Improved system response by passivation

Z. Qu, J. Nwaboh, O. Werhahn and V. Ebert, Flow, Turbulence and Combustion 2020 accepted

Passivation time more than 1000 s for the first filling

@ instrument inner surface coated !

0.05

0.10

0.15

0.20

0.25

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0-6

-3

0

3

6

Absorb

ance / a

.u. Raw data

Fit

Gas sample: HCl in CO2

(b)

(c)

1s=1.010-3

Re

s. / 1

0-3

Relative wavenumber / cm-1


Absorption/desorption of reactive gas - HCl

0 50 100 150 200 250

0

5

10

15

20

25

Pressure

HCl concentration

Time / s

Ce

ll p

ressu

re /

mb

ar

HCl desorption from gas cell inner surface

500

750

1000

1250

1500

1750

HC

l co

nce

ntr

atio

n/ p

pm

HCl gas mixture in closed cell

System inner

surface

Evacuation process

0 500 1000 1500 2000 2500 3000 3500

420

440

460

480

500

HCl concentration

HCl number density

Linear fit

Time / s

HC

l con

ce

ntr

ation / p

pm

8.0x1015

1.0x1016

1.2x1016

1.4x1016

HC

l num

be

r density / c

m-3

Cell closed at 963.8 mbar

21 0C

Slope: -51011 molecule/s×cm3

0 50 100 150 200 250

0

5

10

15

20

25

Ce

ll p

ressu

re /

mb

ar

Pressure

HCl number density

Time / s

HCl desorption from gas cell inner surface

HC

l n

um

be

r d

en

sity /

cm

-3

1013

1014

1015


Stack emission monitoring applications Temperature

Gas composition

0 200 400 600 800 1000-40

-30

-20

-10

0

10

De

via

tio

n /

%

Thot-Tcold / K

T-chioce:

Tmean

Thot

Tcold

Xhot=100 ppm

Xcold=10 ppm

0 50 100 150 200 250

-3

-2

-1

0

1


𝑆(𝑇) ∙ 𝑝 ∙ 𝐿

Heterogeneity effects on dTDLAS

Boundary layersΔT , ΔL , Δx Known? Measurable? Effects?

Heterogeneity effects on dTDLAS depends on:

➢ transition line, S(T) ➢ boundary layers

• temperature • gas compositions

➢ temperature choice • calculation temperature used in• Thot, Tcold, Tmean

2020-11-05 17

MetAMC2 project

WP1: Spectroscopy instrumentation

➢ Develop an OGS system based on a combined dTDLAS/WMS to measure HCl

in cleanrooms (air matrix)

➢ Design a bypass calibration system to bridge dTDLAS and WMS techniques

➢ Target detection of < 1 nmol/mol (ppb) in 1 minute

„Metrology for Airborne Molecular Contaminations 2“



MetMAC2: HCl - in air OGS

Target: trace HCl in Air (Cleanroom)

Instrument C

Multi pass cell 30 m

dTDLAS + WMS

sub nmol/mol range

Single pass cell < 1 m

Instruments A and B

dTDLAS technique

sub µmol/mol range

Detection limit improved by:

• Multipass gas cell

• Wavelength modulation spectroscopy

dTDLAS here:

• As an OGS to calibrate WMS


A. Klein, O. Witzel, V. Ebert. Sensors 2014, 14(11)

MetMAC2: HCl - in air OGS

dTDLAS + WMS

HCl-OGS

By-pass calibration system

non-certified but sufficiently

‘high’ concentration gas mixture

Dilution device >> multi point calibrations


Summary and outlook

➢ We presented the concept of calibration free instruments (dTDLAS based OGS).

➢ dTDLAS OGS instruments can

✓ serve as SI-traceable instruments complying with the TILSAM method;

✓ be used for field measurements, and also provide an alternative field

calibration approach for sticky or reactive gases;

✓ complement calibration gases (CRMs).

➢ Three HCl-OGS instruments have been progressed for different applications.

➢ dTDLAS/WMS HCl-OGS instrument will be validated by comparison to NPL HCl

gas mixtures and dynamic dilution system.

➢ EURAMET 1498 bilateral study with KRISS on 100µmol/mol HCl in nitrogen,

and CCQM HCl key comparison (2021) >> goal HCl CMC(s).

21

Physikalisch-Technische Bundesanstalt

Braunschweig and Berlin

Bundesallee 100

38116 Braunschweig

Zhechao Qu

Telefon: +49 531 592-3112

E-Mail: [email protected]

www.ptb.de

Acknowledgement

Thanks for your attention!

http://www.ptb.de/

Supplementary

progressing optical gas standard concepts

Documents