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Optical reference analyzer for trace water vapor in gases

PTBnews 3.2023
Especially interesting for

manufacturers of specialty and high-purity gases

semiconductor manufacturers/semiconductor industry

the hydrogen and energy gas industry

accredited calibration laboratories for gas humidity; reference and analytical laboratories

An optical gas sensor specially developed by PTB for the trace water vapor range enables calibration-free, SI-traceable measurements for use in climate research and industry.

Launch of a stratospheric research balloon in Kiruna, Sweden. On board is a long-path laser OGS for the calibration-free determination of water vapor (and methane) (Photo: V. Ebert, PTB, 2023)

Atmospheric water vapor is the most dominant contributor to the natural greenhouse effect, and it is by far the most variable of the air components. For example, tropical air at 30 °C and 100 % relative humidity contains more than 4 % by volume (40,000 ppm) of water vapor, compared to only 4 ppm in the stratopause – the coldest and driest layer of the atmosphere. Many industrial processes require even lower, and highly stable gas humidity levels to achieve consistent product quality. In the energy and hydrogen industries, values below 5 ppm (ISO 14687) are needed; for semiconductor or specialty gas production these levels must be less than 0.1 ppm. The quantification of very low (trace) water vapor levels in a wide variety of matrix gases is very important for industry and climate research as it requires highly sensitive analyzers and SI-traceable calibration capabilities.

To date, there are no SI-traceable bottled-gas calibration standards for trace water vapor sensors, especially in the sub-ppm range (similar to other reactive gases such as HCl, see PTB News 3.2022). PTB is therefore developing optical gas standards (OGSs), i.e., gas-cylinder-independent instrument traceability options based on the dTDLAS method (direct Tunable Diode Laser Absorption Spectroscopy). This concept can be adapted (molecule-specific) to a wide variety of target gases and has already been implemented for CO, CO2, HCL, NH3 and water vapor.

The new optical gas standard for water “TwOGaSt” (Trace water Optical Gas Standard) developed at PTB specifically targets the trace water vapor range. Like PTB’s SEALDH water vapor sensor developed for climate research, it is based on a commercial, robust and low-cost diode laser and detector from the telecommunication industry. A 36 meter long-path gas cell extends the H2O measuring range well into the sub-ppm range. The highly accurate, calibration-free OGS concept was demonstrated at PTB for trace water vapor measurements, and TwOGaSt was verified for the first time between 3 ppm and 350 ppm by comparing it with PTB’s primary coulometric trace water vapor standard.

This once again shows that the OGS concept is universal, scalable and broadly applicable to a wide variety of molecules. It opens up SI-traceable quantification and instrument calibration options, especially for reactive or very “sticky” molecules without stable calibration gas

Scientific publication

J. A. Nwaboh, S. Pratzler, V. Ebert: First metrological validation of TwOGaSt, a new, absolute dTDLAS-trace-hygrometer, using the primary, coulometric, trace water vapour generator at PTB. tm – Technisches Messen 90, 57–64 (2023) 

Opens external link in new windowdoi: 10.1515/teme-2022-0024