In previous investigations it was demonstrated that by tunable diode laser absorption spectroscopy (TDLAS) in the infrared it is possible to measure partial pressures in vacuum systems quite accurately. Optical methods for measuring partial pressures compared to the conventional method using quadrupole mass spectrometers have several advantages: they are non-intrusive, no molecules are cracked or produced within the instrument, they can be applied to much higher pressures (100 Pa or more) and so on.
An open question, however, was, whether by this method it is possible to evaluate the results according to the ISO guide to the expression of uncertainty in measurement (GUM), which today is a prerequisite to operate such an apparatus as primary standard.
In this investigation we demonstrate that this is actually feasible. Applying TDLAS we determined molecular line strengths of CO2 in the 2-µm-region on the highest accuracy level. As an example we report the R(12)-line strength GUM-compliant with a relative uncertainty of ±0.5 % (k=1) being in agreement with the HITRAN data base value, but 1.4 % smaller. The results of succeeding partial pressure measurements of CO2 in nitrogen by TDLAS were in agreement with gravimetrically determined values.
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