EVALUATION OF A TUNABLE DIODE LASER ABSORPTION SPECTROMETER AGAINST THE NPL LOW FROST-POINT GENERATOR

摘要

An infrared tunable diode laser absorption spectrometer measuring trace moisture in air has been compared against the Low Frost-point Generator (LFG) of the UK National Physical Laboratory (NPL). The comparison between the two instruments followed on from earlier work described elsewhere at this conference, and took place during the course of the validation of the LFG in a trial phase of its operation down to frost points as low as -100°C (volume fraction of about 15 parts per billion (ppb)). In the form tested, the spectrometer performed with a resolution of better than 2 ppb, and demonstrated high linearity relative to the LFG, making it a highly useful tool for validating the performance of the generator. Conversely, the LFG measurements gave valuable information on the accuracy and repeatability of the spectrometer. The spectrometer and the LFG showed combined reproducibility of better than 12 ppb over a time period of three months. The measurements also demonstrated that the spectrometer had a significant offset of 215 ppb. A possible cause is thought to be captive water vapour within the photodetector of the spectrometer.