Modelling of the dynamic gravimetric preparation of calibration gas mixtures using permeation for trace gas analysis

Permeation is a technique for realising a primary measurement standard for gas composition. A permeation tube is suspended in a permeation chamber and emits a constant mass flow of the nominally pure substance contained in it. This flow is combined with a flow of a carrier gas to obtain a calibration gas mixture with known composition. We used an automated weighing system to monitor the mass loss of the tube, and prepared mixtures of ammonia (NH3) in nitrogen. The advantage of such dynamic gas standards is that unlike static standards they do not have stability issues and the composition of the calibration gas mixture can be chosen more rapidly than with static mixtures.

We revisited and extended existing measurement models describing the standard. We show how dependencies between the input quantities in the extended model can be taken into account. We describe the effect of temperature fluctuations on the permeation rate, temperature and pressure effects on the dispersion of the weighing data. For evaluating the linearity of the balance, a simple Bayesian model was established that takes into account the repeatability and resolution of the balance. We also show that the use of ordinary least squares regression to obtain the permeation rate is justified.