Test Gas Generation from Pure Liquids: An Application-Oriented Overview of Methods in a Nutshell

Table 1

Accuracy of test gas generated based on different methods.

Technique	Accuracy	Remarks
Injection	5–9% (v/v) [14, 15]	(i) The accuracy strongly depends on the accuracy of the injection devices.; (ii) When multicomponents test gas is generated, the accuracy is not as precise as for pure components.
Permeation	2–5% (v/v) [7, 16–18]	(i) The main source of the uncertainty comes from the permeation rate, which is influenced by thermodynamic and physical state variables, such as temperature, pressure and gas flow.; (ii) Permeation methods can be used to generate very low concentrations (ppb level), the uncertainty of the calibration itself also has a strong influence on the overall accuracy.
Diffusion	3–5% (v/v) [9, 19]	(i) The most important uncertainty is the diffusion rate. The variation of it is related to the compound itself, the design of the diffusion device, and thermodynamic state variables.; (ii) In analogy to to the permeation method, the calibration itself contributes to the overall uncertainty.
Evaporation	5–15% (v/v) [4]	(i) The marked deviation results from the dilution system and temperature variation.; (ii) The vapour pressure of the compound is also one considerable factor. The error can be minimized for compounds with a low vapour pressure.