Measurement of the thermostability of PVC and other chlorine-containing polymers with the PVC Thermomat is done by the dehydrochlorination test.
It is based on heating the sample to induce decomposition that results in the release of gaseous HCl.
A continuous stream of nitrogen that passes through the sample transports this HCl from the sample vessel into a vessel containing distilled water whose conductivity is continuously measured.
Conductivity increases in this vessel once HCl emerges. The time at which a 50 µS/cm increase is reached compared to the beginning of the measurement is called the stability time; it is a measure of thermostability.