The Haffmans CO2 Purity Tester – CPT is used for routine controls of the air content of commercial CO2 gas and CO2 gas from fermentation. The principle is based on CO2 absorption by caustic. The measuring burette of the CPT is properly flushed with CO2 gas. The CO2 sample is separated in the measuring burette and the connection is made with the caustic burette. The CO2 is completely absorbed by the caustic and the remaining gas volume is read on the scale of the measuring burette after turning the instrument on its backside.