Practical thermal desorption measurements require an ultrahigh vacuum (UHV) system with a simple method of calibrating the residual gas analyzer for a number of gases. The calibration system should be easy to move between systems. We describe how the gas injection method of Kendall has been used to calibrate an UHV system and present example measurements of the carbon dioxide and carbon monoxide evolution during the decomposition of a commercial triple carbonate used in cathode ray tube manufacture. The activation energy of the BaCO3 component of the triple carbonate was found to be 94.7 kcal/mol.