Abstract: A method and apparatus are disclosed for automatically compensating for the effects of variations in ambient trace gas levels or other causes of drift in trace gas leak detection testing, by generating a stored reference signal corresponding to the trace gas sensor output signal, freezing the reference signal at the beginning of each test and summing the output signal with the frozen reference signal to create a compensated output signal which is then compared with a preset threshold signal to trigger a reject during a given test. The uncompensated output signal is also continuously compared with a preset signal value and the test disabled if the output signal exceeds the preset signal at the beginning of each test.