Abstract: A portable instrument for locating leaks through the wall of a chamber uses a non-dispersive infrared (NDIR) type detector. The instrument includes a gas sampling probe into which a continuous sample of gas is drawn by a small suction pump. The sample is conducted from the probe through a flexible conduit and through a gas sample chamber. The gas sample chamber contains a source of light or infrared radiation and a detector. A narrow pass band filter is interposed between the source and the detector, and the pass band of the filter is centered on an absorption band of the test gas that is to be detected. When the test gas is present, it absorbs some of the radiation. The detector produces an electrical signal representative of the concentration of the test gas. The detector is connected to a signal processing circuit that responds to rapid changes of test gas concentration but ignores slow changes in concentration, thereby reducing the false alarm rate.