A leak detector contains a pump that withdraws gas from a volume into which a tracer gas is leaking. The volume may also contain small amounts of air or other gas in addition to the tracer gas. Downstream from the pump, a flow restriction creates a back pressure between this restriction and the outlet of the pump, the back pressure being determined by the pressure in the test volume, the pumping speed, and the nature of the flow restriction. The flow restriction may be fixed, such as an orifice or a section of a capillary tube, or it may be variable, such as a needle valve. In many cases, the back pressure may be adjusted to approximately one atmosphere, allowing the use there of various conventional gas sensors that are designed to operate at one atmosphere. The invention may also be used for general gas analysis problems.