The conventional pressure method of a leak test is susceptible to temperature change. We propose a new method using lock-in detection for temperature compensation in the leak test. The gas in a pipe is alternately compressed and sucked out by a pump. After each compression and suction the pipe is closed and the pressure variation is measured by a manometer. If a leak is present in the pipe, the pressure variation signal alternates in polarity according to the alternation of compression and suction; the pressure variation due to the leak is modulated by the alternation of compression and suction. The pressure variation due to temperature change is, in contrast, independent of and not correlated with the alternation. A lock-in detector employed, the pressure variation signal is demodulated into an output which is a measure of the amount of gas leakage, while the temperature change term cancels itself out through the time averaging process. An experimental apparatus and model piping were built and several experiments have been performed using air. The results showed the success of the proposed method.

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