Abstract: An apparatus and method for employing the apparatus in testing underground fluid containing lines for leaks. The apparatus comprises (a) a differential pressure transducer mounted to a reservoir for indicating volumetric change in the reservoir; (b) a temperature transducer mounted in the reservoir for monitoring temperature fluctuation in the reservoir; (c) a gauge pressure transducer mounted in the reservoir; (d) a remote temperature sensor and a data acquisition and processing system. The method for employing the apparatus comprises of connecting the apparatus to the line under test and entering certain parameters of a line under test and product temperature into the test computer. The test system then monitors volumetric change in the reservoir as well as pressure and temperature fluctuations in the line under test at 30 second intervals. At the end of the predetermined test period, the system calculates the leak rate during each five minute interval of the test as well as a cumulative leak rate.