Abstract: The apparatus is used for testing heat exchangers having mutually separate first and second flow paths. The apparatus provides a high pressure fluid source (10) to connect with a first pressure-sealed flow path of the heat exchanger under test. A volumetric sensor (VS) is connected to a selected pressure-sealed flow path, having a sensing chamber arranged to receive fluid from the selected flow path. The sensing chamber is connected to a further fluid source such as a compressed air reservoir (16) which provides a high pressure gas supply. Movement of a magnetic float within the sensing chamber is detected by an array of Hall effect sensors, enabling a CPU to calculate the rate of leakage between the two flow paths which may be signaled to a local display and/or alarm or remotely via an internet connection.