An arrangement (10) for controlling the liquid level in a basin (60) by selectively actuating a pump (64) has an environmentally-sealed enclosure (22) with an internal volume subject to thermal and pressure effects. A first (14) and a second (16) pressure sensing means are positioned in the enclosure and a barrier fluid (18) fills a balance of the internal volume. The first sensing means (14) is in a first chamber (22a) of the enclosure (22) and the second sensing means (16) is in a second chamber (22b). The barrier fluid (18) has a first portion (18a) in the first chamber (22a) and a second portion in the second chamber (22b). The first pressure sensing means (14) communicates an "on/off" signal to the pump (64). Diaphragms (26, 40) in the respective chambers (22a, 22b) allow the device to adjust to changes in atmospheric pressures. The barrier fluid (18) is electrically non-conductive, chemically non-reactive with any materials comprising the pressure sensing means and, preferably, an oil.