An expansion tank which comprises a tank having a predetermined volume capacity; a flexible diaphragm in the tank, partitioning tank volume into a liquid-containing portion for holding liquid and a gas-containing portion for holding a gas under a pressure that defines a normal pressurized gas volume when the liquid-containing portion holds a predetermined liquid volume; and a proximity sensor suspended in the gas-containing portion of the tank and adapted to energize an alarm signal when volume of the gas-containing portion is reduced a predetermined amount as indicated by proximity of the diaphragm.