In case of a diaphragm rupture, the pumped liquid can enter the air side of the pump and exit through the air exhaust. Such a leakage can be avoided when using a diaphragm control. Both air chambers have sensors which register entering liquid. These sensors transmit an impulse to a level controller which stops the pump and/or activates an alarm signal. The use of a diaphragm control is only possible with conductive liquids.