Differential pressure switch for water geysers - controls heating using pressure sensitive resistor acted upon by membrane

The differential pressure switch for water geysers, controls the element heating the water by using a pressure sensitive resistor to detect when the water in the switch low pressure chamber has been drained. The water inlet or high pressure chamber lies on one side of the membrane and the low pressure or outlet chamber on the opposite side of the membrane. The pressure sensitive resistor block is situated in the low pressure chamber at the centre of the membrane. With the membrane undistorted the block resistance is relatively high, e.g. 500 Ohms. When the membrane compress the block the latter resistance is reduced to e.g. 1 Ohm.