ll older washing machines and dishwashers use mechanical pressure switches to detect the water level inside the tub. A hose is connected from the base of the tub to the pressure switch, which measures the air pressure being applied by the column of water.

The pressure at the base of a column of water 27.69 inches high is 1psi, or to put it another way, 10.2cm of water exerts a pressure of 1kPa (kilopascals). So, as you can imagine, a switch that’s designed to detect when the water level in a washing machine has reached (say) a depth of 15 inches must be capable of accurately measuring pressures of just 0.5 psi (~3.5 kPa)!

Washing machine and dishwasher pressure switches can be easily scrounged for nothing - you'll find a pressure switch inside nearly every one of these appliances when they're discarded. They are extremely sensitive and their trip points are externally adjustable.

In other words, the pressure switches in washing machines are amongst the most sensitive that you can buy. Well, you don’t really have to buy them – these switches can be scrounged for literally nothing wherever washing machines are being discarded.

Washing machine pressure switches have three main characteristics:

(1) Very high sensitivity;

(2) A "snap" action, to ensure sharp, decisive switching – they’re either on or off; and

(3) Excellent power handling.