Adjustable pressure switch K51 series with membrane execution are dedicated for use at low pressure applications where high accuracy and repeatability are requested. Activates a micro-switch to achieve a predetermined pressure value, the preset pressure is set by turning the knob with graduated index or, in the economical version, the adjustment screw placed at the top of the instrument. Both systems are supplied with a device permits locking the desired set pressure. Mechanical stops protect both the spring and the micro-switch from over pressurization.