How to install a mechanical pressure switch?

How to install a mechanical pressure switch?

This article describes how to install and adjust a mechanical pressure switch. If you want to know more about pressure switches in general, you can read our article about pressure switches. For compressors and water well switches, a dedicated installation guide is available.

Mechanical pressure switch operating principle

Before explaining the installation steps, first the basic operating principle of mechanical switches is explained. The pressure switch opens or closes an electrical circuit when a particular (adjustable) pressure is achieved.

section view of a mechanical pressure switch working principle

Sectional view of a mechanical pressure switch: adjustment screw (A), electrical connection (B), o-ring (C), connection port (D), microswitch (E), piston (F)

In the switch there is a microswitch which is operated mechanically. The medium presses against a diaphragm or piston, which is moved as a result of pressure increase. A spring on the other side of the diaphragm or piston presses in the opposite direction.

If the medium pressure overcomes the spring force and causes a certain displacement, the microswitch will be activated. The spring tension is adjustable with a set screw or knob to increase or decrease the switching pressure.

Before installation

Ensure that the medium conditions are within the specifications of the switch. Always carefully follow the manufacturer’s instructions. Note that some piston pressure switches might be unsuitable for gaseous media such as oxygen.

Installation in the piping system

Pressure switches should not be installed without proper gaskets. Many switches have a parallel male threaded connection.

mechanical pressure switch threaded connection

Male threaded connection on a mechanical pressure switch

The most common sealing method for this connection is to use a standard copper gasket of the appropriate dimensions. These seals are available as flat rings or profiled rings.

flat and profiled seal rings for pressure gauges and sensors

Flat and profiled seal rings.

We recommend profiled seals over flat rings. The advantage of the profiled rings is that after sealing they can rotate one more revolution. Profiled stainless steel seals can rotate approximately 1/2 revolution more. This way, the sensor can be mounted exactly at the desired angle. A profiled seal cannot be reused after installation.

Other sensors might have a flanged connection where the sealing is achieved with an O-ring. Follow the recommended sealing method if this is specified in the datasheet.

Electrical connection

Use the appropriate connector to connect your cable. Make sure that the wiring is according to the connection diagram and that your cable is free of pinching or chafing. In order to prevent short circuits, it is recommended to guide the cable in a way that prevents droplets from sliding towards the sensor.

electrical connection and adjustment screw for a mechanical pressure switch

Electrical connection and adjustment screw for a mechanical pressure switch.

Adjusting the switch pressure

In this paragraph is explained how to adjust a mechanical pressure switch. Usually the switch has no absolute reference point. This means that a test pressure gauge is required to adjust the setpoint to the correct pressure.

Secondly, you need to be able to control the pressure in the system using a pressure regulator or other method.

Thirdly, you will need a method to show the status of the electrical switch. Typically, a continuity tester is used. Most multimeters have this functionality. Alternatively, you can use a testing lamp. In this case make sure to not exceed the switching capacity, which is given in the datasheet.

Here follow basic steps that are common for most switches:

adjustment knob for a pressure switch

For this switch, the adjustment knob needs to be unlocked with a hex key.

  1. If a connector is installed, remove it.
  2. Connect a continuity tester or testing lamp to the pressure switch. Make sure to not exceed the switching capacity. For a NC or NO contact, use the two available terminals. In case of a changeover contact, connect the common with either the NC or the NO contact.
  3. The setpoint can be adjusted with a knob or a set screw, with for example a hex key or slot drive. Some knobs need to be unlocked before they can be adjusted. The image below shows an example of a knob that needs to be unlocked with a hex key.
  4. Screw in the adjustment screw or knob clockwise as far as it will go. Make sure your system is at the desired pressure. Use a test pressure gauge to verify the pressure in the system.
  5. Then slowly adjust the pressure switch to the desired switching pressure. To do so, unscrew (anti-clockwise) the adjustment screw or knob until the continuity tester or testing lamp reacts.

NOTE: The pressure switch hysteresis is usually not adjustable by the user and is set by the manufacturer.

Removing a pressure switch

When you remove a pressure switch, make sure that the system is entirely depressurized. Only exert force on the designated parts of the body. The plastic parts of the pressure switch might get permanently damaged if excessive force is applied.

Sealing rings are often for one-time use. Therefore, use a new sealing ring when installing the switch back into the system.