Air Compressor pressure switch

The pressure switch is an important part on your air compressor, but is also a part that will fail quite often. The pressure switch tells your compressor when to stop and when to start.

The air compressor pressure switch on small and older compressor is a completely mechanical device. With two set-screws you set the upper and lower pressure. Often, there is a blow-down valve (pressure relieve valve) integrated, which will relieve the pressure on the compressors discharge pipe.

Typical air compressor pressure switch

Setting the pressure switch:

On 99% of the pressure switches, the pressure is set as maximum pressure and differential pressure. First you set the maximum pressure, then you set the differential pressure. If you set the maximum pressure at 7,5 bar and the differential pressure at 1 bar, the compressor will start at 6,5 (7,5-1) bar and stop again at 7,5 bar. Always set the pressure switch when there is pressure on it, otherwise you might damage it. Step-by-step:

Start with an empty air tank (0 bar) and turned-off compressor.

1. Turn the pressure setpoint screw completely to "+" (maximum pressure). Turn the differential screw also completely to "+" (maximum differential pressure = lower setpoint).
2. Start the compressor.
3. When the compressor reaches your desired pressure, quickly turn the pressure setpoint to "-" until the compressor stops. Danger: be careful not to let the pressure rise too high!
4. The compressor is now stopped with maximum pressure in the air receiver.
5. Open some valve to slowly drain the compressed air.
6. When the minimum required pressure is reached, quickly turn the differential screw to "-" until the compressor starts up.
7. Let it go back-and-forth a few times (start compressor, stop, drain air, start compressor etc) to fine-tune the settings.

We have now successfully set the pressure switch. The procedure can be a different depending on your air compressor pressure switch type. Always check the manual!

On / Off switch

On some types of air compressor pressure switches, there is also an on / off button or buttons. This is basically an all-in-one solution for small compressors. When you press 'on' it will automatically start and stop te compressor according to the pressure setpoints. When you press 'off' it will ignore the setpoints and shut down the compressor.

Buying an air compressor pressure switch

When your pressure switch is broken, it�??s best to get a new one that is identical to the old one. If this is not possible, look for the following:

• Pressure band: the minimum and maximum pressure that you can set. Some pressure switches might only work from 0 to 3 bars, others might work from 3 to 10 bars. Get one that�??s right for you. Most compressors will work around 6 to 7 bar. Also, make sure the delta-P (pressure differential) suits your needs. This is the minimum differential pressure that can be set (for example, to start at 6 bar and stop at 7 is 1 bar differential pressure).
• Mechanical connection: You need some way to connect the pressure switch to the air receiver. Your old pressure switch used a particular connection (thread, diameter). Buy a pressure switch with the same kind of connection.
• Power rating: Some pressure switches are used to directly switch on and off the electro motor. Make sure the power rating (amps, or kilowatts/horsepower) is big enough for your motor. Other compressors may use an auxiliary relay to switch on/off the motor.
• Integrated blow-down valve: On the smaller (home workshop / garage) reciprocating piston compressors, the blow-down valve is usually integrated in the pressure switch. The blow-down valve will blow off any pressure in the pipe between the compressor and the receiver when the compressor stops. This is to make sure that the compressor can easily start (the electromotor is not strong enough to push the piston against the pressure at start).

Pressure sensor

On newer and bigger machines, there is no old-fashioned mechanical pressure switch. It is replaced by a pressure-sensor. The sensor tells the central controller the current pressure. When the pressure drops below of above the setpoint, the controller takes action (start or stop the compressor). The same sensor value is used to display the pressure on the LCD screen. The setpoint can be changed on the central controller (using the menu).