How to calibrate a Pressure switch?

instrumentation

#1

##Pressure switch:

Pressure switches are devices that are configured to sense a change in pressure and respond in a specified manner.

pressure switch

The inlet pressure is applied to the bottom of the operating piston. This piston is forced upwards by the inlet pressure against the range spring. The tension of the range spring can be adjusted so that it is compressed at a certain pressure. When this pressure is reached the operating pin will hit the trip button on the micro-switch and change it over. The normally open contacts (NO to C) will become closed and the normally closed contacts (NC to C) will open.

The pressure at which the micro-switch changes over is set by adjusting the trip setting nut. This nut adjusts the tension of the range spring

##Calibration Procedure:

  • Connect the pressure switch to the air supply via a hand pressure regulator and test gauge, as shown in the diagram.

  • Use an Ohmmeter to check that the switch contacts are as indicated; NO and NC.

  • Connect the Ohmmeter to the normally open contacts. The meter should read “open circuit”. Adjust the hand pressure regulator to increases the pressure to the switch until the contacts change over. The meter should now read “short circuit”. Note down the pressure reading on the sheet provided. This pressure is the switch setting for a “rising” pressure.

  • Increase the pressure on the switch to its maximum rating. Slowly reduce the pressure to the switch until the switch changes over from closed to normally open again. Note down this pressure reading. This pressure is the switch setting for a “falling” pressure.

  • From the readings you have taken work out the pressure difference between the rising and falling pressure settings. This is called the “dead band” of the switch.

  • The maximum dead band is usually stated by the manufacturer. The switch is unserviceable if the maximum dead band is more than the manufacturer’s recommendation.

  • We can clear the error by adjusting the trip setting nut.