Calibration of Pressure Switches


#1

A pressure switch is a form of sensor that closes or opens an electrical contact when a certain pressure is obtained through an increase in pressure or a pressure drop. Before knowing how to calibrate and adjust a pressure switch, let’s look at some basics with pressure switch calibration:

Setpoint:

This is the pressure at which the pressure switch is required to operate.A pressure switch can be configured to operate with a rising pressure (high-level alarm) or a down pressure (low-level alarm). Most switches are designed to operate with a “pressure gauge” pressure set point, that is, in relation to atmospheric pressure.

Some applications require an ‘absolute’ pressure set point, that is, relative to the absolute zero pressure, and for this, an absolute pressure switch is required. Ideally, the switch range should be chosen so that the setpoint is between 25% and 75% of this range.

Dead-band or Reset::

This is a configuration that determines the amount of pressure change required to reset the switch to its normal state after it has been triggered. The dead band, the reset or the switching differential is the difference between the rising and falling pressures at which the pressure switch operates.

In the case of a fixed differential output switch, this is usually about 1% to 3% of the switch range. For an adjustable differential output switch, it can be adjusted from approximately 5% to 12% of the switch range.

Trip Type:

This is the address to which the change of state should occur.

If the type of shot is low, this means that the change of state occurs when the pressure is falling.

If the type of shot is high, this means that the change of state occurs when the pressure increases.

Set-Up before calibrating the Pressure switch:

  • The setpoint of the pressure switch

  • The dead-band of the switch

  • Depressurize and isolate the pressure switch from the process

Calibration Procedure:

Required tools:

  • Pressure calibrator or Multimeter (DMM) to determine the current state of the switch, Pressure generation device (Handpump), pressure calibrator or Pressure gauge to measure the pressure.

Procedure:

  • Connect the pressure switch to a pressure source, for example, the air supply through a manual pressure regulator and a test meter, as shown in the diagram above.

  • Use an ohmmeter or digital multimeter (DMM) configured for the continuity range to verify and verify that the switch contacts are as follows: NO (normally open) and NC (normally closed).

  • Connect the ohmmeter or DMM between the normally open contacts (NO) and the common terminal © of the switch. The meter should read “open circuit”. Set the manual pressure regulator to increase the pressure to the set point of the pressure switch until the contacts change. The meter should now read “short circuit”. Write down the pressure reading and write it down. This pressure is the switch set point for an “up” pressure.

  • Increase the pressure to the switch to its maximum value. Slowly reduce the switch pressure until the switch changes from closed to normally open again. Write down and record this pressure reading. This pressure is the switch setting for a “down” pressure.

  • From the readings you have taken, solve the pressure difference between the ascending and descending pressure settings. This is called the “dead band” of the switch. The dead band calculated must be equal to or less than the dead band of the manufacturer.

The maximum deadband is generally declared by the manufacturer. The switch can not be used if the maximum dead band is greater than the manufacturer’s recommendation (dead band on the switch nameplate).

To calibrate the switch for a low pressure alarm, go through the steps in this order: Step 1 to Step 2 to Step 4 to Step 3 to Step 5