Equipment Required:

·         Pressure Source

·         Accurate Gauge

·         Microswitch contact monitoring device.

·         For high-pressure applications Hand pump is more suitable.

·         For low-pressure shop air or simple regulated pressure can be used or an electronic pressure source can be used.

·         Pressure should have an accuracy of 0.25 %.

 




Procedure of the Pressure Switch Bench Calibration

·         Connect the pressure switch to a hand pressure regulator and test gauge, as shown in the diagram above.

·         With a Multimeter, set to the continuity range to check and verify that the switch contacts are as indicated: NO (Normally open) and NC (Normally closed).

·         Connect the Ohmmeter or DMM between the normally open contacts (NO) and the common terminal (C) of the switch. The meter should read ―open circuit‖. Adjust the hand pressure regulator to increase the pressure to the set point of the pressure switch until the contacts change over. The meter should now read ―short circuit‖. Note the pressure reading and write it down. This pressure is the switch set point for a ―rising‖ pressure.

·         Increase the pressure to 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 and write down this pressure reading. This pressure is the switch setting for a ―falling‖ pressure.

·         When error found, adjust set point by screw adjustment and repeat until desired value obtained.

·         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 dead-band calculated should be equal to or less than the manufacturers‘ dead-band. The maximum dead- band is usually stated by the manufacturer.


Following information must be present in Pressure Switch Calibration Certificate:

·         Unique instrument identifier .E.g.: P&ID Tag.

·         Mfg., Model, Serial Number, Location.

·         Set point with units of pressure

·         Reset point (if necessary) or some way to indicate ―trip on increasing input‖

·         NO or NC contacts

·         Tolerance with units of measure

·         Test equipment used with unique identifier and next due date

·         As found and As left data

·         Results

Technician signature / Date