How to Calibrate and Adjust a Differential Pressure Switch ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Calibrate and Adjust a Differential Pressure Switch

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Just like pressure switches, a differential pressure switch can be calibrated to a known set point. You can do a quick calibration of a differential pressure switch the same way you calibrate a pressure switch. However for more accurate calibrate, the procedure is slightly modified.
Before going through the calibration steps please read: How a Differential Pressure Switch Works

Quick Calibration Procedure for a Differential Pressure Switch
Equipment required includes:
(a) A variable pressure source
(b) A digital multimeter or continuity test lamp
(c) A test gauge
Calibration Equipment Setup
Calibrating a Differential Pressure Switch
Step 1
Connect the variable pressure source to a test gauge and the HI side pressure port of switch.

Step 2
Connect the test light or multimeter (set to the ohmmeter setting) across C – Common and NO – Normally Open switching element contacts as shown above

Step 3
Raise pressure and note test gauge reading when circuit closes.

Step 4
Slowly drop pressure and note test gauge reading when circuit opens.

Step 5
Adjust set point screw to increase or  decrease set point.

Step 6
Repeat Steps 3, 4 and 5 until contacts change at desired increasing or decreasing
differential pressure set point.

Accurate Calibration Procedure
To accurately calibrate a differential pressure switch, we need to simulate the required service conditions.

Equipment required for this calibration process are:
(a) Differential pressure gauge (preferably a digital type)
(b) Variable pressure source
(c) Block, bleed and equalizer valves
(d) Continuity test lamp or digital multimeter

First determine whether the set point occurs on increasing or decreasing differential pressure and calibrate using either of: 
(a) Set point on Increasing Differential Pressure
(b) Set point on Decreasing Differential Pressure

Calibration Steps for Set point on Increasing Differential Pressure:
Connect the setup as shown below and then proceed with the steps
Calibrating a Differential Pressure Switch on Increasing or Decreasing Differential Pressure
Calibration Steps
Steps 1
Connect the continuity test lamp or digital multimeter across the C - Common and NO -Normally Open switching element / contacts.

Steps 2
Close the bleed valves, open the equalizer valve and raise pressure equally on both HI and LO sides to the static pressure that the differential pressure switch will see under normal operating conditions.

Step 3
With static pressure stable, close the equalizer valve to isolate the HI side from the LO side.

Step 4
Keeping HI side pressure steady, slightly open the LO side bleed valve to reduce the LO side pressure (increase differential pressure) until desired differential pressure set point appears on differential pressure gauge. Close bleed valve to stabilize differential pressure. Check the status of the electrical contacts against the following possible scenarios and follow the instructions that match the status of the contact:
Different set point conditions during calibration - Increasing Differential Pressure
(a) Set point is Okay if (see diagram above):
Contacts make precisely at increasing differential pressure set point, repeat Steps
2 – 4 as desired to verify calibration. Calibration is complete.
(b) Contacts are Open – Set Point Too High
If contacts are open when increasing differential pressure is reached, adjust set point screw until contacts make. Repeat Steps 2 – 4
(c) Contacts Closed – Set point Too Low
If contacts are closed when increasing differential pressure is reached, adjust set point screw until contacts break. From this point, adjust set point again until contacts make. Repeat Steps 2 - 4

Calibration Steps for Set point on Decreasing Differential Pressure:
Steps 1
Connect the continuity test lamp or digital multimeter across the C - Common and NO – Normally Open contacts.

Step 2
Close the bleed valves, open the equalizer valve, and raise pressure equally on both HI and LO sides to the normal operating static pressure.

Step 3
With normal HI side pressure stable, close the equalizer valve to isolate the HI side from the LO side.

Step 4
Slightly open the LO side bleed valve to reduce LO side pressure (increase differential pressure) until the normal operating differential pressure appears on the differential pressure gauge. Close the bleed valve to stabilize differential pressure. Contacts should close (make) by the time normal operating differential pressure is reached. If the contacts are still open at normal operating differential pressure, adjust the set point screw until the contacts make.

Steps 5
Keeping the HI side pressure steady, slightly open the equalizer valve to increase LO side pressure (decrease differential pressure) until the desired differential pressure set point appears on the differential pressure gauge. Close the equalizer valve to stabilize differential pressure. Check the status of the electrical contacts against the following differential pressure scenarios and follow the instructions that match the status of the contacts:
Different setpoint conditions during calibration - Decreasing differential pressue
(a) Set point is Okay if (see diagram above):
Contacts break precisely at decreasing differential pressure set point, repeat steps
2 - 5 as desired to verify calibration. Calibration is complete.
(b) Contacts Open – Set Point Too High
If contacts are open when decreasing differential pressure is reached, adjust set point screw until contacts make. From this point, adjust set point screw again until contacts break. Repeat Steps 2 – 5
(c) Contacts Closed – Set point Too Low
If contacts are closed when decreasing differential pressure is reached, adjust set point screw until contacts break. Repeat Steps 2 – 5