Calibrating Differential Pressure Switches: A Guide

How to Calibrate and Adjust a Differential Pressure Switch
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.
I.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
Calibration Steps
Step 1:
Connect the variable pressure source to a test gauge and the HI side pressure port of switch
Step 2:
Connect the test lamp or multimeter (set to the ohmmeter setting) across C-Common and NO-Normal
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
II.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
Calibration steps
Step 1:
Connect the continuity test lamp or digital multimeter across the C-Common and NO-Normally Open
switching element/contacts
Step 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:
Keep HI side pressure steady, slightly open the LO side bleed valve to reduce the LO side pressure
(increase diffrential pressure) until desired diffrential pressure set point appears on diffrential pressure
gauge. Close bleed valve to stablize 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:
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
Step 1:
Connect the continuity test lamp or digital multimeter across the C-Common and NO-Normally Open
switching element/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.

Step 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:
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 are Open – Set point too High: If contacts are open when deecreasing 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.

Basics of The 4 - 20mA Current Loop
The 4-20mA current loop is a very robust and popular sensor signalling standard. Current loops are ideal
for data transmission because of their inherent insensitivity to electrical noise. In a 4-20mA current loop,
all the signalling current flows through all devices. All the devices in the loop drop voltage due to the
signal current flowing through them. The signalling current is not affected by these voltage drops as
long as the power supply voltage is greater than the sum of the voltage drops around the loop at the
maximum signal current of 20mA
As shown in the diagram above, current supplied from the power supply flows through the loop wires
with resistance, RW, to the trasmitter and the 4-20mA transmitter regulates the current flow within the
loop. The current allowed by the transmitter is called the loop current and it is proportional to the
parameter that is being measured. The loop current flows back to the controller through the wire, and
then flow through resistor, R, to ground and returns to the power supply. The current flowing through R
produces a voltage that is easily measured by the analog input of a controller. For a 250 ohm resistor, the
voltage will be 1VDC at 4mA and 5VDC at 20mA.
As the diagram above showns, there are four basic components in the 4-20mA current loop namely:
a. The power supply
b. The 2-Wire transmitter
c. A receiver resistor, R that converts the loop current into a voltage
d. The loop wires that interconnects all devices or components in the loop.