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INSTRUMENTATION TOOLS
Pressure Switch Calibration Procedure
are commonly used in the process industry for a wide
range of applications. A pressure switch is a form of sensor that closes or
opens an electrical contact when a certain pressure has been obtained either
through a pressure rise or a pressure drop.
Pressure switches
Pressure switches are used to monitor, control, or provide a caution or warning
for a pressure related process. The repeatability, accuracy, and functionality of
a pressure switch often tie directly into the saftey or efficiency of a process and
thus it becomes important that pressure switches are verified and calibrated to
ensure their proper function in the process.
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There are several terms that describe the function of a switch which need to be
understood when testing a pressure switch:
The pressure which the switch will change state.Set point:
The state of the switch when at barometric pressure or ideal
state.
Normal State:
Depends on Pressure switch type.(typically it would be either OPEN or
CLOSED).
The pressure at which the switch resets back to the normal state.Reset:
The allowable variation from the set point pressure.Tolerance:
The closeness of agreement between the results of
consecutive measurements.
Repeatability:
The pressure difference between the change-of-states.Dead Band:
(i.e. OPEN to CLOSE or CLOSE to OPEN).
This is the direction to which the change-of state should happen.Trip Type:
If the trip type is low, this means the change-of-state happens when the
pressure is falling.
If the trip type is high, this means the change-of-state happens when the
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pressure is rising.
Calibrating or Verifying a Pressure Switch
Before implementing the steps, you need to confirm the and the
of the Pressure switch. In addition, The pressure switch must be
depressurized and isolated.
setpoint
dead-band
Pressure Switch Calibration Procedure
, so we will need
to test when the switch changes between open to close. Next, we’ll test the
reset pressure (from closed to open) and lastly, repeat the test at least once todetermine repeatability.
Consider our Pressure Switch is a normally open switch
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Required Tools:
1. we must use a pressure calibrator or Multimeter (DMM) to determine the current
state of the switch.
2. we need to have a
pressure generation device (preferably a hand pump ).
3. we will need a way to measure the true
pressure using a pressure calibrator or Pressure gauge.
Calibration Procedure
1. First we connect the pressure gauge and Pressure switch to the hand pump as
shown in above figure.
2. Then we need to measure the present status (NO/NC) of electrical output of the
switch with the multimeter.Put multimeter in Continuity mode.
3. Note: Here in this example our pressure switch considered is Normally open type.So when you connect the multimeter before calibration we have default Normally
Open (NO) signal as output.
4. With our connections complete, we are ready to
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perform the calibration. We do this by increasing pressure until we detect a change
in the switch.
5. Now increase the pressure with the help of hand pump.
6. When we see the switch change state from open to close, we record the result.
7. Next, we
decrease the pressure till we see the switch reset (change from close to open) and
record the results. Also record the dead band.
8. Lastly, we would
repeat this test at least once to determine repeatability. The difference between set
point and rest pressures is the dead
band.The dead band must be within the range. The deadband & setpoint of
pressure switch available on nameplate of pressure switch.
For Normally closed Pressure switch calibration also follows the same above
mentioned procedure. But the default status of Normally closed type Pressure
switch will have N.C. signal as output. When testing with increasing pressure
the output state changes from N.C. to N.O and while on decreasing pressure
the output state changes from N.O. to N.C. Remaining all steps are same.
Thats it. very simple.
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POSTED BY S BHARADWAJ REDDYInstrumentation Professional Working in a reputed Oil & Gas
Company.
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