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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.

    Also Read: Alarm Simulator using basic gates

    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

    Also Read : How to convert current 4-20mA to Percentage

    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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