Calibration of pressure measuring instruments


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Pressure instrument calibration is the process of adjusting the instrument’s output signal to match a known range of pressures. Proper calibration provides the desired beginning and ending pressures, and produces an output signal that is proportional to the process pressure.

Calibration of the instrument is carried out by applying to it an air or liquid pressure whose value is accurately known. The method used depends upon the range of the instrument.

Calibration of Low-Pressure Gauges:

For very low pressures, up to 3 inches of water gauge, an inclined water gauge may be used. For pressure up to 144 in. of water gauge or 12 in. of mercury gauge, a water manometer may be used,
although at the upper part of this range a mercury manometer becomes more manageable.

For higher pressures up to 72 in. mercury gauge, a mercury manometer may be used. For still higher pressures, a dead-weight piston tester or hydraulic pumps must be used.

Calibration of Pressure Transmitters:

Typical tools required:

  • 24 VDC power supply

  • Multimeter digital

  • Pneumatic hand pump (up to 600 psig) / Hydraulic hand pump (up to 10.000 psig) / Low pressure hand pump / High precision digital test gauge

  • HART communicator

  • Screwdriver toolkit

Procedure:

Isolate the pressure transmitter and bleed off pressure as per the site Lockout/Tagout Procedure.

Do not perform any adjustments until all “as found” calibration data has been recorded. Note any deviation from this procedure in the remarks section of the calibration form.

  • Make connections between the pressure source, pressure standard, and the pressure transmitter sensing line.

  • Connect the digital multimeter to the current output of the transmitter.

  • Turn on the transmitter and allow the internal components to reach normal operating temperature

  • Using a dead-weight tester or a portable regulated air supply, exercise the transmitter from zero to full scale and back to zero.

  • Adjust the pressure (vacuum) source to 0, 25, 50, 75,and 100% of the calibration range and record the following at each data point
    a. Digital pressure standard reading
    b. Current output

  • Adjust the zero to set exactly 4mA output, indicated on a precision meter.

  • Supply 100% pressure to the instrument.

  • Adjust the span to get a reading of exactly 20 mA.

  • Repeat steps 6 to 8 till no adjustment is required.

  • Apply 50% pressure (half-scale input pressure).

  • Adjust the linearity to bring the output signal within the specified tolerance of 12 mA (halfscale
    output current).

In most instruments, adjusting the span may throw off the zero adjustment or vice-versa. Therefore, it needs to repeat the entire calibration procedure several times to achieve a properly balanced calibration.

-Disconnect the test equipment from the switch under test.

-Complete the calibration form and affix the applicable calibration label.

-If all checks are within tolerance, restore the pressure switch to operational condition and notify
appropriate personnel of work performed