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Calibration of Pressure Gauge
Following procedure should be followed for calibration of pressure gauges.
- Pressure gauges shall be checked by means of Hydraulic Pressure Gauge Comparator. This is a means of testing a gauge against a Standard Pressure Gauge.
- The gauge Comparator shall be firmly fixed to a bench. A test gauge of range comparable to the gauge under test is fitted to one branch of the comparator. The gauge to be tested is fitted to the other branch and the hand pump on comparator in order to check the gauge against the readings of the test gauge.
- Readings shall be checked for pressures corresponding to 0%, 25%, 50%, 75% and 100% of the range of the gauge under test. Actual gauge reading shall be noted for both rising and falling signals.
- Test gauges shall have accuracy better than 0.25% of full scale and shall be periodically checked per Standard specification for accuracy against a dead-weight tester.
- After the test has been completed, the instrument shall be identified with a color code sticker.
- Results shall be filed and recorded on applicable forms.
Pressure Gauge Calibration Using Deadweight Tester
How deadweight tester works?
The pressure which is produced in deadweight tester through capstan operated hydraulic ram and hydraulic fluid is oil, which is balanced by dead weight acting upon piston of known area.
Set up the equipment’s needed for calibration. Know first the maximum capacity of the deadweight tester whether it can accommodate the range of the gauge to be calibrated.
Please read also: Pressure switch calibration and I to P Converter
- A spirit level and adjustable feet shall be provided to enable the operator to level the instrument. A flotation indicator mounted on the top plate removes the guesswork when floating the piston.
- Prepare the manual and data sheet that will be used for details of the instrument to be calibrated. Some information needed for calibration to be inputted to data sheet are:
- a) Equipment name/ID no.
- b) Ambient Temperature
- c) Relative Humidity
- d) Date of Calibration
- e) Date of Next Calibration
- Wrap the gauge fitting with Teflon tape (clockwise direction).
- Pace the gauge to be tested to the manifold assembly (with gauge fitting match to the manifold joint). Use correct size of wrench, and make sure there is no leak.
- If the gauge to be tested has for example, 0 to 100 psi range, use 5-point scale as reading reference. Other gauges may have higher ranges. This rule holds to kinds of pressure gauges.
- 5 POINTS OF CALIBRATION
|
PERCENT(%)
|
EQUIVALENT(psi)
|
|
0%
|
0 psi
|
|
25%
|
25 psi
|
|
50%
|
50 psi
|
|
75%
|
75 psi
|
|
100%
|
100 psi
|
0% – low pressure (zero reading) 100% – high pressure (Full scale reading). The pointer’s initial reading shall be 0 psi. If not, zero adjustment shall be made. Place weights on the cylinder corresponding to each point scale of the pressure gauge. Increase the actuating pressure equivalent to 0%, 25%, 50%, 75% and 100% of the assigned range.
- Tightly close the release valve by turning it clockwise.
- Pump gently until its weights floats to an ideal level.
- Rotate weights clockwise very gently and continuously while pumping.
- If cylinder base exceeds above or below the ideal level, rotate fine pressure adjustment lever.
- Record the pointer indication values for each point-scale. This is the “AS-FOUND” data. a) Upscale (0%, 25%, 50%, 75% and 100%) b) Downscale (100%,75%, 50%, 25% and 0%).
- If the “AS-FOUND” data exceeds the required accuracy, calibration shall be done. A zero or span adjustment shall be made. h) Zero adjustment – minimum scale value i) Span Adjustment – maximum scale value SPAN= Maximum Range-Minimum range.
- Repeat the procedure until the readings for each point scale is within the specified limits of accuracy.
- Record the readings for each point-scale and this is the “AS-LEFT” data.
- When done, release the built system pressure by opening the release valve.
- After the test has been completed, the instrument shall be identified with a color code sticker.
Results shall be filed and recorded on applicable forms.
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