An Overview
Pressure calibration is one of the important processes in instrumentation
and control engineering, and it makes sure that the field instruments,
including pressure gauges, transmitters, and sensors, are used to take
measurements accurately and reliably. The process simply compares the output of
the instrument being tested against a reference standard whose pressuren is known and very precise.
Why Pressure Calibration is Important
Accuracy: Ensures that the instrument measures pressure correctly,
which is vital for process control and safety.
Compliance: Meets industry standards and regulatory requirements.
Performance: Improves the efficiency and reliability of process
operations.
Safety: Reduces the risk of accidents caused by incorrect pressure
readings.
Steps for Pressure Calibration
1. Preparation
Check Specifications: Review the instrument's operating range, accuracy,
and calibration frequency as per manufacturer guidelines or process
requirements.
Choose a Standard: Utilize a pressure calibrator or deadweight tester with
an accuracy higher than the device under test.
Environmental Requirements: The temperature, humidity, and vibration
should be stable so that there is no error.
2. Preparing Equipment
Isolate: The field instrument is removed from the process for the purpose
of eliminating interference.
Connect:
Instrument: It is connected to the pressure calibrator through suitable
fittings and tubing.
Teflon tape or thread sealant is used to prevent leaks.
Bleed Air: Make sure that all lines contain no air for correct adjustment.
3. Calibration Process
Zero Adjustment:
Inflate the instrument to pressure and zero adjust when allowed.
Apply Pressure:
Step all known pressure increments up with the calibrator at least to 0%,
25%, 50%, 75%, 100% of the range, respectively.
Record the data produced by the calibrator and DUT.
Hysteresis Check:
Decrease pressure incrementally to 0%, recording data to establish
potential hysteresis present.
Repeatability Test:
Repeat the calibration procedure several times to achieve stable results.
4. Compensation (if necessary)
Compensating Correction: Modify the device settings when measurements are
not in the expected range.
Span Compensating Correction: Calibrate the high end of the measuring
range to match the accepted value.
5. Report
Calibration Certificate:
Note the instrument identification, date of calibration, information
regarding the reference standard used, and measurements.
Traceability: Verify that the reference standard is traceable to a
well-known authority such as NIST or NABL.
Static Pressure Calibration:
For devices measuring steady pressures, such as pressure gauges and
transmitters.
Dynamic Pressure Calibration:
For devices used in fluctuating pressure environments, such as
piezoelectric sensors.
Differential Pressure Calibration:
For devices measuring the difference between two pressure points.
Common Errors in Pressure Calibration
Connection Leaks: Can result in wrong readings.
Temperature Variations: The reference standard and DUT are affected
differently.
Poor Setup: Misalignment or poor fittings can distort results.
Instrument Drift: Older instruments do not hold calibration for a long
time.
Conclusion
Pressure calibration ensures the reliability and accuracy of field
instruments, forming the backbone of process control systems. Proper
calibration procedures, high-quality standards, and detailed records are
essential to achieve optimal performance and compliance.
