Pressure Calibration of Field Instruments

 

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.