Pressure Calibration – Field Procedure

Pressure transmitter calibration must always be performed using proper isolation, equalization, and safe handling practices. Most calibration errors occur due to incorrect manifold operation.

Before Starting Calibration

• Obtain work permit if required
• Verify process isolation
• Confirm manifold valve condition
• Use calibrated pressure source (dead weight tester / pump)
• Verify loop power (24V DC)

Never attempt calibration without proper isolation.

Step 1 – Safe Isolation

1. Close HP and LP isolation valves
2. Open equalizing valve
3. Ensure transmitter reading drops to zero
4. Vent process pressure safely

Always equalize before disconnecting any impulse line.

Step 2 – Zero Check

• With equalizing valve open, output should be 4 mA (for 0% LRV)
• If offset exists, perform zero trim
• Allow signal to stabilize before adjustment

For DP transmitters, zero shift is common after shutdown.

Step 3 – Span Calibration

1. Close equalizing valve
2. Apply known pressure using calibration pump
3. Verify output matches expected mA value
4. Adjust span if required

Example: If range is 0–10 bar, applying 10 bar should produce 20 mA.

Step 4 – Linearity Check

• Apply 25%, 50%, 75%, and 100% test points
• Record actual vs expected output
• Ensure error within acceptable tolerance

Always increase and decrease pressure to check hysteresis.

Common Calibration Mistakes

• Adjusting zero without equalizing first
• Leaving equalizing valve open during span test
• Using uncalibrated pressure source
• Ignoring temperature effects
• Not verifying DCS scaling

After Calibration – Restoration

1. Open equalizing valve
2. Slowly open HP and LP isolation valves
3. Close equalizing valve
4. Verify stable reading
5. Confirm DCS value matches field reading

Sudden valve opening can damage sensor diaphragm.

Field Reality

Many transmitters are replaced unnecessarily. In most cases, incorrect manifold handling or impulse imbalance is the true cause of calibration failure.