3.3 Calibration & Maintenance of Pressure & Temperature Transmitters

Calibration & Maintenance of Pressure & Temperature Transmitters

Calibration is the process of comparing a measuring instrument against a known, accurate standard to determine its deviation and, if necessary, adjusting it to bring it within acceptable tolerances. For CCST Level I technicians, performing accurate calibrations on pressure and temperature transmitters is a core competency that ensures process safety, product quality, and regulatory compliance.

Calibration Test Equipment

Calibration equipment must meet the uncertainty, traceability, range, compatibility, and environmental requirements in the approved procedure. A 4:1 test-uncertainty ratio is a common planning goal, not an automatic rule; when it cannot be achieved, the procedure must account for measurement uncertainty and any required guard band. In the United States, traceability is often established through NIST, but an unbroken documented chain to the applicable SI realization or recognized national metrology institute is the governing concept.

Pressure Calibration Standards

1. Deadweight Tester: This is considered a primary standard for pressure calibration. It operates on the fundamental definition of pressure ($P = F / A$). It consists of a precise hydraulic piston of known cross-sectional area loaded with calibrated metal masses (weights). By operating the piston/masses at the specified float condition, a pressure derived from calibrated mass, effective piston area, local gravity, fluid head, temperature, and other corrections is applied to the device under test. Deadweight testers are highly accurate but are generally confined to laboratory or bench use due to their weight and sensitivity to levelness and gravity.
2. Pneumatic Hand Pump: For field calibration of low to medium pressure pneumatic instruments or DP cells, a hand-operated squeeze pump is used to generate pressure. It is coupled with a precision digital pressure gauge (the standard) to verify the applied pressure.
3. Hydraulic Test Rig: For high-pressure field calibrations, hydraulic pumps (using water or oil) generate the necessary pressure, which is again referenced against a high-accuracy digital test gauge.

Temperature Calibration Standards

1. Temperature Dry-Well Calibrator: A dry-well (or dry-block) calibrator is a portable field device that precisely heats or cools a metal block. The block has drilled holes where the technician inserts both a certified reference temperature probe and the RTD or thermocouple being tested. The built-in controller stabilizes the block at specific temperature setpoints.
2. Precision Multimeter / Communicator: In many cases, it is impractical to remove a temperature sensor from the process. Instead, technicians calibrate the transmitter by disconnecting the sensor and using a precision loop calibrator or HART communicator to inject a simulated millivolt (for thermocouples) or resistance (for RTDs) signal corresponding to specific temperatures.

The 5-Point Calibration Procedure

A common industrial procedure uses a five-point check to evaluate performance across the calibrated range at 0%, 25%, 50%, 75%, and 100%. The approved calibration procedure and device specification determine the actual points, repetitions, direction, tolerance, and uncertainty treatment.

Ascending and Descending Checks

To thoroughly test the instrument, the pressures or temperatures are applied in an ascending order (0% up to 100%) and then in a descending order (100% back down to 0%).

• Linearity is verified by ensuring the transmitter output forms a straight line relative to the input across all 5 points.
• Hysteresis is checked by comparing the ascending readings to the descending readings. If the transmitter outputs 12.0 mA at 50% pressure while going up, but 12.2 mA at 50% pressure while coming down, the device exhibits hysteresis (mechanical memory or lag), often caused by worn elastic elements or linkages.

As-Found vs. As-Left Data

Documentation is a critical aspect of calibration.

• As-Found Data: Before any adjustments are made, the technician performs the 5-point check and records the readings. This documents how the instrument was performing while it was in service. If the as-found data is wildly out of tolerance, a root cause analysis might be required to determine if product quality was compromised.
• As-Left Data: If the instrument was found to be out of tolerance, the technician adjusts it. After adjustment, another 5-point check is performed. This new data is the "As-Left" reading, proving the instrument was returned to service operating correctly within acceptable limits.

Bench Calibration vs. Field Calibration

Bench Calibration is performed in the instrument shop. The environment is clean, temperature-controlled, and vibration-free. Bench calibration is ideal for initial configuration of new transmitters, extensive repairs, or when highly precise primary standards (like deadweight testers) are required.

Field Calibration is performed out in the plant, with the instrument remaining mounted on the pipe or vessel. The process must be isolated (using block and bleed valves or manifolds), but the instrument is not removed. Field calibration is faster and subjects the instrument to its actual ambient operating conditions (vibration, mounting position, ambient temperature), but it can be more challenging due to weather, accessibility, and the limitations of portable calibration gear.

Zero and Span Adjustments

When a transmitter is out of calibration, two primary adjustments bring it back into alignment:

1. Zero Adjustment: Shifts the entire calibration curve up or down. If a pressure transmitter is supposed to read 4 mA at 0 psi but actually reads 4.5 mA, adjusting the Zero will bring it down to 4.0 mA. Crucially, a zero adjustment shifts all other points equally (e.g., the 100% point will also drop by 0.5 mA).
2. Span Adjustment: Changes the slope of the calibration curve. Span represents the difference between the upper range value and the lower range value. Adjusting the span acts as a multiplier, stretching or compressing the output range.

Conventional interacting adjustments: Follow the manufacturer procedure; many analog instruments call for zero, then span, followed by repeated endpoint and intermediate checks because the adjustments interact. Modern smart transmitters separate these functions electronically (Sensor Trim vs. Output Trim), minimizing interaction, but the principle of checking 0% and 100% repeatedly remains valid.

Summary Table: Calibration Concepts