Pressure transducer & RTD verification
Verify and correct compressor package instrumentation — pressure transducers, RTDs, thermistors, and ΔP sensors.
This guide is general field reference for trained, qualified service technicians. It is not a substitute for the original equipment manufacturer's service manual, applicable codes (NEC, OSHA, ASME, local), or the judgment of a licensed professional. Procedures, torque values, pressures, refrigerants, and safety requirements vary by make, model, jurisdiction, and revision. Always verify against current OEM documentation and follow your employer's safety program. Working on compressed-air, electrical, pressure-vessel, and refrigerant systems can cause serious injury or death. By using this guide you accept all risk; AirCompDX and its authors disclaim all warranties and are not liable for any damage, injury, loss, or code violation arising from its use.
Removing a transducer from a live line releases pressure at the fitting. Isolate and vent before breaking any instrument connection. Control circuits may remain live with the main disconnect open — verify.
- Calibrated reference pressure gauge or hand pump with test gauge
- Precision decade box or RTD simulator
- True-RMS multimeter with mA range and a loop calibrator
- Ice bath and boiling water for two-point temperature checks
- Small flat screwdriver for terminal blocks
- Replacement transducer matched to range and output (4–20 mA vs 0–5 V)
- 100 Ω platinum RTD or OEM thermistor
- Sealing washers / bonded seals
- 1Compare against a reference
Tee a calibrated gauge into the discharge line and compare with the controller display at two or three pressures across the operating band. More than 2% of span error is worth correcting.
- 2Check the loop, not just the sensor
Measure the raw signal at the sensor and again at the controller input. A difference means wiring, a shield ground loop, or a failing analog input — not a bad sensor.
- 3Verify the 4–20 mA zero and span
At 0 psi the loop should read 4.00 mA; at full range, 20.00 mA. Drifted zero with correct span usually means a strained diaphragm from an overpressure event.
- 4Check RTDs cold and hot
A 100 Ω platinum RTD reads 100.0 Ω at 32°F and about 138.5 Ω at 212°F. Substitute a decade box at the controller input to prove whether the error is in the sensor or the controller scaling.
- 5Verify separator ΔP
Compare the controller's separator ΔP with a manual gauge reading across the element. A false-high ΔP has sent many good separators to the trash.
- 6Correct the scaling
Apply the controller's offset/span correction rather than replacing a good sensor. Record the applied offset in the maintenance log — an uncorrected offset later looks like a new fault.
- 7Confirm alarm setpoints
After correcting scaling, re-verify high-temperature, low-oil-pressure, and high-pressure trip setpoints. Calibrating a sensor without re-checking trips can leave a machine unprotected.