Pneumatic Temperature Transmitter Commissioning and Field Fault Diagnosis

How Pneumatic Temperature Transmitters Work

A pneumatic temperature transmitter converts a temperature measurement into a proportional air pressure signal between 3 psi (lower range value) and 15 psi (upper range value). First, the sensing element — either an RTD (Pt100, 100 Ω) or a thermocouple (Type J or K) — produces a millivolt or resistance change. Second, an internal Wheatstone bridge circuit converts this into a mechanical beam deflection that positions a flapper plate relative to a nozzle. Third, the nozzle-flapper gap controls the backpressure in the output pneumatic circuit. Finally, a pneumatic relay amplifier converts the nozzle backpressure into a stable 3–15 psi output with 20 psi instrument air supply.

The nozzle orifice diameter is typically 0.010–0.015 inches. Contamination in the instrument air — oil droplets, rust particles, or moisture — can partially block the nozzle and cause output high bias. This is the most common field fault. Install a 5-micron coalescing filter at the transmitter air supply inlet and check the element during every scheduled maintenance visit.

Commissioning Procedure

• Step 1: Connect a calibrated pressure gauge (0–30 psi, 0.1% accuracy) to the transmitter output port. Connect instrument air supply at 20 psi ±0.5 psi. Apply the LRV temperature using a decade resistance box (e.g., 100.00 Ω for 0°C with Pt100 per IEC 60751 linearization).
• Step 2: Check the output. It should read 3.00 psi ±0.06 psi (±0.5% of span). Turn zero screw counter-clockwise if above 3.06 psi, clockwise if below 2.94 psi. Make quarter-turn adjustments and allow 30 seconds for stabilization.
• Step 3: Apply the URV resistance (e.g., 177.05 Ω for 200°C). Output should read 15.00 psi ±0.06 psi. Adjust the span screw. Clockwise increases output. Iterate zero and span adjustments until both endpoints are within ±0.06 psi.
• Step 4: Apply midpoint temperature (50% of range). Verify output reads 9.00 psi ±0.12 psi. A midpoint error exceeding 0.5 psi indicates nonlinearity in the flapper mechanism or worn pivot bearing — replace the transmitter.
• Step 5: Document as-found and as-left values on the calibration record, including supply pressure, ambient temperature, and sensing element resistance values. This satisfies the IEC 61511 proof test documentation requirement.

Integrating with Allen-Bradley ControlLogix and Invensys I/A Series

Allen-Bradley ControlLogix requires 4–20 mA input, so convert 3–15 psi using a P/I converter (Moore Industries SPA2 or Rototherm PT-I) configured for 3–15 psi input and 4–20 mA output. The conversion formula: mA = ((psi – 3) / 12) × 16 + 4. Configure the 1756-IF16 input module impedance at 250 Ω and set over-range alarm at 20.8 mA and under-range alarm at 3.8 mA.

For Invensys I/A Series FBM04, connect the P/I converter output to the FBM04 channel terminals. In Foxboro Control Software, configure the AI function block with HSCI and LSCI parameters for URV and LRV temperature values. Set ITYPE to 1 (4–20 mA mode). Use an isolation barrier (Phoenix Contact MCR-SL-CUR-I-I) if the two devices do not share a common signal ground — ground loop noise introduces 0.04–0.1 mA of error, translating to 0.5–1.25°C on a 200°C span.

Six Common Field Faults

• Fault 1 — Output Pegged High (15+ psi): Nozzle blocked by oil mist. Disconnect supply air and clean with dry nitrogen at 5 psi. Replace the supply filter element. If fault recurs within 90 days, install a desiccant dryer upstream.
• Fault 2 — Output Pegged Low (below 3 psi): Supply pressure has dropped below 18 psi. Check the regulator and filter differential pressure indicator. Replace the filter if differential exceeds 5 psi.
• Fault 3 — Output Hunting (±0.3 psi oscillation): Relay amplifier ball valve seat worn. Replace the relay assembly — do not attempt to lap the ball seat in the field.
• Fault 4 — Zero Drift After 6 Months: Spring metal fatigue in ambient temperatures above 60°C. Insulate the transmitter body. If zero drift rate exceeds 0.5% per month, shorten calibration interval to 6 months.
• Fault 5 — Cold Junction Compensation Error (thermocouple types): Ambient temperature changes above 20°C between seasons. Install a thermal enclosure or switch to an RTD element, which has no cold junction effect.
• Fault 6 — Nonlinear Mid-Span Output: Pivot bearing wear in the flapper mechanism. Zero and span calibrate correctly but midpoint error exceeds 1% of span. Replace the transmitter body — this mechanism is non-serviceable in the field.

Conclusion and Action Advice

Pneumatic temperature transmitters are reliable instruments when maintained correctly. First, always commission with a calibrated portable gauge — permanent gauges are not accurate enough for setpoint verification. Second, convert the 3–15 psi output to 4–20 mA using a calibrated P/I converter before connecting to Allen-Bradley ControlLogix or Invensys I/A Series modules. Set module under-range and over-range alarms to detect P/I converter failures. Reduce calibration intervals to 6 months for transmitters in ambient temperatures above 60°C or in poor-quality air systems. Track midpoint error trend across calibration cycles — a midpoint error growing beyond 0.5% of span per year signals mechanism wear and justifies proactive replacement.

Author: Tan Jianming is an industrial automation engineer with over 10 years of experience in PLC, DCS, and control systems.