1. Resistance Thermometers (RTD)
Industrial RTDs commonly fail by open circuit or short circuit. Open circuits are more frequent because RTD sensing wire is relatively fine.
Open and short circuits are easy to diagnose using a multimeter on the 1Ω range. If the measured resistance is less than the nominal resistance R0, a short circuit is possible; if the multimeter indicates infinite resistance, the sensing element is open. Short-circuit faults are usually straightforward to repair: if the repair does not change the sensing wire length or diameter, locate the short, dry the area and restore insulation. Open-circuit repairs typically alter the sensing wire length and therefore the resistance, so replacing the sensing element is preferable. If soldering is used for repair, the instrument must be recalibrated and verified before use.
2. Thermocouples
Correct use of thermocouples yields accurate temperature readings and reduces material waste. Besides common errors from reversed compensation wires, incorrect wire types, or loose wiring (remedy: use the correct compensation wire and tighten terminals), major sources of thermocouple error include improper installation, thermal conductivity effects, and time lag.
2.1 Installation-related errors
Errors arise when thermocouple position or immersion depth does not represent the true furnace temperature. Thermocouples should not be installed too close to doors or heating elements. Insertion depth should be at least 8 to 10 times the protective sheath diameter. Gaps between the thermocouple sheath and the furnace wall should be filled with insulating material such as refractory cement or asbestos rope to prevent heat loss or cold air ingress. The cold junction should not be too close to the furnace body where temperatures exceed 100°C. Installations should avoid strong magnetic or electric fields, so thermocouples and power cables should not share the same conduit to prevent interference. Do not install thermocouples in regions with very low fluid flow; when measuring gas temperature in a pipe, install the thermocouple facing against the flow and ensure adequate contact with the gas.
2.2 Insulation deterioration
Insulation breakdown can occur when the thermocouple insulation, sheath, or lead-out board is contaminated with dirt or salt deposits, producing poor insulation between thermocouple poles and the furnace wall. This problem worsens at high temperature and can cause thermoelectric potential loss or introduce interference, producing errors that may reach hundreds of degrees.
2.3 Thermal inertia errors
Thermal inertia causes the indicated temperature to lag changes in the measured temperature. This is especially significant for rapid measurements. Use thermocouples with fine sensing wires and small-diameter sheaths to minimize inertia; where environment permits, remove the protective sheath. Because of measurement lag, the detected temperature fluctuation amplitude is smaller than the actual furnace temperature fluctuations. Larger time constants reduce the thermocouple fluctuation amplitude and increase deviation from the actual temperature. For accurate measurement choose thermocouples with small time constants. Time constant is inversely proportional to heat transfer coefficient and proportional to sensing tip diameter, material density, and specific heat. To reduce time constant, the most effective method is to minimize the sensing tip size and use materials with good thermal conductivity and thin sheath walls. In precise measurements, un-sheathed bare-wire thermocouples are used, but they are vulnerable and require frequent calibration and replacement.
2.4 Thermal resistance errors
At high temperatures, layers of ash or dust on the sheath increase thermal resistance and hinder heat transfer, causing indicated temperatures to be lower than the true temperature. Keep the thermocouple sheath clean to reduce this error.
3. Bimetal Thermometers
Bimetal thermometers operate by bonding two metals with different thermal expansion coefficients; one end is fixed while the other end twists when temperature changes, converting deformation into pointer rotation to indicate temperature.
If linearity errors occur in service, adjust the pointer via the adjustment knob on the rear of the thermometer. The instrument must be calibrated and verified after adjustment before returning to service.
4. Pressure-type Thermometers
Pressure-type thermometers measure temperature via thermal expansion of a liquid. A sealed system composed of the bulb, capillary tube and bourdon tube is filled with the sensing liquid. When the bulb senses temperature change, liquid volume changes, altering system pressure and causing the bourdon tube curvature to change; this displacement is transmitted via linkage to the pointer, which indicates temperature on the dial. These instruments have linear scales, small bulb volume, fast response, high sensitivity and direct reading.
Common faults for pressure-type thermometers include stuck pointers and large indication errors. For a stuck pointer, use a pointer puller to remove and reseat the pointer, then recalibrate before use.
4.1 Common faults in temperature control instruments
Temperature control instruments use RTDs or thermocouples to regulate the controlled object. Common faults include:
- Poor installation location, preventing proper thermal exchange and causing low indications.
- Poor thermal insulation at the measurement point, causing local heat loss and readings lower than system temperature.
- Loose wiring or poor contact causing inaccurate readings: RTDs may indicate high, thermocouples may indicate low.
- Short circuits causing RTDs to indicate low or minimum, and thermocouples to indicate low or fail.
- Open circuits producing maximum RTD reading and no or minimal thermocouple indication.
Note that most temperature control systems use motor-driven instruments for measurement, indication and control, which exhibit significant measurement lag.
4.2 Troubleshooting methods
- Check the instrument display. If the display is at maximum or minimum and remains there, this indicates a system fault since measurement systems generally have large lag and do not change abruptly. Common causes are open circuits in thermocouples, RTDs or compensation wires, or failure of the transmitter or amplifier.
- If the instrument value oscillates rapidly, the likely cause is improper tuning of the PI(D) controller.
- If the instrument value shows large, slow oscillations, the cause is usually changes in the process; if process conditions are steady, then the control system itself is likely faulty.
- After diagnosing a control-system fault, check the control valve input signal to the valve. If the input signal does not change but the valve moves, the valve diaphragm may be leaking. If the positioner input signal is constant while its output changes, the positioner is likely faulty. If the controller output changes while its input is constant, the controller itself may be faulty.
