Why sensor readings change over time
Every temperature sensor changes slowly as it ages. That gradual shift, called drift, means the reading moves away from the true value even though nothing looks wrong. Controllers faithfully hold whatever the sensor reports, so a sensor that reads two degrees low can lead to a process that is actually two degrees high with no alarm at all.
Typical contributors to drift include:
- Thermal cycling and exposure to high temperatures, which can alter the materials in a thermocouple or the elements in a resistance temperature detector.
- Contamination, where chemicals, moisture or vapors get into the sensor or thermowell and change its electrical behavior.
- Mechanical stress, from vibration, bending or repeated installation and removal.
- Electrical effects, such as degraded insulation, corroded terminals or poor reference junction compensation in the measurement electronics.
- Transmitter and input electronics, since the converter and the controller input card can also shift over time.
How fast drift occurs varies widely with sensor type, temperature, environment and quality, so any fixed interval is a starting assumption rather than a rule.
Calibration and verification
Calibration compares an instrument against a more accurate reference and records the difference. Verification is a lighter check that confirms the reading is within an accepted tolerance. Both support the general idea behind regular calibration of industrial equipment: an instrument is only useful if you know how far to trust it.
Common methods include:
- Comparison in a dry-block or liquid bath. The sensor under test and a reference thermometer are placed in a stable heated environment and readings are compared at one or more temperatures.
- In-situ comparison. A reference probe is placed next to the installed sensor, sometimes in a spare well. This tests the sensor in its actual condition, though stability of the process limits accuracy.
- Ice-point or other fixed-point checks. For a quick check near a known physical point, though they test only that point.
- Electrical simulation. A calibrator supplies a known signal to the transmitter or controller input to check the electronics, separately from the sensor.
Documenting the results matters. An as-found reading (before adjustment) shows how much the device drifted since the last check, and an as-left reading shows the result after adjustment. A history of as-found values lets a team set sensible intervals: if a sensor barely moves between checks, the interval can often be lengthened, and if it moves a lot, it can be shortened or the sensor type reconsidered. Where the product is regulated, follow the required procedures, use references that are themselves calibrated, and keep records as the rules demand.
Common faults and their symptoms
Troubleshooting is easier if you separate the loop into parts: sensor and wiring, controller, final control element and process.
| Symptom | Possible causes to look at |
|---|---|
| Reading jumps or flickers | Loose or corroded terminals, damaged cable, electrical noise, moisture in the sensor head |
| Reading stuck or frozen | Failed sensor, broken wire, input card problem, fallback value in the control logic |
| Reading steadily offset | Sensor drift, wrong scaling or units, wrong sensor type configured, poor thermowell contact |
| Temperature oscillates around setpoint | Poor tuning, sticking valve, oversized actuator, changed process conditions |
| Cannot reach setpoint | Failed heater element, low steam or hot water supply, fouled exchanger, blocked strainer, saturated output |
| Overshoots on start-up | Aggressive tuning, integral windup, sensor placed too far from the heat input |
| Uneven temperature across a zone | Failed element in one section, poor mixing, partial blockage, sensor not representative |
Valves and heaters are frequent culprits. A control valve that sticks, perhaps because of wear, a dry packing or contamination, makes the loop hunt in a slow cycle. Solenoid valves that act as on/off heating or cooling switches can also stick or fail to seal. Heater elements may open-circuit, short to ground or lose output gradually, and contactors can wear from frequent switching. In heat exchangers, scale and deposits cut capacity; heat exchanger fouling is a leading reason a loop that used to work now struggles at full output.
Diagnosing methodically
A few habits save time:
- Check the simplest things first: power, fuses, tripped breakers, visible damage, loose connections.
- Compare the reading with a second source such as a handheld reference, an adjacent sensor or a historian trend.
- Look at the controller output. If it sits at 100 percent and the temperature is still low, the actuator or supply is the issue. If the output is steady and the temperature wanders, look at disturbances or sensor problems.
- Review recent changes: new product, replaced parts, altered settings, changed supply conditions.
- Simulate or force signals only under procedures that keep the process safe.
When the same fault returns, a structured root cause analysis helps avoid treating symptoms again and again.
Repair or replace
The choice depends on cost, risk and availability. Considerations that often point toward repair or adjustment:
- The fault is a loose terminal, a failed connector or a sensor that can be re-calibrated within tolerance.
- The device is otherwise reliable and spare parts are available.
- A valve needs new packing or a trim kit rather than a full replacement.
Signs that replacement may be wiser:
- Drift is large, erratic or keeps recurring after re-calibration.
- A sensor has visible damage, corrosion or moisture ingress.
- A controller or input card is obsolete, unsupported or has repeated unexplained faults.
- The repair cost approaches the price of a new part, or repeated repairs cause lost production.
- The device no longer meets the accuracy or safety needs of the current process.
These ideas overlap with the general replacement decision signs used for any equipment. Sensors and thermowells are relatively cheap compared with the product they protect, so many plants replace them on a schedule or at the first sign of doubt, particularly for critical loops.
Spares and records
Keeping a small stock of common sensor types, fuses, contactors and a spare controller of the type in use shortens outages. Which parts to hold depends on how critical the loop is and how long a replacement takes; thinking in terms of critical spares helps focus the list. After any replacement, check scaling, verify the reading against a reference and confirm that tuning and alarms still work. Save the controller configuration so it can be restored.
Safety
Servicing temperature equipment can expose people to hot surfaces, steam, stored heat, electrical energy and fuel. Follow manufacturer manuals and plant rules, isolate energy sources with lockout and tagout style procedures, let equipment cool where needed, and use trained and certified personnel. Do not bypass safety limits to keep production running. This article is general information, not engineering advice for a specific system.