Sensor Reading Drift Decision Tree

Why this matters

A sensor that reads wrong makes a healthy system behave badly and a faulty system look fine. The control acts on what the sensor reports, so a drifted temperature, pressure, or level sensor causes short cycling, poor performance, and chasing ghosts. The hard part is that the system may be fine and the sensor lying, or the sensor honest and the system genuinely off. This tree teaches how to tell a drifted sensor from a real condition across HVAC, appliances, pool, and process equipment in any trade.

Start here: is the reading even plausible

Before trusting or condemning, sanity-check the number against reality.

• Compare to a known-good independent reading. Put a calibrated handheld meter on the same point. A large gap between the sensor and your reference is the drift you are hunting.
• Compare to common sense. A water temperature reading below freezing on a running system, or a pressure that defies the gauge next to it, is an obviously bad sensor or connection.
• Compare against a second sensor if the system has one. Two sensors at similar points that disagree widely point to one of them being off.

If the sensor agrees with your reference, the sensor is fine and the condition is real - stop blaming the sensor.

If the reading is stuck or pinned

A value that never moves, or sits at the extreme high or low end, is usually not drift but a hard fault.

• Pinned high or low (open or short): an open sensor circuit often reads full-scale one way, a shorted one reads full-scale the other. Check the sensor and its wiring for an open or short.
• Frozen at one value: a sensor that reports the same number through real changes is failed or disconnected, or the control has lost the input.
• Check the connection. A loose, corroded, or wet terminal changes the resistance the control sees and throws the reading off without the sensor itself being bad.

If the reading is off by a consistent offset

The sensor tracks changes correctly but is shifted high or low by a steady amount. That is classic drift or a placement problem.

1. Check placement. A temperature sensor in the wrong spot, in a draft, against a hot surface, or in dead air reads a real-but-irrelevant condition. Many "drift" calls are a sensor reading exactly what it sees in a bad location.
2. Check for fouling. A sensor coated in scale, grease, dust, or biofilm reads slow and offset. Clean it and recheck.
3. Check the reference or calibration. Some sensors and controls allow a calibration offset. Verify it has not been set wrong or drifted. Recalibrate against your known-good meter.
4. Aging. Sensing elements drift with age and exposure. A sensor that no longer matches a reference after cleaning and correct placement is worn out.

If the reading is erratic or jumpy

Bouncing values that swing around point to a connection or interference problem more than the element itself.

• Loose or intermittent connection makes the reading jump as contact changes. Reseat and secure terminals.
• Moisture intrusion at the sensor or junction causes erratic resistance. Dry and seal.
• Electrical noise on a low-level signal line (running beside power conductors) induces jumps. Reroute or shield the sensor wiring.

Diagnostic summary

The trap to avoid

Do not adjust the system to make the sensor reading look right, and do not replace the sensor on a hunch. Prove the drift with an independent calibrated reference first. Half of "bad sensor" calls are a fouled element, a bad connection, or a sensor sitting in the wrong place reading exactly what is there. Confirm before you condemn.

References

• Instrumentation and sensor manufacturer general calibration practice
• HVAC and process measurement practice on sensor placement and verification
• See related: The Control Isn't Responding Decision Tree
• See related: Intermittent Control Fault Decision Tree