7 common mistakes in temperature sensor installation – 90% of people have encountered them

The sensor is installed adjacent to the heating tube, motor, and frequency converter; it measures not the actual temperature of the medium but rather radiant heat and environmental interference. Correct installation requirements: maintain a straight-line distance of ≥50 mm between the probe and the heating element, a distance of ≥80 mm from the motor/frequency converter, and at least 100 mm away from hot air outlets. Prioritize straight pipe sections, avoiding areas downstream of valves or elbows as well as fluid stagnation zones to ensure measurement of a "representative temperature."

When the probe is inserted only into the insulation layer or air layer without contacting the medium, the measured value will inevitably be low. For pipeline temperature measurement, the insertion depth should be 1/2 to 2/3 of the pipe diameter, with the sensing end positioned at the point of maximum flow velocity in the center of the pipe. The minimum insertion depth must be no less than 10–15 times the outer diameter of the protective pipe. For small-diameter pipes below DN80, a 45° inclined insertion or use of an expansion fitting is recommended. For furnace and flue gas temperature measurements, the insertion depth should typically be ≥1 m; beyond this depth, a support frame must be installed to prevent bending.

An air gap exists between the sensor and the measured surface; since air is a poor conductor of heat, this can cause response delay and inaccurate measurements. When performing surface-mounted temperature measurement, the measured surface must be cleaned first, followed by application of thermal grease to fill the gap, which should then be secured with a pressure block or clamp. A thin layer of thermal grease is sufficient—excessive thickness may impair heat transfer. For PT100 precision sensors, spring-loaded compression installation is recommended to ensure full contact between the probe and the medium.

The three-wire configuration is the standard connection method for industrial PT100 sensors, yet wiring errors are extremely common. Common issues include: inconsistent wire specifications, connecting two wires together at a single terminal, and reversing the polarity between the compensation wire and signal wire. These errors can cause bridge compensation failure and introduce additional resistance errors. The correct procedure requires all three wires to have identical material, diameter, and length, with a resistance difference ≤ 0.05 Ω; each wire must be connected independently to its respective terminal, and parallel or twisted connections are strictly prohibited.

If the shielding layer is not grounded, electromagnetic interference will directly propagate into the signal line, causing reading fluctuations or jumps; grounding both ends creates a ground loop that introduces circulating interference. The correct approach is to use shielded twisted pair cables with single-end grounding (typically at the instrument/control cabinet end), ensuring a grounding resistance of ≤4 Ω. Signal lines should be installed parallel to power cables with a minimum spacing of 30 cm, and crossed at a 90° angle. In high-interference environments, it is recommended to use a temperature transmitter to convert resistance signals into 4–20 mA current signals for transmission.

The temperature measurement signal cable and the power cables for the frequency converter and high-power motor are installed in the same conduit or cable tray, causing power-frequency interference to interfere with the signal, resulting in reading drift or even complete accuracy loss. Specifications require that signal cables be routed independently through conduits and installed separately from power cables in different trays, maintaining a minimum spacing of 300 mm. For long-distance transmission, PT1000 thermocouples (with higher resistance values and minimal voltage drop effects) should be preferred, or direct use of temperature transmitters outputting standard 4–20 mA signals is recommended.

Many people assume sensors are "maintenance-free" and can be left unattended after installation. In reality, temperature sensors gradually degrade and develop drift under high-temperature, vibrating, or corrosive conditions. The resistance-temperature relationship of PT100 platinum wires deviates due to oxidation at elevated temperatures, while the thermoelectric potential of thermocouple wires diminishes with prolonged use. Recommendation: Key measurement points should be calibrated every 6–12 months using a standard reference source (e.g., an incubator); in high-frequency usage scenarios, the calibration interval should be shortened. The instrument's graduation settings must also be verified—missetting PT100 to Cu50 will result in significant measurement errors.