How to determine if a furnace temperature sensor has poor contact?

Observing temperature display jumps and unstable responses, as well as testing signal continuity by comparing with a conventional thermocouple and using a multimeter, are the main methods for figuring out whether a furnace temperature sensor has poor contact. The particular standards for judging are as follows:

 

1. Common signs of inadequate contact

The phenomenon

An explanation

Risk Repercussions

Temperature fluctuations or flashes

The control system shows a value that varies significantly in a brief amount of time (e.g., 580℃→520℃→600℃).

This results in imprecise temperature management, which may lead to either excessive or insufficient tempering.

Unpredictable sporadic display

The temperature shows "---" or an over-range alert sporadically.

interruption of the sensor signal, making closed-loop control impossible.

The heating curve has numerous spikes.

Instead of a steady climb, the furnace temperature curve exhibits sawtooth variations.

Uncontrolled heating power is the outcome of the PID system's frequent adjustments.

After cooling, the reading either drifts or does not return to zero.

After the furnace is shut down, abnormal readings are still there on the sensor, and they continue to be unstable even after it is restarted.

shows signs of severe oxidation or loose wiring.

Practical Case: During a holding period at 580°C, the temperature of an H13 tempering furnace in a manufacturing abruptly plummeted to 530°C. Three molds were rejected because of low hardness after an investigation showed that the thermocouple connectors had increased contact resistance as a result of high-temperature oxidation.

 

2. Rapid Testing Techniques on-site

Standard Thermocouple Comparison Method: Link the original sensor in parallel with a standard thermocouple that has been approved by metrology. Poor contact is indicated if the standard value stays constant but the control system reading varies.

Multimeter Dynamic Testing: Determine the thermocouple output millivolt value using a multimeter. During heating, keep an eye out for any abrupt fluctuations or signal interruptions (e.g., around 14.2mV for a K-type thermocouple at 580°C).

Tap Test Method: While the apparatus is operating, gently tap the sensor terminals. A loose or inadequate connection is indicated if the temperature display changes correspondingly.

Infrared Thermometry Assisted Verification: To determine the furnace's real temperature, use a handheld infrared thermometer. More research is necessary if the sensor reading deviates by more than 10°C.

Suggestion: To prevent interference from workpiece heat absorption, testing should be done in empty or lightly loaded settings.

 

3. Typical Reasons and Remedies

Because

The answer

Oxidation at the Terminal

Retighten after removing the oxide layer and applying anti-oxidant paste.

Unreliable Connection

Look for loose screws and use spring washers to keep them from coming free.

Carbon Deposits or Water Infiltration in the Protective Tube

To ensure that the thermocouple may expand and contract freely, remove and clean any internal pollutants.

Broken and Aging Cable

Replace with compensating wire that can withstand high temperatures; do not use a bending radius that is too tiny.

Safety Rule: "Check fluctuations immediately, verify by comparison, and tighten promptly." When coupled, these three ideas guaranty consistent and trustworthy temperature readings.