What are the symptoms of poor contact in a furnace temperature sensor?

The temperature control precision of the tempering process can be significantly impacted by poor contact in a furnace temperature sensor, which may result in uneven workpiece hardness or performance failure. It can also cause temperature display variations, unstable response, and signal interruptions.

 

1. Common Symptoms

Symptoms

Particular Description

Risk Repercussions

Regular Variations in Temperature or Flashing

Readings from the control system can change significantly in a little amount of time (e.g., 580℃→530℃→600℃).

Heating power is out of control and PID temperature control is imprecise.

Unpredictable and Sporadic Display

Temperature displays sporadically, accompanied by "---" or over-range alerts.

Process interruption prevents closed-loop control.

An uneven heating curve and lots of burrs

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

The workpiece is susceptible to deformation and cracking due to thermal stress superposition.

After cooling, readings do not drift or return to zero.

After the furnace is shut down and restarted, the sensors continue to exhibit aberrant results.

shows loose connections or ongoing oxidation of the wire.

Practical Case: Three batches of molds were destroyed owing to low hardness during the tempering of H13 molds in a manufacturing because oxidation of the thermocouple connector increased contact resistance and caused an abrupt temperature decrease of 30°C.

 

2. How Can I Spot Bad Contact Quickly?

Tap Test: While the terminals are operating, gently tap them. A loose connection is indicated if the temperature display changes correspondingly.

Multimeter Dynamic Testing: Check for sudden changes or pauses in the signal by measuring the output millivolt value (for example, 14.2mV for K-type 580℃).

Comparing Standard Thermocouples: Attach a standard thermocouple in parallel. Poor contact is indicated if its reading remains constant while the original sensor varies.

Infrared Temperature Measurement Assisted Verification: To determine the furnace's real temperature, use a handheld infrared thermometer. A variation more than 10°C necessitates careful examination.

To prevent interference from the workpiece absorbing heat, it is advised to test in an empty furnace.

 

3. Typical Reasons and Solutions

Reasons and Remedies:

Terrain Oxidation: Retighten after removing the oxide layer and applying an antioxidant paste.

To stop connecting screws from becoming loose, check them and use spring washers.

In order to allow the thermocouple to freely expand and contract, remove any carbon deposits or water intrusion from the protective tube.

Aging and Broken Compensation Wire: Avoid extreme bending and replace with a high-temperature resistant wire.