Step 1
First, do a 5-minute self-check.
If two or more of the following items apply to you, there is a high probability that there is an "invisible discrepancy" in your current temperature measurement method or equipment.
- Thermocouples have not been replaced for a long time, or they disconnection frequently.
- The quality of the workpiece changes depending on its position within the furnace.
- instrument 's temperature display is stable, yet a malfunction occurs.
- The hunting doesn't stop even after adjusting the PID settings.
- The door seal is damaged, or outside air can easily enter.
- I have never performed a temperature distribution measurement (TUS).
Step 2
Three often overlooked truths
The greatest danger in controlling the temperature inside a furnace is relying solely on the “display” reading from the “control” instrument.
① display temperature ≠ Workpiece temperature
control only monitors the "temperature at the sensor tip." The actual product temperature will vary depending on the material, size, and quantity of the workpiece.
② There are always temperature variations inside the furnace.
Measuring at only one point won't reveal temperature differences between the heater area and the door area. To eliminate variations in quality, it's necessary to understand the "space" rather than just a "point."
③ Sensors are "consumable items".
When thermocouples are continuously exposed to high temperatures, their electromotive force gradually shifts due to thermal degradation. It's not okay just because disconnection.
For those who want to understand the mechanisms of temperature control and controllers.
Before considering the causes of unstable temperatures, understanding the basics of controllers and PID control will make it easier to distinguish between control settings and equipment-related problems.
Step 3
Main causes of temperature fluctuations
The causes of the problem can be broadly divided into three categories. Please check if any of these apply to you.
Step 3
10 main reasons why temperature fluctuates
The reason the furnace temperature is unstable is due to measurement systems such as sensors,
If we consider it in terms of equipment systems such as heaters and control, and environmental systems such as workpieces and operations,
This makes it easier to pinpoint the cause.
[A] Sensor and measurement problems
-
1. Thermocouple degradation and lifespan: When used for extended periods in high-temperature environments, characteristic of thermocouples change.
display temperature may be lower or higher than the actual temperature.
Even if disconnection, the measurement may be inaccurate. -
2. Sensor placement and insertion depth: If the sensor is too close to the heater or too far from the workpiece,
There will be a difference between the temperature control is observing and the temperature you actually want to manage.
The insertion depth and mounting condition also need to be checked.
[B] Equipment and control issues
-
3. Heater deterioration over time: When the heating capacity changes due to heater deterioration or partial disconnection of the wires,
This can lead to uneven temperature distribution within the furnace, making it difficult to reach the target temperature. -
4. Deterioration of insulation: When the insulation performance deteriorates, heat can easily escape to the outside.
It may take a long time to heat up, or it may be difficult to maintain the set temperature. -
5. PID setting mismatch: Due to aging of equipment and changes in work conditions,
The PID parameter you previously set may no longer match the current equipment characteristic.
This can sometimes cause hunting or over. -
6. Changes in heat capacity and load conditions: When the quantity, material, and loading method of the workpiece change,
heat quantity removed from the furnace and the temperature response will change.
Even with the same control settings as before, the temperature may become unstable.
[C] Operational and environmental issues
-
7. Workpiece quantity and arrangement: Changes in the quantity and arrangement of workpieces will alter how heat is transferred and the thermal balance within the furnace.
Even with the same settings, quality may vary depending on the placement location. -
8. Opening and Closing the Door and Outside Air Influx: When the door is opened or closed, outside air enters the furnace,
causing the temperature to drop temporarily.
If the door is opened and closed frequently, or if you proceed to the next step process before the temperature has recovered,
temperature fluctuations will be more significant. -
9. Changes in the surrounding environment and disturbance: Changes in ambient temperature, ventilation, cooling conditions, etc., can alter the thermal balance of the furnace.
Even with the same settings, the temperature response may vary. -
10. Temperature distribution inside the furnace: The temperature inside the furnace is not necessarily the same everywhere.
Temperature differences can occur depending on the heater's location, insulation, and airflow.
If you only measure one point, you may overlook these temperature variations.
Step 4
Specific solutions and recommended measuring instruments
Basic principles for isolating the cause
Instead of immediately resorting to PID settings or equipment replacement, checking the following in order will make it easier to pinpoint the cause: "Are the measured values correct?", "Is there temperature variation inside the furnace?", and "Have there been any changes in equipment or control conditions?".
Even if you know the cause, if you're unsure "how to fix it," introducing specialized measurement and control equipment suited to your application can quickly resolve the problem.
| Problems on site | Chino's solutions (products) | Benefits |
|---|---|---|
| The sensor deteriorates and disconnection easily. Accuracy decreases in high-temperature and corrosive environments. |
Solid-pack thermocouples (sheathed thermocouples) | The protective tube is densely filled with inorganic insulation material. This ensures a long lifespan even in harsh environments and minimizes accuracy drift. |
| I want to know the temperature of the workpiece itself. Temperature variations inside the furnace are not visible. |
thermal image measurement (thermography) | It captures the inside of the furnace and the workpiece as a "surface (image)" without contact. You can see at a glance where it is hot and where it is cold. |
| I want to perform a temperature distribution measurement (TUS). I want to manage data from multiple points together. |
Paperless recorders/data loggers | It allows simultaneous connection of multiple thermocouples, complies with stringent heat treatment standards such as AMS2750, and automates the recording and analyzing of measurement data. |
Investigate the cause in detail.
Learn more about control
FAQ
FAQ
If adjusting the PID doesn't improve the situation, what should I look at?
First, we suspect factors other than control. Please check for things like "deterioration of the thermocouple itself (replacement with a solid pack is recommended)," "disconnection heater wire," and "deterioration of the door gasket."
When is temperature distribution measurement necessary?
This is performed when we feel that the quality of the workpiece changes depending on where it is placed, after replacing heaters or insulation materials, or at the time of periodic quality assurance (such as TUS requirements).
Next Step
For everything from identifying the cause to resolving the issue, please consult Chino.
Even if you're still at the stage of "I don't know what the cause is" or "I'm unsure whether I should upgrade the equipment or change the sensors," that's perfectly fine. Our expert technicians will inquire about your furnace specifications and operating conditions and propose the optimal solution.
I need advice regarding a problem with the furnace temperature.
We offer consultations on everything from selecting measuring instruments and measuring temperature distribution to periodic calibration.
Technical support contact information is available here.
