In the aging test of the electric oven, the deviation in temperature control accuracy increases significantly, and the difference between the set temperature and the actual temperature sometimes exceeds 20°C. How is it adjusted?

1. Analysis of the Cause of the Problem

The temperature control accuracy of electric oven appliances is one of the key indicators for evaluating their performance. The situation you encounter—a temperature difference exceeding 20°C, which is far beyond the normal deviation range of 10°C ± typical household ovens—indicates systemic issues with aging testing equipment or the oven itself. The main reasons are as follows:

• Thermocouple aging or improper temperature measurement point position: Long-term use of thermocouples in high-temperature environments can cause oxidation, causing thermoelectric potential signal drift and low readings. Additionally, if the thermocouple probe does not reach the actual temperature measurement point inside the oven, or is fixed near the heating tubes and is affected by radiant heat interference, false temperatures can also be measured.

• Temperature controller PID parameter imbalance:P the proportion band, integration time, and differential time in the ID parameters may drift after long-term operation, causing the temperature control system to respond slowly and preventing timely adjustment of heating output.

• Solid-State Relay (SSR) Damage: SSR controls the oven's heating tubes to maintain the set temperature. If a thyristor inside the SSR breaks down or breaks the circuit, it can cause abnormal heating power output and control logic disorder.

• Load changes cause temperature control parameters to be mismatched: When testing multiple ovens simultaneously and testing a single oven, the PID parameters of the controller need to be set separately. If multiple loads are controlled using PID parameters for a single unit, temperature control deviations will inevitably occur.

• The oven's insulation performance declines: aging or damaged sealing strips or collapsed insulation cotton accelerate heat loss, forcing the heating tubes to operate at full power for long periods but still struggling to maintain the set temperature.

2. Detailed Inspection and Operational Methods

Step 1: Use a standard thermometer to verify the actual temperature (rule out issues with the thermocouple itself)

Operating method: Place a calibrated K-type armored thermocouple (accuracy ± 1.0°C) and a precision thermometer near the temperature measurement point inside the oven. Close the oven door, set it to 200°C, and after 30 minutes of stability, compare the precision thermometer readings with the temperature displayed by the equipment controller. If the readings are the same (difference ≤± 2°C), it means the temperature measurement system is accurate; the problem may lie in the temperature control logic or the oven's own insulation performance. If the precision thermometer reads 200°C but the controller shows 180°C, it indicates a negative deviation in the thermocouple and needs to be replaced or recalibrated.

Step 2: Check the installation position of the thermocouple

The thermocouple probe should be placed at the geometric center of the oven cavity or a standard test point (usually in the oven center area), and must not touch the heating tubes or inner wall. If the probe is placed close to the heating tube, it will be strongly affected by radiant heat, and the reading temperature will be significantly higher than the actual heating temperature of the food. Adjustment method: Extend the thermocouple probe out with ceramic insulated sleeves, ensuring at least 5cm distance between the probe and the heating tube, and fix it on the sample rack with fine metal wire, avoiding direct contact with the oven's inner wall.

Step 3: PID parameter self-tuning or manual optimization

Most temperature controllers have auto-tuning capabilities. How to operate: Enter the controller parameter settings interface, find the PID self-tuning function (usually labeled as "AT" or "Auto-Tune"), and start the auto-tuning program. The controller automatically performs the complete process of heating—overshoot—stabilization, calculates PID parameters suitable for the current heating system, and saves them automatically. Self-alignment is usually completed within 20-40 minutes. Please keep the equipment running steadily during this period and do not interrupt midway.

If the device does not have self-tuning or the results are still unsatisfactory, you can try manual adjustment:

• Appropriately reduce the proportional band (P) (for example, from 20 to 15) to improve the system's response speed to deviation.

• If the temperature overshoot is severe after adjustment (exceeding the set value by a lot), appropriately increase the integration time (I).

• For specific parameter adjustments, refer to the PID parameter table in the original factory manual, prioritize restoring the factory default values before resetting.

Step 4: Check the SSR and heating circuit

Use a clamp meter to measure the current at the SSR output terminal and confirm that when the temperature is below the set value, the SSR can continuously conduct and supply power to the heating tubes; When the temperature approaches the set value, the SSR should be able to switch on and off at a high frequency (about several to dozens of times per second) to achieve precise temperature control. If the SSR remains in a conduction or disconnected state, it indicates damage and requires a replacement model of the same specification (it is recommended to use an industrial-grade SSR with a rated current of no less than 25A).

Step 5: Check the oven door seal

Using the previously mentioned "A4 paper method": with the box door closed, clamp the paper between the door seal and the box body and try to pull it out. If it is easily removed, it indicates poor sealing. Check whether the door seal is deformed, hardened, or damaged; if found, replace it promptly. Additionally, check whether the insulation layer of the oven casing has collapsed or been damaged—if the insulation cotton collapses after long-term high-temperature operation, it will cause a large amount of heat to escape from the cabinet surface, seriously affecting temperature control accuracy.

Step 6: Special handling for batch testing of multiple loads

If the equipment is used for testing multiple ovens aging simultaneously, the following recommendations are made:

1. Each time the load type is changed or the number of loads changed, PID self-tuning is re-executed to adapt the controller to the heating characteristics of the current load.

2. Each oven should be equipped with an independent temperature sensor and a separate temperature control circuit to avoid mutual interference.

3. Establish load profiles to record recommended PID parameters for different load types, which can be directly called during the next test without repeated tuning.

Step 7: Daily maintenance and calibration recommendations

• Perform precision calibration of the thermocouple every 6 months; if deviations exceed ±2°C, replace them promptly.

• Check the SSR indicator light monthly to ensure it is functioning properly, and regularly measure the heating current.

• After each load type change, be sure to re-verify the temperature control accuracy.

• After the aging test, let the oven cool naturally to room temperature before closing the door to reduce thermal stress damage to the sealing strip caused by high temperatures.