Drying oven temperature sensor calibrated and over temperature protection inspection

introduction

As a widely used equipment in laboratories and industrial production, the accuracy of temperature control and operational safety of Drying Ovens are crucial. Temperature sensors are key components for precise temperature control in Drying Ovens, and their value accuracy directly impacts the reliability of processes such as heating, drying, or aging. At the same time, the over-temperature protection system is the last barrier to prevent sample damage or safety accidents caused by the failure of the equipment due to temperature control failure. Therefore, regularly calibrating the temperature sensor of the Drying Oven and checking its over-temperature protection function is a necessary procedure to ensure the accuracy of experimental data and the safety of equipment operation. This article will systematically explain the principles, methods and precautions of calibration and inspection.

Calibration principle:

The temperature sensors commonly used in Drying Ovens are mainly thermocouples or platinum resistors (such as PT100). Its calibration essentially takes the sensor's indicative value (T_Show) with a standard of known accuracy at multiple temperature points (T_Mark) to determine the sensor's error or establish a correction value. Calibration is typically performed within the effective working area of the dryer's internal space, where the consistency of the space temperature field is assessed by uniform dotting. Key evaluation indicators include value error and temperature uniformity.

The calculation formula for the indicative error ΔT is:

ΔT = T_Show - T_Mark

Temperature uniformity is usually characterized by the difference between the maximum and minimum temperatures measured at each measurement point at the same time under steady-state conditions.

Calibration method

The calibration process follows a rigorous process, and the main steps are summarized as follows:

1. Preparation: Select a qualified/calibrated multi-channel temperature logger or data collector as the standard, and the extended uncertainty of its sensor (standard platinum resistance) should be better than 1/3 to 1/5 of the required tolerance of the calibrated Drying Oven. Prepare at least 9 standard sensors (which can be increased or decreased according to the volume of the box) to measure the temperature at each point in the space.

2. Placement: The standard sensor is arranged on the upper, middle, and lower layers of the Drying Oven workshop, with at least three points (front, middle, and back) on each floor. All sensor temperature sensing ends should not be in direct contact with the chamber wall or sample holder, and should be in a free air environment. The sensor of the calibrated Drying Oven itself is placed near the center point of the middle layer.

3. Calibration point selection: Usually select at least two calibration points, such as the lower limit of the common temperature range, the upper limit point, and one middle common point. For a universal Drying Oven at 105°C, 80°C and 105°C are available as calibration points.

4. Data Acquisition: Set the temperature of the Drying Oven to the first calibration point. After the temperature display stabilizes (usually more than 30 minutes), start recording all standard sensors and calibrated sensor values at intervals of 1 minute for at least 30 minutes. Calculate the average temperature of each point during this time period.

5. Data processing: calculate the difference between the average value of the calibrated sensor and the average value of the center point standard at the middle level, which is the value error of the calibration point. At the same time, the difference between the maximum and minimum values of the average value of all standard sensors over the same time period is calculated to evaluate the temperature uniformity.

6. Repeat steps 4-5 to complete the testing of the remaining calibration points.

Calibration results are processed

The calculated indicative error is compared to the maximum allowable error specified in the dry oven manufacturer's technical specifications or relevant usage standards (e.g., some industry process regulations). If the error exceeds the allowable range, the temperature control instrument of the Drying Oven needs to be adjusted (such as correcting the sensor offset) or the sensor should be replaced. After adjustment or replacement, recalibration is required. The temperature uniformity results can be used to evaluate whether the Drying Oven is suitable for applications where there are specific requirements for temperature field uniformity.

Over-temperature protection inspection principle

The over-temperature protection system is usually independent of the main temperature control system and is a redundant safety device. It typically consists of a separate temperature sensor (over-temperature protection probe) and a set-point controller (over-temperature protector). When the temperature in the chamber unexpectedly exceeds a preset safety limit (over-temperature set point), the controller should be able to immediately cut off the power to the heater and may trigger an audible and visual alarm. The purpose of the inspection is to verify the functional integrity and action accuracy of the independent loop.

Inspection method

Over-temperature protection checks should be performed after the temperature sensor is calibrated to ensure that the reference for checking ambient temperature is reliable. The main steps are as follows:

1. Confirm the set value: Consult the equipment manual or records to clarify the original set temperature value (T) of the overtemperature protector_{Over-temperature setting}(。 Usually it is set in the range of 5°C to 30°C above the operating temperature.

2. Connection monitoring: Place the temperature sensing end of a standard temperature sensor close to the temperature sensing end of the over-temperature protection probe to monitor its perceived temperature.

3. Functional test: Set the main temperature control of the Drying Oven below T_{Over-temperature setting}temperature (e.g. below 10°C) and initiate heating. Then, slowly increase the setpoint of the overtemperature protector until it is slightly below the actual temperature inside the current box. Observe whether the overtemperature protector operates within the specified time (cut off the heating power supply and alarm).

4. Action point test (optional, need to be careful): In order to accurately measure the actual operating temperature of over-temperature protection, the main control temperature setting value can be higher than T_{Over-temperature setting}(This operation may cause the temperature in the box to exceed the normal range, and it is necessary to ensure that there are no samples in the box and that personnel are on duty). When the temperature inside the box rises, the standard sensor reads the temperature at the moment of action, which is the actual operating temperature. Record the deviation from the set value.

5. Reset check: After triggering the over-temperature protection, check whether the reset function is normal. It usually takes the temperature to drop to a safe zone and a manual reset before the unit can restart heating.

Notes:

Safety must always be a top priority during calibration and inspection: operators should be trained; When checking the over-temperature protection, it is strictly forbidden to place any samples or flammable materials in the box; Protective equipment should be worn when conducting high-temperature testing to prevent burns; When the equipment is powered on, it is forbidden to touch the electrical components; Any equipment with abnormal functions should be immediately deactivated and repaired. It is recommended that the calibration and inspection cycle be set once a year, and for equipment that is used frequently or for critical processes, it should be appropriately shortened.

summary

Temperature sensor calibration and over-temperature protection checks in Drying Ovens are two important independent but related maintenance activities. Systematic calibration ensures the accuracy of process temperature and is the basis of data reliability. Strict over-temperature protection inspection is the guarantee of the safe operation of the equipment. By establishing and implementing standardized operating procedures, the good condition of the equipment can be continuously maintained, and the quality loss and potential risks caused by temperature loss can be effectively prevented, thereby supporting the stable progress of experiments and production activities.

References

JJF 1101-2019 Specification for calibration of temperature and humidity parameters of environmental test equipment

GB/T 30435-2013 Electric heating Drying Oven and electric heating blast Drying Oven

IEC 61010-2-010 Safety requirements for electrical equipment for measurement, control, and laboratory use

NIST Technical Note 1297 Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results

Compilation of temperature calibration papers related to the industry authoritative journal "Metrology Technology"