In the hygienic process industry, the most common standard calibration procedure for temperature sensors is two-point or three-point calibration. Calibration can be performed in the facility's laboratory or near the access point. This reflects the state-of-the-art approach to obtaining the clearest temperature profiles for the sensors and detecting deviations. This practice also aligns with the expectations of auditors and regulatory bodies.
In the healthcare industry, standard resistance temperature detectors (RTDs) for measuring process temperatures can last for years. Even so, the greatest risk to temperature instruments in healthcare systems lies in the calibration process. Opening the equipment, removing plugs, connecting and disconnecting power cords, introducing the thermometer into an oil bath or blocking the calibrator, or transporting the thermometer to a laboratory all increase the likelihood of mechanical damage.
Removing the sensor from the process or thermal bushing is the leading cause of RTD (Resistor Temperature Detector) failure. One of the most pressing questions for users is: "What is the best way to reinstall the sensor in the exact same measurement location in the process after removing it for calibration?" The risk is significantly reduced if the temperature sensor can be calibrated in situ.
Temperature calibration timing
The main limitation of traditional three-point calibration is the calibration cycle time. How does the performance of the RTD change between calibration cycles? Most factories determine the cycle time based on risk management and cost analysis. More calibrations (shorter cycles) can reduce risk but increase costs.
Fewer calibrations (longer cycles) have the opposite effect. Sensors with self-calibration or self-verification capabilities provide continuous health monitoring of thermistors. The sensor performs calibration each time sterilization is performed, without requiring:
• Stop the production process;
• Remove the thermal resistor from the process;
• Any efforts by maintenance or measurement personnel.
At this point, calibration will identify or eliminate the risk of inconsistencies with the thermal resistor immediately before the calibration week begins, thereby ensuring the highest product quality.
According to the sensor's design specifications, the minimum temperature threshold required to initiate self-calibration is 118°C. This typically occurs during each sterilization cycle.
However, since not all hygiene applications involve a sterilization process, other methods can be used to utilize self-calibration. One method involves removing the sensor from the process and placing it in a ceramic dry block heat source to reach a temperature of 118°C.
Once this temperature is reached, the drying block can be shut off and allowed to cool, while the sensor remains in the drying block heater. When the temperature drops below 118°C, self-calibration will occur, ensuring the thermal resistance remains within tolerance, which will significantly reduce process downtime.
Key concepts:
■ Sensor components may fail when removed from the process for calibration.
■ In-situ calibration can reduce the risk of sensor failure.
Think about it
Can in-situ calibration save time and resources in the process?
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