In this article, we will explain the steps necessary to calibrate a temperature probe or a temperature logger, such as a Thermo Button. This guide covers preparation, procedure, comparison of results, the calculation of measurement uncertainty, and the drafting of a calibration certificate.
Equipment Preparation
To perform precise calibration, it is essential to use a reference that is more accurate than the device being calibrated, such as a high-precision PT100 probe. Ideally, this reference should be accompanied by a calibration certificate issued by a metrology laboratory, ensuring traceability and compliance.
Calibration Conditions
Calibration must be performed in a stable and homogeneous environment. It is crucial that the ambient temperature does not fluctuate to avoid any disturbances in the measurements. If the devices are placed in environments where temperature fluctuates, it can lead to incorrect results. Using a calibration bath or a controlled environment is ideal.
It is essential to place both the device being calibrated and the reference probe in the same location, side by side. This precaution prevents temperature differences between the two devices, which could skew the results. If the two devices are placed in slightly different areas, there could be local temperature variations, compromising the accuracy of the calibration.
If you do not have a calibration bath, there are simple methods to perform calibration yourself. However, this should not replace calibration done by a metrology laboratory equipped with the appropriate Machine, Manpower, Machinery, Methods, Measurements, Materials (5M Method).
Simple Method for Calibration at 0°C
A simple and effective way to calibrate a device at 0°C is to use melting ice. Melting ice maintains a stable temperature around 0°C, making it perfect for quickly verifying the accuracy of temperature probes.
- Fill a bowl with crushed ice.
- Add some water to create a melting ice mixture.
- Place both devices (the reference probe and the device being calibrated) in this bowl of melting ice, ensuring they are not in direct contact with the walls of the bowl but fully submerged in the mixture.
- To maintain this stable temperature and avoid fluctuations caused by ambient heat, place the bowl in a refrigerator. This will ensure that the surrounding air temperature does not disrupt the process.
- Wait for both devices to stabilize at 0°C before comparing the values.
This method is commonly used to calibrate thermometers at temperatures close to 0°C and does not require expensive equipment such as calibration baths, but it will not provide results as accurate as a calibration bath.
Simple Method for Calibration at 37°C
If you have an incubator, you can perform your calibration at 37°C by carefully placing both the device being calibrated and the reference probe in the same location inside the incubator, preferably in a container (such as a box or beaker) to avoid temperature variations caused by air circulation in the incubator.
Measurement Procedure
Once both devices (the reference and the device being calibrated) are placed in this stable environment, allow them to measure the temperature for several minutes, typically 30 minutes after the temperature has stabilized. This ensures that the measured values are reliable and do not fluctuate over time.
If you are calibrating a Thermo Button, make sure to set the measurement frequency to 1 minute and the resolution to 0.1°C (for Thermo Button 22L, 22T, or Hygro Button).
Comparison of Values
After the stabilization period, record the values measured by the reference and by the probe or Thermo Button being calibrated. Calculate the average of the measurements taken by each device. This step is crucial to identify any discrepancies between the reference and the device being calibrated.
Measurement Uncertainty Calculation
The calculation of measurement uncertainty allows you to determine the margin of error in your calibration result and evaluate its reliability. It indicates how much a measurement can deviate from the actual value. For example, a temperature reading of 25°C with an uncertainty of ±0.5°C means the actual temperature could be between 24.5°C and 25.5°C.
The lower your uncertainty, the more confidence you can have in your calibration result.
You can use this Excel file to calculate calibration uncertainty, considering:
- The dispersion of values during calibration (standard deviation).
- Uncertainty of the reference, as provided by the calibration certificate of the reference instrument.
- Resolution of the device being calibrated and the reference probe.
These uncertainties are squared, summed, and the square root is extracted, then multiplied by a factor K = 2.
Drafting the Calibration Certificate
The results of the calibration must be formalized in a calibration certificate. This document will include the discrepancies observed between the devices, the measurement uncertainties, as well as mandatory information such as the traceability of the reference, the measurement conditions, and the dates. You can customize this certificate with your company’s information and logo, and it can be stored in a database or printed.
