For temperature monitoring, the thermometer is an indispensable device. It is used in laboratories and several food operations and manufacturing industries. It should work correctly, showing only the correct temperature, so functions can be carried out smoothly.
But usual wear and tear might render it incorrect or dysfunctional. So to keep it working properly, thermometer calibration is an unavoidable process that you must undertake regularly.
This guide outlines everything concerning thermometer calibration, which will aid you in learning about this essential preventive maintenance practice.
Types of Thermometer Calibration
Different types of thermometers are used for different operations, and their calibration depends on their usage and manufacturing. For example, a Bi-metal coil thermometer differs entirely from an infrared thermometer; thus, their calibration methods differ.
To check whether the thermometer is displaying the correct temperature, the following methods are used for its calibration.
Boiling Water Method
This popular method is used to calibrate a bi-metal coil thermometer.
Materials required:
- A sturdy, heat-tolerant container
- A hot plate
- Distilled water
- Reference Thermometer (It can be a NIST or ISO thermometer, depending on the company size or lab)
Steps To Calibrate Thermometer
- Fill the container with distilled water. Heat it to the reference temperature. Since it is the boiling water method and the boiling temperature of the water is 100⁰C, the reference temperature will be 100⁰C or 212⁰F for sea level. The boiling temperature of the water also changes with the altitude. So, determine whether you are at sea level or above.
- Once the water is boiling, immerse up to 2 ½ inches of the underlying thermometer to be calibrated in it. Also, immerse the reference thermometer. Ensure that none of the thermometers touch the bottom of the container.
- Let them be for one minute. Then record the readings of both thermometers. Compare the deviation of the underlying thermometer against the reference thermometer.
- If the underlying thermometer is digital, you may find a calibration button. For others, you might have to adjust the needle using the hex nut. If no manual calibration is allowed, you are advised not to use the thermometer if the accuracy deviation is more than +/-0.5C.
- Once calibrated, repeat step number 2 and 3. Record calibration adjustments.
- Mark the underlying thermometer to use for measuring temperature within a specified range.
Ice Water Method
This method is used for thermometers which are used for cold temperatures. It is quite similar to the boiling water method.
Materials required
- A sturdy, heat-tolerant container
- Crushed Ice
- Distilled water
- Reference Thermometer (It can be a NIST or ISO thermometer, depending on the company size or lab)
Steps To Calibrate Thermometer
- Pour crushed ice into the container. Pour distilled water to make a slushy so thermometers can be dipped.
- Now, immerse up to 2 ½ inches of the reference thermometer in the container for a minute. Ensure that the thermometer is not touching the bottom of the container.
- Check whether the reference thermometer is reading 0⁰C (32⁰F) or not. If not, put some more ice in the baker or use another slushy mixture. Check the temperature again.
- Once the reference thermometer is equilibrated, immerse up to 2 ½ inches of the underlying thermometer to be calibrated. Ensure that the thermometer is not touching the bottom of the container.
- Then record the readings of both thermometers. Compare the deviation of the underlying thermometer against the reference thermometer.
- If the underlying thermometer is digital, you may find a calibration button. For others, you might have to adjust the needle using the hex nut. If no manual calibration is allowed, you are advised not to use the thermometer if the accuracy deviation exceeds +/-0.5C.
- Once calibrated, repeat step number 4 and 5. Record calibration adjustments.
- Mark the underlying thermometer to use for measuring temperature within a specified range.
These are the two methods that are popularly used for thermometer calibration. Other than these, if your lab has any electronic temperature instruments, read the user manual from the manufacturer to learn the calibration method.
Conclusion
The abovementioned methods are useful for thermocouples, thermistors, and bi-metal coil thermometers. Resistance Temperature detectors are hard to calibrate, and their replacement is recommended instead of calibration. Infrared thermometers require a black body instrument for calibration, which is expensive and is only found at certain labs.