Thermometer Calibration by the Comparison Method
When employing thermometer calibration by the Comparison Method, readings from a thermometer with unknown accuracy are compared to those from a standard device. The standard device is calibrated to meet the quality requirements of the National Institute of Standards and Technology (NIST) or a similar governing body.
Typically, this method of calibration is used for liquid-in-glass thermometers. This technique often applies to Standard platinum resistance thermometers (SPRT) and resistance temperature detectors (RTD) for industrial equipment as well.
Common Thermometer Types
Two types of liquid-in-glass thermometers exist:
- Mercury thermometers contain a bulb filled with mercury attached to a narrow tube. Changes in temperature yield changes in the volume of mercury. These small volume changes drive the mercury up the tube or pull it down the tube.
- Alcohol or spirit thermometers look and act like mercury thermometers. However, they use ethanol instead. Ethanol is less toxic than mercury, and cleans up more easily after breakage because ethanol evaporates quickly. Since ethanol costs less than mercury, replacing broken thermometers also incurs less cost.
The three categories of mercury and spirit thermometers include:
- Complete Immersion Thermometers, which show temperature correctly when completely covering the entire thermometer with fluid (gas or liquid).
- Total Immersion Thermometers, which show temperature correctly when covered by fluid except for a small portion of the column [6 to 12 mm (0.24 to 0.47 in )].
- Partial Immersion Thermometers, which show temperature correctly when immersed to a specific depth. A line on the thermometer usually indicates required depth.
Thermometer calibration by the Comparison Method includes the following steps:
- Review the device to be calibrated.
- Prepare the calibration bath.
- Test the device to be calibrated.
- Record any difference and reset if possible.
Next, let us take a closer look at the processes involved in each step.
1. Review the device to be calibrated.
- Document the level of accuracy required for the tasks the device will be used to complete.
- Look at the device to be calibrated. Ensure that the column and bulb are not cracked and the legibility of the scale.
- Note any identification numbers. These numbers often trace back to manufacturer’s specifications that could help in making calibration decisions.
2. Prepare the calibration bath.
- Set the calibration bath to the desired calibration temperature.
- Ensure the immersion of the NIST standard thermometer in the calibration bath at the proper depth.
- If calibrating more than one device, begin with the device with the lowest temperature.
- Wait until the calibration bath stabilizes at the desired temperature.
- Use the NIST standard thermometer to measure the temperature.
- When the temperature remains unchanged for at least three readings, taken thirty (30) seconds apart, the bath has stabilized.
3. Test the device to be calibrated.
- Insert the thermometer to be calibrated into the calibration bath at the proper immersion depth.
- Allow the thermometer to be calibrated to achieve a stable temperature. The temperature is stable when the reading on the device has not changed for at least three readings, taken thirty (30) seconds apart.
4. Record any difference and reset if possible.
- Use a magnifying glass to look at the tested device.
- Identify the difference between the device’s output and the calibration bath.
- Document the difference.
- Perform any reset using manufacturer’s instructions.
- Repeat steps 2-4 for remaining devices being calibrated at higher temperatures.
Thermometer calibration by the Comparison Method enjoys popularity as one of the most widely used techniques. Restaurants employ this method via ice bath to ensure proper temperatures for food storage, and home chefs may use it in conjunction with oven thermometers to get the known offset of an oven’s settings.
As we’ve seen here, however, laboratory-grade thermometer calibration by the Comparison Method involves a much more rigorous procedure and complies with the standards of the appropriate governing bodies.
Thermometer Calibration by the Fixed Point Method
What is the Fixed Point Method?
The Fixed Point Method of thermometer calibration assures the quality and accuracy of a thermometer’s measurements. Typically, only national metrology laboratories use it. Thermometer calibration by the fixed point method focuses on instruments that must measure accurately within ±.001℃.
This method uses the ITS-90 international temperature scale developed in 1990. Based on the thermodynamic or absolute temperature scale, ITS-90 is not truly a scale. It is a set of fixed points that defines an international equipment calibration standard. It helps scientists know that a temperature measured in Asia will be the same as a temperature measured in Europe. Scientists can repeat results regardless of location with properly calibrated equipment.
In creating the ITS-90 standard, a body of scientists selected seventeen fixed points. These points are based on temperatures where various elements or compounds achieve equilibrium. These points include :
Although Freezing Point and Melting Point are common terms, Triple Point and Vapor Pressure Point might not be. The Triple Point is the temperature of a substance where all the phases of matter – solid, liquid, and gas – exist in equilibrium. The Vapor Pressure Point is the temperature of a substance where the gas and liquid phases are in equilibrium.
How is the Fixed Point Method used in thermometer calibration?
To achieve fixed point calibration, the thermometer to be calibrated is placed in a specially designed flask. This flask maintains a constant temperature through both heating and cooling. The technician selects a limited number of fixed points from the ITS-90 with the goal of using as few as possible. The actual procedure varies depending on the fixed points selected. 
The points of the ITS-90 temperature scale have been identified, documented, and globally accepted. As a result, technicians with proper training can reproduce standard calibration conditions.
 B.W. Magnum. “International Temperature Scale of 1990 (ITS-90).” John R. Rumble, Editor-in-Chief. CRC Handbook of Chemistry and Physics, 98th Edition, 2017-2018. CRC Press, Boca Raton, FL (USA): 2017. 1-17.
 http://www.evitherm.org/files/795/ThermometerCalibrationMethods.pdf, last visited 01/18/2018