Thermometer Calibration

Thermometer calibration is the process of ensuring that a thermometer provides accurate temperature measurements. This is accomplished by comparing the thermometer’s readings with a reference standard (for example, a temperature fixed point) or a known temperature source (comparison calibration). The goal of calibration is to identify any deviations or errors and, if necessary, make adjustments so that the thermometer delivers accurate measurements.

Methods for Calibrating Thermometers

Fundamentally, there are two different methods for calibrating thermometers: calibration using the comparison method and calibration at temperature fixed points.

Thermometer Calibration Using the Comparison Method

The Zeroth Law states that if two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. Based on this law, one can conclude that if a calibrated thermometer indicates the same temperature as the actual temperature in a calibration bath and the thermometer to be calibrated also indicates this temperature, both thermometers measure the same temperature. However, this only applies under the condition that an equilibrium state exists, which is not always the case in the real world.

The comparison method is one of the most common and efficient techniques for calibrating thermometers. In this method, the thermometer under test is compared with a high-precision reference thermometer whose measurement accuracy and traceability to national or international standards is ensured.

Thermometer Calibration Procedure

1. Calibration Preparation:
   • The thermometer under test and the reference thermometer are prepared. Both must be clean and in perfect condition.
   • A stable calibration device, such as a calibration bath or a dry-block calibrator, is set to the target temperature. These devices ensure homogeneous and stable temperature distribution.
2. Define Temperature Points:
   • Calibration is typically performed at several calibration points within the thermometer’s measurement range, e.g., at 0°C, 50°C, and 100°C. The points may vary depending on the application.
3. Perform Comparison Measurement:
   • Both thermometers are simultaneously placed in the calibration device, ensuring that they are at the same temperature. This minimizes temperature gradients and ensures precise results.
   • Once the temperature in the calibration device is stable, the readings of the reference thermometer and the thermometer under test are recorded.
4. Analyze Deviations:
   • Calibration means “determining a deviation.” The difference between the displayed values of the reference thermometer and the thermometer to be calibrated is determined. These deviations are documented and can be used for adjustment if necessary.

Advantages of the Comparison Method

• High Accuracy: Thanks to the use of a precise reference thermometer and stable calibration devices, the results are very reliable.
• Flexibility: The method is suitable for various thermometer types, including liquid-in-glass, resistance, or infrared thermometers.
• Efficiency: Multiple thermometers can be calibrated simultaneously, making the method particularly economical in practice.

Typical Applications

The comparison method is frequently used in testing laboratories, industry, and calibration laboratories, particularly when high accuracy is required. It is ideal for routine calibrations and quality assurance in processes where precise temperature measurements are critical.

Thermometer Calibration at Temperature Fixed Points

In thermometer calibration at temperature fixed points, the temperature standard is not a calibrated reference thermometer but rather a so-called temperature fixed point. These temperature fixed points are used as defining temperatures of the ITS-90 temperature scale and for calibrating thermometers.

The International Temperature Scale of 1990 (ITS-90) defines the temperature scale using nine fixed points in the range from -189.3442°C (triple point of argon) to 961.78°C (freezing point of silver). These fixed points are thermodynamic equilibrium states during phase transitions of pure substances.

An example of a fixed point is the triple point of water, where high-purity water exists at 0.01°C in solid, liquid, and gaseous states. This state can then be used in a water triple point cell to calibrate thermometers at this defined temperature.

How Often Must a Thermometer Be Calibrated?

There is no absolute or definitive answer to this question. The frequency of thermometer calibration generally depends on various factors:

1. Intended Use of the Thermometer: A thermometer used in critical applications such as medicine or the food industry may need to be calibrated more frequently than a simple household thermometer.
2. Accuracy Requirements: Some processes require very high temperature accuracy. In such cases, regular calibration is essential.
3. Environmental Conditions: Thermometers used under extreme conditions (e.g., very high temperatures) may be more susceptible to inaccuracies and should therefore be checked more frequently.
4. Previous Calibration Results: If the last calibrations showed no or only minor deviations, the time between calibrations may possibly be extended. However, if significant deviations are found at each calibration, the calibration frequency should be increased.

As a general rule, critical thermometers should be calibrated at least once annually. However, in some industries or for certain applications, more frequent calibration may be required, e.g., every three or six months. It is always a good idea to include regular inspection and calibration in the maintenance schedule to ensure that the thermometer is functioning correctly.

One can also refer to standards and guidelines when establishing calibration cycles. For example, DIN EN ISO/IEC 17025 is an international standard that specifies requirements for the competence of testing and calibration laboratories. If a laboratory is accredited to this standard, it means that it possesses the technical competence and has established a management system that ensures the generation of consistent and valid results.

The calibration of measuring instruments, including thermometers, is an important component of this standard. Some key points regarding calibration from DIN EN ISO/IEC 17025 are:

1. General Requirements: Laboratories must ensure that all equipment that could influence the results is calibrated and/or qualified.
2. Intervals: The standard does not prescribe specific calibration intervals. Instead, laboratories should use their risk management to determine the frequency of calibrations.
3. Traceability: Calibrations should be traceable to national or international standards.
4. Records: Laboratories must maintain records of calibrations, including details on the method, operator, environmental conditions, confirmation intervals, results, and any deviations.

The German Accreditation Body (DAkkS) has additional documents and regulations that contain specific requirements and recommendations for calibration in various areas. If a laboratory in Germany is accredited to DIN EN ISO/IEC 17025, it must also comply with the relevant DAkkS rules.

A laboratory accredited to DIN EN ISO/IEC 17025 must have a clear policy and procedures for calibrating its equipment. However, the exact frequency of calibration is determined by the laboratory itself, based on its risk management and the specific requirements of its accreditation.

National and International Standards for Thermometer Calibration

Thermometer calibration plays an important role in metrology, particularly in industries where precise temperature measurements are necessary. National and international standards provide clear guidelines for conducting and documenting these calibrations.

International Standards

• ISO/IEC 17025: This standard defines the general requirements for the competence of calibration laboratories. It ensures that calibration results are internationally comparable and traceable.
• ITS-90: The International Temperature Scale of 1990 provides a reference for calibrating thermometers through defining temperature fixed points. It also defines Standard Platinum Resistance Thermometers (SPRT) as interpolation instruments.
• IEC 60584: This standard governs thermocouples and their thermoelectric voltages, which are important for accurate temperature measurements.
• IEC 60751: This standard describes requirements for platinum resistance thermometers (RTDs), which are frequently used in high-precision applications.

National Standards and Guidelines

• DIN EN ISO 9001: This standard focuses on quality management systems and also affects calibration processes in various industries.
• DKD Guidelines: The German Calibration Service guidelines provide specific instructions for calibration laboratories in Germany. These guidelines supplement ISO 17025 and define specific procedures and methods.
• EURAMET Guidelines: At the European level, EURAMET provides metrological guidelines that support laboratories and calibration facilities in performing consistent and traceable calibrations.

Importance of Traceability

All the standards mentioned emphasize the importance of traceability in thermometer calibration, i.e., the ability to trace calibrations back to international standards. This ensures that temperature measurements are comparable worldwide.

By complying with these standards and guidelines, companies ensure that their thermometers meet the highest standards of accuracy and reliability—an important prerequisite for quality assurance in many industries.

Tips for Successful Thermometer Calibration

Tip 1: A Thermometer Only Measures Its Own Temperature

The statement “A thermometer only measures its own temperature” points to a fundamental principle in thermometer calibration.

When you use a thermometer to measure the temperature of something, whether it is air, a liquid, or a solid body, what you are actually measuring is how warm or cold THE THERMOMETER itself is. The thermometer reaches thermal equilibrium with the medium it is measuring. This means that it assumes the same temperature as the medium.

A simple example is a mercury thermometer. The mercury in the thermometer expands and contracts based on how warm or cold it is. If you immerse it in warm water, the mercury will expand because it is getting WARMER. If you immerse it in cold water, it will contract because it is getting COLDER. In both cases, the thermometer is actually measuring how warm or cold THE MERCURY is, not directly the water. But because the mercury quickly reaches thermal equilibrium with the water, the thermometer effectively indicates the temperature of the water.

The same principle applies to digital thermometers, PT100 (resistance thermometers), thermocouples, and others. They all respond to temperature changes by changing their own temperature and then displaying or measuring that value.

It is important to note that for an accurate temperature measurement, the thermometer and the object to be measured must have sufficient time to reach thermal equilibrium. Otherwise, the measurement could be inaccurate.

Tip 2: Thermocouples Always Perform a Differential Measurement

A thermocouple consists of two different metals that are joined together at one end. When a different temperature exists at this junction (called the “measuring point” or “hot junction”) than at the other end of the two metals (called the “reference junction” or “cold junction”), a voltage is generated between these two points. This voltage is called the thermoelectric voltage and depends on the temperature difference between the two ends and the specific material properties of the two metals.

This means that a thermocouple always measures the temperature difference between the measuring point and the reference junction. To determine the absolute temperature at the measuring point, the temperature at the reference junction must be known. Often this reference junction is cooled to a known temperature (e.g., 0°C with a so-called external reference junction) or, for example, with miniature connectors, room temperature is used as a reference.

A thermocouple therefore does not directly measure an absolute temperature, but rather a temperature difference between two points. To obtain an absolute temperature measurement, the temperature at one of the two points must be known.

Tip 3: Resistance Thermometers Always Measure Too High

Resistance thermometers, often referred to as Pt100 or Pt1000 (where the numbers indicate the nominal resistances at 0°C), use the temperature-dependent resistance of a metal, usually platinum, to measure temperatures. When an electric current flows through a resistor, that resistor is heated. This is a direct result of Ohm’s law, where the electrical power P through a resistor R is described as P = I² × R, where I is the current flowing through the resistor.

In resistance thermometers, a measuring current is passed through the platinum resistor to measure the resistance (and thus the temperature). But precisely this measuring current can—especially if it is too high—lead to significant heating of the measuring resistor. This heating then distorts the measurement result, as the sensor becomes warmer than the actual environment to be measured. As a result, the resistance thermometer indicates a temperature that is too high.

For precise measurements or applications, the heating caused by the measuring current must be taken into account and compensated for.

Sources

• Walter Blanke: The International Temperature Scale of 1990: ITS-90
• Frank Bernhard: Handbook of Technical Temperature Measurement, 2nd Edition
• Thomas Klasmeier: Reference Book “Temperature”, Edition 3