How water triple points save costs and minimize risks in precision thermometry

In the world of precision thermometry and calibration, where accurate temperature measurements are essential for a wide range of applications, water triple point cells and gallium melting point cells play a key role.

The use of these fixed temperature points in temperature laboratories, especially for users of standard platinum resistance thermometers (SPRT) or high quality industrial platinum resistance thermometers (PRT) can save costs and minimise risks.

The value of regular checks

While the use of these precision thermometers relies on external calibration by specialised temperature calibration laboratories, the question is: what happens between calibration cycles? Thermometers can be affected during transport or improper handling, which can lead to changes in their values (thermometer drift). Such a change, which is only detected during the next calibration, can have serious consequences, such as the possible invalidation of all previous measurements, which would require a call-back of all calibrated instruments. Such incidents can severely affect confidence in temperature calibration and cause significant costs. However, these risks can be minimised by regular checks of the thermometers at the water triple point and the gallium melting point.

Checks at the water triple point

The calibration certificate of an accredited temperature calibration laboratory will show the last value of the water triple point. After receiving a calibrated thermometer, it should be checked at the water triple point and the result compared with this value. Such a check allows an uncertainty of measurement of less than 0.001°C and is an essential step in ensuring the reliability of the thermometer between calibrations.

The gallium melting point: another way to evaluate thermometer reliability

The Gallium Melting Point allows the resistance value of a thermometer to be measured at 29.7646°C. It is easy to use and offers very low measurement uncertainties. The combination of the gallium melting point and water triple point enables the calculation of the resistance ratio (WGA), a crucial value for assessing the reliability of the thermometer.

This so-called W-value is calculated from the current resistance — in our case the resistance at the gallium melting point R(GA) — and the last known resistance value of the thermometer at the water triple point R(WTP):

W(GA) = R(GA) / R(WTP)

The benefit of the W-value calculation

The W-value is an important quantity in temperature measurement because it practically calculates the characteristic curve of a thermometer. The purer the platinum of the temperature detector of the resistance thermometer, the higher this W-value.

There are certain scenarios that can lead to changes in the performance of the thermometer. Suppose the resistance value of the thermometer at the water triple point increases, but the W-value at the gallium melting point (WGA) remains constant. In such cases, this indicates that the characteristic curve has a parallel shift. This is the classic thermometer drift. This drift effect can happen due to mechanical and thermal stresses. The good news is that such changes are often reversible, and calibration can correct these effects and is therefore useful.

However, if the W-value changes (typically it decreases), this is an indication that the thermometer is contaminated. Unfortunately, such changes are often not reversible and in many cases the thermometer can no longer be calibrated.

Regular checks of the WGA can determine how the thermometer is being stressed in everyday use. These checks can therefore provide a valuable aid to deciding whether or not calibration is reasonable. They can also help to maximise the life and accuracy of the thermometer and prevent unexpected failures.

Advantages of the regular measurement at the water triple point and W-value

The regular measurement of the values R(WTP) (resistance at the water triple point) and W(GA) (ratio of the resistance at the water triple point to the resistance at the gallium melting point) has a number of advantages:

Cost savings: Regular in-house checks allow users to extend calibration periods in accredited, external laboratories, resulting in significant cost savings.

Increased confidence: Regular checks, confirming the accuracy and reliability of the thermometers, will increase the level of confidence in the laboratory.

Avoidance of errors: By identifying and correcting potential problems, erroneous measurements can be avoided that could otherwise lead to serious consequences.

In addition, the use of gallium melting points and water triple points in temperature laboratories allows significant risk minimisation by reducing the need to transport thermometers for external calibrations. This not only saves transport costs, but also minimises the risk of damage to the thermometers.

It is advantageous for users of SPRTs and PRTs to have access to their own water triple point and gallium melting point cells. They allow regular checks and adjustments, increase confidence in the reliability of the thermometers and can reduce the need for external calibrations and associated costs and risks. In a world where precision is critical, water triple points and gallium melting points provide an effective way to ensure accuracy and reliability in thermometry.