Stable isotopes help calibrate thermometers

The work of University of Groningen PhD student Vahideh Faghihi will make the calibration of thermometers even more accurate. No big deal for the average user, but important to climate scientists.

The official standard for the Kelvin, the base unit of temperature according to the International System of Units (SI), is taken from what is known as the triple point of water. This is the temperature at which water is present as a gas, liquid and solid, some 0.01 degrees Celsius. The accuracy with which the triple point is measured defines the accuracy of temperature measurements, says Harro Meijer, supervisor to Faghihi at the Energy and Sustainability Research Institute Groningen.

‘The triple point is affected by the purity of the water’, explains Meijer. So the water for the ‘triple point cells’ used for calibration is distilled several times over to remove impurities to below 0.0001 percent. But even then, there is another confounding factor: ‘Water will contain a small quantity of the heavier stable isotopes of oxygen and hydrogen. And the quantity of these isotopes affects the triple point.’ For example, oxygen has an atomic weight of 16, but is also present in heavier forms of 17 or 18.

Margin of error

Now the lab over which Meijer presides is called the ‘Center for Isotope Research’, and that includes stable isotopes. ‘We decided to investigate the effect of these isotopes on the triple point, and see if we could improve the calibration process, so Faghihi produced mixtures with different quantities of hydrogen and oxygen isotopes, and characterized these accurately. Our colleagues at VSL, the Dutch Metrology Institute in Delft, who ensure that SI units in the Netherlands conform with international standards, measured the triple points of these different mixtures. This resulted in a formula that will enable anyone to “normalize” the measurement to a standard isotopic composition.’

How important is this? ‘It is a matter of perhaps tens of microkelvins. Still, there are fields of science and technology where the improvements matter. For example, climate scientists monitor deep sea water temperatures at millikelvin levels.’ However, there are several steps between the official standards and a working thermometer in the field. ‘With each step, the margin of error increases slightly, so the more accurate the first standard, the better.’ The metrological community values the improvement: Meijer has been invited to present the results at the coming international temperature measurement conference.

But the triple point cells may disappear in the foreseeable future. ‘The Kelvin is one of the few SI standards that is still based on a measurement with an “arbitrary” material rather than a constant of nature. Efforts are now being made to define temperature on the basis of the Boltzmann constant.’ But until then, the Faghihi formula – made in Groningen – will provide the most accurate measurement.

Vahideh Faghihi will defend her PhD thesis ‘ Water enriched in the rare stable isotopes: Preparation, measurement and application ’ on 17 June 2016