Temperature Fixed Points and Resistance Thermometry

The thermodynamic temperature of a system of mutually interacting particles is proportional to the average of the distribution of kinetic energy within the system. A true measurement of system temperature would require a detailed knowledge of this distribution and how it changes when heat is added or removed; clearly difficult if not impossible. This implies that practical temperature measurement must rely upon inferred rather than discretely calculated values as would be obtained through direct observation.

Phase change phenomena in pure materials are unique in that phases of different density co-exist in equilibrium. When small amounts of heat are added or removed only the relative phase proportions vary, leaving the overall energy distribution and thus the temperature of the system unchanged. Boiling points, freezing points and triple points are typical of constant temperature phenomena. Ideally, the temperature at which these transitions occur is unique to the material and completely reproducible. If realized under controlled conditions, these phase change equilibriums can provide fixed, reference points for thermodynamic temperature. The absolute thermodynamic temperature of such points has been established through a variety of measurement techniques and by utilizing years of historical comparison data. They are updated as necessary when a new international temperature scale is promulgated.

When two bodies are brought into intimate thermal contact, they will reach the same temperature. If some property of one of the bodies is well behaved in its reaction to changes in temperature and this property can be conveniently measured, such a body can be used as a thermometer. The measured value of a thermometer at a fixed point is a direct representation of a known thermodynamic temperature. Temperatures in between the fixed points can be interpolated once the typical thermal properties of the thermometer have been established.


Standard Platinum Resistance Thermometer (SPRT)


The electrical resistivity of pure platinum wire increases monotonically with temperature. An excellent thermometer can be made by winding coils of very fine platinum wire on a specially designed form and sealing the structure inside a suitable container with an inert gas. Although the resistance of two different coils of identical construction will rarely be the same at any given temperature, the ratio of resistance at any temperature to that at a chosen reference temperature will behave in roughly the same way for all thermometers made of high purity, strain free platinum. The behavior of an SPRT at temperatures between the fixed points can be adequately described by a curve whose characteristics are defined by the measured fixed point values specific to that thermometer. A set of interpolation equations (which are functions of SPRT resistance ratios) solved at the fixed point temperatures defines the temperature scale as represented by a specific SPRT.

The reference point for determining the resistance ratio of an SPRT at any other temperature is the triple point of water at 0.01 °C (273.16 K). Specially designed and constructed Triple Point of Water Cells have been in use for many years at standards labs across the world. They are used on a regular basis to determine and re-verify the reference resistance for SPRTs.

The International Temperatuire Scale of 1990 (ITS-90) defines the SPRT as the accepted instrument for the practical realization of the Kelvin Thermodynamic Temperature Scale (KTTS) from the triple point of Hydrogen at -259.35 °C (13.80 K) to the freezing point of Silver at 961.78 °C (1234.93 K ). The specifics of thermometer contruction, necessary fixed point devices and the overall techniques needed for approximating ITS-90 can be found here. There are several manufacturers of fixed point systems, SPRTs and measurement instrumentation as well as many companies who provide calibration services.

Please feel free to contact Zero K Designs for advice and guidance in creating and/or maintaining temperature calibration systems.