Fixed temperature points

Chapter 8. Temperature scales and temperature fixed points... 

Even if nowadays, the MCT may be considered a primary thermometer only on a narrow temperature range, it is considered the best dissemination standard in the millikelvin range [56-59], In fact, the 3He melting pressure is a good thermometric property because of its sensitivity over three decades of temperature with a resolution A T/T up to 10 5 [56], The good repeatability, the insensitivity to magnetic fields up to 0.5 T [60] and the presence of temperature-fixed points allow for the control of possible shifts in the calibration curve of the pressure transducer. The usefulness of these fixed points is evident, considering that the ITS-90 is based just on the definition of fixed points. 

Table 6.2. A selection of temperature fixed points according to the (1990) Revision of the International Temperature Scale.

The measurement of temperature is one of the most common physical measurements routinely made. It is so common that it is often overlooked as a variable when complex biochemical reactions are being studied. This is unfortunate, because an error in the temperature of a reaction may produce a large error in the results that becomes apparent when the results are compared with those of known standard reactions. For example, if the rate of reaction of an unknown enzyme is being studied at a temperature that is different by 0.1°C from the temperature at which the standard reaction was measured, an error as large as 2-5% in the observed rate of reaction can occur. The experimental data would not correlate then with the known enzyme reaction rates. Such errors lead to confusion in determining mechanisms and to the large variations that occur even in normal values from one clinical laboratory to another. This article seeks to bring the importance of accurate temperature measurements to the attention of biomedical scientists. We will identify the latest methods of temperature measurement and control as well as new temperature fixed-point standards that are or will shortly become available. 

Whatever thermometer is used, in either routine or in standardization applications, it must be calibrated periodically against a primary standard, or at least its calibration must be checked periodically at some temperature fixed points (defining fixed points or well-characterized secondary fixed points) in the range of the thermometer. 

Other, accurately known temperature fixed points, conveniently spaced throughout the interval from —40°C to as high as 160°C, are currently being developed and when work on them is completed, they will permit the user to calibrate thermistors, as well as other types of thermometers, in this range. To be most useful to the biomedical community, these temperature reference points should be well-defined, stable, and reproducible to 0.005°C. 

What are some of the features of high-quality (defining and/or secondary) temperature fixed points that make them suitable for use in calibrating precision thermometers ... 

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