Temperature measurement practical standards

The ITS is an artifact scale, designed to relate temperature measurements made with practicable instruments as closely as possible to the thermodynamic scale. The scale is established and controlled by the International Committee of Weights and Measures (BIPM) through its Consultative Committee on Thermometry, which was established in 1937. The BIPM itself is established to maintain and implement the Treaty of the Meter, to which most nations of the wodd subscribe thus the ITS has not only scientific but legal status in most nations. Within nations, the Temperature Scale is maintained by national standards establishments, eg, in the United States the National Institute for Standards and Technology (NIST), in England the National Physical Laboratory (NPL), and in Germany the Physikalisch-Technische Bundesanstalt (PTB). 

The primary reference electrode for aqueous solutions is the standard hydrogen electrode (SHE), expressed by H+(a=l) H2(p=105 Pa) Pt (see 11 in Section 4.1). Its potential is defined as zero at all temperatures. In practical measurements, however, other reference electrodes that are easier to handle are used [24]. Examples of such reference electrodes are shown in Table 5.4, with their potentials against the SHE. All of them are electrodes of the second kind. The saturated calomel electrode (SCE) used to be widely used, but today the saturated silver-silver chloride electrode is the most popular. 

The standard temperature selected for the values given in this book is 18° Centigrade, following the procedure of the thermochemistry section (Bichowsky1) of the International Critical Tables. The authors have been reluctant not to use the almost universally accepted standard temperature of 25° Centigrade for thermodynamic calculations but the selection of 18° as the standard temperature is practically necessary in this case because all of the monumental work of Julius Thomsen and of Marcellin Berthelot was done at or near 18° and there are not now available sufficient heat capacity data with which to make accurate conversion to 25° (this is especially important for reactions involving substances in aqueous solution where the temperature coefficient is usually very large). In later years, as the data on heat capacities become available, or as the heats of many of the reactions, which have until the present time been measured only by Thomsen or Berthelot or both, are redetermined, it will be quite feasible to use 25° as the standard temperature. 

The International Practical Temperature Scale of 1968 (IPTS-68) is currently the internationally accepted method of measuring temperature reproducibly. A standard platinum resistance thermometer is the transfer medium that is used over most of the range of practical thermometry. 

Accurate temperature calibration using the ASTM temperature standards [131, 132] is common practice for DSC and DTA. Calibration of thermobalances is more cumbersome. The key to proper use of TGA is to recognise that the decomposition temperatures measured are procedural and dependent on both sample and instrument related parameters [30]. Considerable experimental control must be exercised at all stages of the technique to ensure adequate reproducibility on a comparative basis. For (intralaboratory) standardisation purposes it is absolutely required to respect and report a number of measurement variables. ICTA recommendations should be followed [133-135] and should accompany the TG record. During the course of experiments the optimum conditions should be standardised and maintained within a given series of samples. Affolter and coworkers [136] have described interlaboratory tests on thermal analysis of polymers. 

The emphasis in this work has been on the acquisition of simultaneously-obtained instantaneous values of temperature and concentration, with as high a spatial resolution as practical for such experiments. The temporal and spatial resolution requirements result from the necessity to probe within (if at all possible) characteristic turbulence time and length scales. The accuracy of our experiments (which, in any case, utimately depends upon a trade-off with resolution (1)), is considered to be adequate to achieve the diagnostic goal of providing data of value to flame modelers this can be seen by comparison of the fluctuation temperature measurement uncertainty (characterized by a 5-7% standard deviation) with the broad temperature spread of the measured pdf s (extending, in Fig. 4, from values near ambient temperature to values in the vicinity of the adiabatic flame temperature). ... 

In practice it is the International Practical Temperature Scale of1968 (IPTS-68) which is used for calibration of scientific and industrial instruments-t This scale has been so chosen that temperatures measured on it closely approximate ideal-gas temperatures the differences are within the limits of present accuracy of measurement. The IPTS-68 is based on assigned values of temperature for a number of reproducible equilibrium states (defining fixed points) and on standard instruments calibrated at these temperatures. Interpolation between the fixed-point temperatures is provided by formulas that establish the relation between readings of the standard instruments and values of the international practical temperature. The defining fixed points are specified phase-equilibrium states of pure substances, t a given in Table 1.2. 

Calorimetric methods are infrequently used for routine quality control purposes because of their non-specific nature and relatively slow speed. However, data from calorimetry experiments are commonly presented in applications for new product licenses and in support of patent applications. To ensure the integrity of all calorimetry data, normal procedures for good laboratory practices, standard operating procedures, appropriate calibration methods, and regular instrument servicing are necessary. The use of DSC for the measurement of transition temperatures and sample purity is described in the United States Pharmacopoeia, and standard procedures for DSC analyses are also suggested by the ASTM (100 Barr Harbor Dr., West Conshohocken, Pennsylvania 19428). 

In view of the elaborate experimental techniques usually required to make accurate thermodynamic temperature measurements, the need for a practical scale above about 0.5 K that is close to the Kelvin thermodynamic temperature scale remains great. There are several modifications that can be anticipated for a future IPTS. They include assigned values of fixed points that are in closer agreement with thermodynamic temperatures (as determined by recent experiments), extension of the range covered to lower temperature, improved standard instruments for interpolation procedures. It is expected that there will be a scale revision which will encompass the above, and that the new scale will be adopted by about 1987. 

Primary standards are those developed and maintained by national standards laboratories such as the National Bureau of Standards. These laboratories develop, maintain, and disseminate standards, such as the International Practical Temperature Scale. The IPTS-68 is disseminated to the users through secondary standards such as calibrated thermometers, fixed point references, and so on (see Table II). Some of these thermometers are calibrated directly against the defining fixed points and others are calibrated over the range of need by a comparison calibration against a standard interpolating thermometer. This ensures that the basis for temperature measurement, the IPTS-68, is the same everywhere throughout the world. 

The standardized scale now used in temperature measurement is the International Temperature Scale of 1990 (ITS-90) [1-3]. ITS-90 has been designed to give values as close to the corresponding thermodynamic temperatures as practically possible. It covers the range of... 

Thermogravimetry needs a check of the accuracy of temperature, mass, and time measurements. Practically all thermobalances are capable of producing good data with only infrequent checks of the calibration via a standard mass. Since changes in volume of the sample take place, a buoyancy correction should be done routinely. The mass, m, of the displaced gas can easily be calculated from the ideal gas law (m = pMAV/RT). 

The flash point, denoted PP or jp, expressed in K( C), is the temperature at which a pool of a flammable liquid will generate sufficient vapors to form an ignitable vapor/air mixture. It can also be seen as the temperature at which a liquid will reach its lower flammabihty limit (LFL) in air. However, flammable liquids can also ignite below their flash point if the surface area is increased either by dispersion (e.g., aerosol) or by mechanical activation (e.g., spraying) that raises the concentration of vapor in air above the lower flammability limit. In practice the flash point of a liquid is measured following standardized laboratory test protocols such as the Continuously Closed Cup Test (ASTM D6450), the Pensky-Martens Closed Cup Test (ASTM D93) or by the Tag Closed Cup Test (ASTM D56). 

The discussion of DTA in the previous section describes the precautions and recommended practices for calculating the melting point from DTA or DSC curves. The advent of instrumentation for simultaneous DTA/TG or DTA/EGA allows the TG and EGA instruments to be calibrated directly as well. Many secondary standards for temperature measurements have also been developed for thermal analysis. These have been based on solidj—solid2 crystallographic transitions or magnetic transitions, that is, Curie or Neel temperatures. 

Note 6— It is good practice to calibrate the thermocouple or other temperature-measuring device against a standard thermocouple or reference standards about once a week, when the furnace is in constant use, the actual frequency depending on experience. 

The density determination may be carried out at the temperature of the laboratory. The liquid should stand for at least one hour and a thermometer placed either in the liquid (if practicable) or in its immediate vicinity. It is usually better to conduct the measurement at a temperature of 20° or 25° throughout this volume a standard temperature of 20° will be adopted. To determine the density of a liquid at 20°, a clean, corked test-tube containing about 5 ml. of toe liquid is immersed for about three-quarters of its length in a water thermostat at 20° for about 2 hours. An empty test-tube and a shallow beaker (e.g., a Baco beaker) are also supported in the thermostat so that only the rims protrude above the surface of the water the pycnometer is supported by its capillary arms on the rim of the test-tube, and the small crucible is placed in the beaker, which is covered with a clock glass. When the liquid has acquired the temperature of the thermostat, the small crucible is removed, charged with the liquid, the pycnometer rapidly filled and adjusted to the mark. With practice, the whole operation can be completed in about half a minute. The error introduced if the temperature of the laboratory differs by as much as 10° from that of the thermostat does not exceed 1 mg. if the temperature of the laboratory is adjusted so that it does not differ by more than 1-2° from 20°, the error is negligible. The weight of the empty pycnometer and also filled with distilled (preferably conductivity) water at 20° should also be determined. The density of the liquid can then be computed. 

In discussing Fig. 4.1 we noted that the apparent location of Tg is dependent on the time allowed for the specific volume measurements. Volume contractions occur for a long time below Tg The lower the temperature, the longer it takes to reach an equilibrium volume. It is the equilibrium volume which should be used in the representation summarized by Fig. 4.15. In actual practice, what is often done is to allow a convenient and standardized time between changing the temperature and reading the volume. Instead of directly tackling the rate of collapse of free volume, we shall approach this subject empirically, using a property which we have previously described in terms of free volume, namely, viscosity. 

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