<P> In spite of these restrictions, most generally used practical thermometers are of the empirically based kind . Especially, it was used for calorimetry, which contributed greatly to the discovery of thermodynamics . Nevertheless, empirical thermometry has serious drawbacks when judged as a basis for theoretical physics . Empirically based thermometers, beyond their base as simple direct measurements of ordinary physical properties of thermometric materials, can be re-calibrated, by use of theoretical physical reasoning, and this can extend their range of adequacy . </P> <P> Theoretically based temperature scales are based directly on theoretical arguments, especially those of thermodynamics, kinetic theory and quantum mechanics . They rely on theoretical properties of idealized devices and materials . They are more or less comparable with practically feasible physical devices and materials . Theoretically based temperature scales are used to provide calibrating standards for practical empirically based thermometers . </P> <P> The accepted fundamental thermodynamic temperature scale is the Kelvin scale, based on an ideal cyclic process envisaged for a Carnot heat engine . </P> <P> An ideal material on which a temperature scale can be based is the ideal gas . The pressure exerted by a fixed volume and mass of an ideal gas is directly proportional to its temperature . Some natural gases show so nearly ideal properties over suitable temperature ranges that they can be used for thermometry; this was important during the development of thermodynamics and is still of practical importance today . The ideal gas thermometer is, however, not theoretically perfect for thermodynamics . This is because the entropy of an ideal gas at its absolute zero of temperature is not a positive semi-definite quantity, which puts the gas in violation of the third law of thermodynamics . The physical reason is that the ideal gas law, exactly read, refers to the limit of infinitely high temperature and zero pressure . </P>

What kind of data is a temperature reading