Thermometer scales, comparison

Comparison of thermometers Showing the relative indications of the Fahrenheity Centigrade and Reaumur thermometer scales... 

The temperature of a gas is related to the kinetic energy of its particles. For example, if we have a gas at 200 K in a rigid container and heat it to a temperature of 400 K, the gas particles will have twice the kinetic energy that they did at 200 K. This also means that the gas at 400 K exerts twice the pressure of the gas at 200 K. Although you measure gas temperature using a Celsius thermometer, all comparisons of gas behavior and all calculations related to temperature must use the Kelvin temperature scale. No one has yet achieved the conditions for absolute zero (0 K), but we predict that the particles will have zero kinetic energy and exert zero pressure at absolute zero. 

The most satisfactory and the most accurate method for the calibration of a thermometer is to compare it directly with short-scale thermometers, each of limited range, which have been standardised at the National Physical Laboratory.f The comparison should be made as far as possible under the same conditions as the thermometer is to be used. For experiments demanding the highest precision e.g., in research work), it is a good plan to have two similar sets of short-scale thermometers, one of which has been standardised at the N.P.L. the unstandardised set is... 

Any instrument which can be used for measuring temperatures is called a thermometer. Thermometers may be, and are, constructed which utilise any property of a body such as those mentioned above. To evade the difficulty of comparison of scales, they are usually all referred to a gas thermometer, with Centigrade scale as standard. The ice and steam-points on the latter are taken as 0° and 100° respectively. 

The utility of c) as a thermometer gains further support from direct measurements showing correlation of temperature and 3 of precipitation through time over seasonal cycles (Shuman et al, 1995). At longer temporal scales, temperature measurements in boreholes and gas composition measurements both provide temperature information which can be compared to 3. Results from such comparisons have so far... 

When a hot body and a cold body are brought into physical contact, they lend to achieve the same warmth after a long lime. These two bodies are then said to be at thermal equilibrium with each other. The zeroth law of thermodynamics (R.H. Fowler) states that two bodies individually at equilibrium with a third are at equilibrium with each other. This led lo the comparison of the states of thermal equilibrium of two bodies in lei ms ol a third body called a thermometer. The temperature scale is a measure of state or thermal equilibrium, and tw-o systems at thermal equilibrium must have the same temperature. 

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 value of 1/a for nitrogen at 1 atm is 272. Experiments with other gases indicate that the ice point, 0°C, is equivalent to a value of near 273 K. The Kelvin scale is now defined with high accuracy such that the triple point of water (where ice, water and water vapour are all in equilibrium, at 0.01°C) has the temperature 273.1600° on the Kelvin scale. The triple point is more accurately defined than the ice point. On this basis 0°C is 273.15 K. Measurement on this ideal gas scale is best conducted with a constant pressure helium thermometer, although there are small deviations from the absolute scale. A comparison of the four temperature scales discussed above is given in Table 1.1. 

Table 1.1 Comparison between thermometers, calibrated linearly from 0° to 100°C and the perfect gas scale...

---