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TEMPERATURE:

(MEASUREMENT  AND CONTROL)

            The concept of  TEMPERATURE  defines a measure of internal energy associated to the movement (randomly for fluids and vibrations/rotations for solids) of constituent particles of a physical system. This assumes the system is in stationary condition so called thermodynamic equilibrium.

            Absolute Thermodynamic Scale, T has as reference points " 0 "(temperature corresponding to the lowest internal energy), and  " 273.16  " for Triple Point of Water, temperature where pure water, its vapors and ice coexist. Unit is Kelvin (K). Other scales in use are:

                        Celsius,                   T_C = T - 273.15;      ( ^oC )

                        Fahrenheit               T_F = (9/5)T_C + 32    ( ^oF ).

            A system changes its temperature by exchanging heat with its surroundings. Different ovens or cryostats are used to provide desired temperature for sample under study. Devices mentioned above are assisted by Temperature Controllers, units that measure the temperature in the interested area furnishing the right amount of energy to assure the programmed conditions for experiment

            Below is a basic schematic which can be found to most thermo-units:

T  - Thermostat, any device needing controlled temperature;

 H / C  E   - Heating/Cooling element, sometime both are present;

         Tc    - Thermocouple (temperature sensor)

         T.C.   - Temperature Controller;

          L -  Line (providing power to H/C element using the temperature    signal as feedback).

            Thermocouple, the temperature sensor, is made from two wires of different metals, like those in table below, welded at one end. This end will be placed as close as possible to the point of interest, and it provides at output a voltage signal which is a function of temperature in that point. Wires are electrically isolated from the proximity of weld point to their ends, which are connected with right polarity to the temperature controller (or meter).

            The Table below shows the basic properties of some common thermocouples:

 Thermocouple                 Room temperature                 Useful temperature

    thermal e.m.f./mV K^-1                   range/K

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Copper/Constantan               .040                                        100 - 600

Iron/Constantan                   .050                                        100 - 1000

Chromel/Alumel                     .040                                        100 - 1500

Pt-10% Rd/Pt                       .006                                        100 - 1700

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