RESISTANCE TEMPERATURE MEASUREMENT
Resistance thermometry is perhaps one of the most uncomplicated of the range of electrically actuated temperature measuring systems. The principle of measurement is based on the simple fact that all conductors offer a degree of resistance to the passage of electric current and that the temperature of the conductor is, for practical purposes, directly proportional to the resistance it offers.
The resistance at any temperature t is (Rt) and this is found by adding 1 to µt (which is the temperature coefficient ( µ ) of the material used, multiplied by the (temperature), the result of this is then multiplied by Ro (which is the original resistance (Ro) at the ice point, (0°C)
The temperature coefficient of resistance ( µ ) is very sensitive to small amounts of impurities and mechanical stress over the range 0oC to 100oC ( R0 – to – R100 ).
Common forms of resistance elements are shown below.
Measuring Circuit
The bridge is in balance at only one value of thermometer resistance. At all other resistance values of the thermometer the out of balance current is proportional to the temperature.
The value of the potential difference across B.C. (fig. above) for any value of Rt (resistance due to temperature) is found from:
Rt R2
Vg = V ( ______ _ ______ )
Rt + R3 R1 + R2
You can see that the value of V (the supply voltage) must be a fixed value. At frequent intervals the supply voltage should be checked and "standardised".
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