[Concept] : Basic Type of Thermometer

Concept: A thermometric property like length of a wire, resistance of a metallic and volume of a gas proportional to temperature.

In this article let X0, X100, Xt be the parameters at ice point, steam point and unknown temperature, respectively. So

t = (X0-Xt/X100-X0)1000C

Thermometer: The instrument used to measure the temperature is called is called Thermometre.

Classification: The classification is depending upon the physical property of the substance that varies with temperature.

1. Liquid Thermometres: These thermometers are based on the thermal expansion of liquids.

Principle: The increase in volume of the liquid in the glass bulb is directly proportional to increase in the temperature.

If l0, l100, lt are the parameters of length. Then temperature

t = (l0-lt/l100-l0)1000C

Range: For mercury thermometer upto 3000C.

2. Constant volume gas Thermometer: These thermometers are based on the thermal expansion of gases at constant volume.

Principle: The increase in pressure of a gas at constant volume is directly proportional to increase in the temperature.

If P0, P100, Pt are parameters of pressure. Then temperature

t = (P0-Pt/P100-P0)1000C

Range: Constant hydrogen gas thermometers has different range with different bulbs used

With Platinum bulb: -2000C to 5000C.

With Porcelain bulb: 11000C.

3. Constant pressure gas thermometer: These thermometers are based on the thermal expansions of gases at constant pressure.

Principle: The increase in volume of a gas at constant pressure is directly proportional to increase in its temperature.

If V0, V100, Vt are parameters of volume. Then temperature

t = (V0-Vt/V100-V0)1000C

With air as a thermometric substance can be used to measure temperature upto 6000C.

4. Resistance Thermometers: These thermometers are based on the variation of electric resistance of metals with temperature. These thermometers usually employ platinum as the thermometric substance.

Principle:  The increase in resistance of a platinum wire is directly proportional to increase in its temperature.

If R0, R100, Rt are parameters of resistance. Then temperature

t = (R0-Rt/R100-R0)1000C

Range: -2720C to 12000C.

Questions:

1. The resistances of a platinum resistance thermometer at lower and upper fixed points are 3.50ohms and 3.65ohms respectively. Then find the resistance of platinum wire at -370c? Ans: 3.55ohms

2. The readings corresponding to the ice and steam points for a certain pressure gas thermometer are 500cc and 545cc. If reading at certain temperature be 510cc then find that temperature? Ans: 22.220C