The construction and mechanical operation of all types of pressure spring thermometers are identical. The thermal system of pressure spring thermometer contains a metal bulb, capillary and receiving element. Metal bulb contains a thermometer fluid, a liquid or a gas or a liquid vapour and is inserted at the point at which the temperature is to be measured. The bulb comes in thermal equilibrium with its surroundings, thereby developing a given pressure or displacement of fluid. A metal capillary is connected to the bulb and transmit the pressure at the bulb to the receiving element at the instrument. The receiving element is a form of bourdon tube or pressure spring. It is used to convert the pressure of displacement of fluid in the thermometer bulb into a motion. This motion is used to operate a pointer for the indication of temperature.
Pressure spring thermometers are of three types:
(i) Liquid expansion thermometer
(ii) Gas expansion thermometer and
(iii) Vapor actuated thermometer.
1.Liquid Expansion Pressure Spring Thermometers
It utilizes the volumetric expansion of a liquid caused by the temperature changes to operate the pressure spring and indicate the temperature. The relation between volume of expansion of a liquid and its temperature is given by the law of cubical expansion: Vf = V0(1+BT) Where Vf is the final volume, V0 is the initial volume, B is the mean coefficient of volumetric expansion and T is the temperature. The equation indicates a linear relation which is not quite true as the coefficient of volumetric expansion, B varies slightly, with temperature. Working The bulb is filled with the thermal liquid at a high pressure. A temperature increase at the bulb results in an expansion of the liquid which causes expansion of the bourdon tube or pressure spring and thus indicates the temperature.
Mercury is most widely used thermal liquid because of its wide temperature range. Ethanol or Toluene may also be used. The thermal expansion of these liquid is about six times greater than that of mercury and from that standpoint their use is advantageous. The temperature limits of the mercury filled pressure thermometer are about -35 to 10000F.
- Greater sensitivity
- Linear scale shape
- No head effect
- No barometric effect
- Immersion effect
- Ambient temperature effect.
2.Gas Expansion Thermometer (Gas Thermometer)
It utilizes the expansion of a gas caused by the temperature changes to operate the pressure spring and indicate the temperature. The most commonly use gas is nitrogen. Hydrogen and helium are also used as thermometric fluids in gas thermometer. There are two main types of gas thermometer, one operating at constant volume and the other at constant pressure. The constant volume gas thermometer is more widely used.
It is based on the ideal gas equation: PV = RT, for one mole of an ideal gas; where P, V and T are the pressure, volume and temperature of the gas, respectively and R is the universal gas constant.
Since it operates at constant volume, the equation can be written as: T or P1/T1 = P2/T2µP
Where P1 and T1 represent the reference pressure and temperature, respectively and P2 and T2 represent the unknown pressure and temperature, respectively. If P2 can be calculated using a manometer, the unknown temperature T2 is given by T2 = P2T1/P1.
A simple form of constant-volume gas thermometer is shown in Figure .
The gas is enclosed in the bulb B and the pressure recorded by the difference in levels (h) of the mercury columns. The mercury level at R is always adjusted so that it coincides with the mark. The pressure of the gas within the bulb is then given by P2 = P1 + h, where P1 is the atmospheric pressure. By using different gas thermometers a wide range of temperatures can be measured:
Hydrogen -200 oC to +500 oC
Nitrogen +500 oC to + 1500 oC
Helium -270 oC to + 1500 oC
These thermometers can be very accurate, to within 0.005 oC from 0 oC to 100 oC, 0.1 oC around 500 oC and to within 2 oC at 1500 oC.
- The coefficient of expansion of gases is very large as compared to liquids. Therefore, gas thermometers are sensitive.
- The coefficient and the rate of expansion of all gases are the same under similar conditions.
- The coefficient of expansion of the material of the bulb of the thermometer is negligible in comparison to the coefficient of expansion of a gas.
- The gases expand uniformly and regularly over a wide range of temperature.
- The thermal capacity of a gas is low as compared to liquids. Hence even small changes of temperature can be recorded accurately.
- Gases can be obtained in a pure form.
- Gas thermometers can be used over a wide range of temperature. They are suitable to measure high and low temperatures.
- The temperatures measured with a gas thermometer agree with the temperatures on the thermodynamic scale.
- Gas thermometers are not suitable for routine work. They are large and cumbersome. They can be used only in one position. They are mainly used to standardize and calibrate other thermometers.
- Ambient temperature effect
- Immersion effect
3.Vapour – Actuated Thermometer
It operates from the vapour pressure of a liquid that partially fills the bulb. The vapour pressure is measured by a pressure spring and the instrument is calibrated in terms of temperature.
Since the vapour pressure depends solely on the temperature at the free surface of the liquid, the vapour actuated thermometer indicates only the temperatures existing at the free surface. The most commonly used fluids for vapour actuated thermometer include methyl chloride, sulphur dioxide, ether, toluene, propane, butane and hexane. Its range depends entirely on the filling medium. However, temperatures as low as -500F and as high as 6000F may be measured.
- Low cost
- Easy to maintain
- It requires no ambient temperature compensation.
- Good speed of response
- Scale shape is non-linear
- Head effect
- Barometric effect