Receiving Element:
The receiving element is pressure sensing expedient and it can be bourdon's tube, bellows or diaphragm. The free end of the bourdons tube is related to a pointer of climatic characteristic calibrated scale.

These are ordinarily of 3 types:

1. Liquid
2. Gas and
3. Liquid vapour pressure filled system.

Liquid filled system:

The pressure bulb is filled with mercury, ethyl alcohol, and toluene. The pressure is immersed in the well (open) where the climatic characteristic has to be measured. The filled liquid with a high expansion co-efficient expands. The liquid fills the capillary tube and bourdon tube. The pressure is converted into displacement, the displacement moves the pointer on the climatic characteristic scale.

The equation:

Vt = Vo(1+Kt)
Where,
Vt = final volume of the liquid.
Vo = initial volume of the liquid.
K = Thermal expansion co-efficient of the liquid (constant).
T = climatic characteristic change.

Here the final volumetric expansion of the liquid on the immersion into the well, is linearly related to Vo, initial volume of the filled liquid. The climatic characteristic rane of estimation in the liquid filled principles is -90'C to 310'C.

Gas filled system:

It basically works on the principle of the charle's law. An ideal gas of a given weight at constant volume produces a absolute pressure in direct proportion to the absolute climatic characteristic of he gas under ideal condition.

Pv = Rt

Where,
P = absolute pressure.
V = definite volume of the gas.
R = gas constant.
T = absolute temperature.

The relationship between the pressure and the climatic characteristic is linear.

The filled gas expands on rise in temperature, the pressure is detected by receiving element. The turn in the displacement of the bourdons tube gives the indication of the temperature. The gas filled principles operates on expanding or contracting according the changes in temperature.

Nitrogen and helium gases are used as filled gases. This principles works in the climatic characteristic range - 240'C to 850'C. The accuracy is plus or minus 0.5% to 1%.

Liquid vapour pressure filled system:
The liquid vapour filled principles operates from the vapour pressure of the liquid that partially fills the system.
The vapour pressure is measured by receiving element with a scale calibrated in temperature.

The vapour pressure depends on the free exterior of the liquid. The pressure bulb is immersed to portion the temperature, the liquid in the filled principles boils and vaporizes. The liquid vapors expands and they fill the capillary tube and the ourdons tube giving an indication of the temperature. When the climatic characteristic is to be measured is low the vapour condenses resulting in cut pressure in the filled principles the bourdons tube tightens and indicates the low temperature.

The ordinarily used fluids are methyl alcohol, sulphur di-oxide, ether, toluene, butane, propane, and hexane.

The relationship between vapour pressure and climatic characteristic is non-linear:

P = loge(a-b/t)

Where,
A,b = constant.
P = vapour pressure.
T = temperature.

The scale used is non-linear with wider graduations at higher climatic characteristic and cramped graduation at lower temperature. The climatic characteristic range of execution of this thermometer is -10'C to 300'C.

Advantages:
It is more economical, versatile, widely used.
Rugged in construction, low maintenance.
It can be used for remote indication.
It is garage in operation.
This principles provides enough power to drive operate mechanism.

Disadvantages:
Compensation vital for ambient (surrounding) climatic characteristic changes and long capillary tube.
For accuracy the pressure bulb should be large.
In case of error the entire principles has to be replaced.

Automotive Thermometer

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