The filled theory essentially consists of:
Pressure tube:
This is made up of copper, monel, or stainless steel. It is filled with the liquid or gas or the vapour pressure and the liquid. Hence this thermometer is known as a filled system. The fluids are filled in at absolute pressure and temperature.
Capillary Tube:
The capillary tube connects the filled tube with the receiving element.
Receiving Element:
The receiving element is pressure sensing gadget and it can be bourdon's tube, bellows or diaphragm. The free end of the bourdons tube is connected to a pointer of temperature calibrated scale.
These are generally 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 temperature 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 temperature scale.
The equation:
Vt = Vo(1+Kt)
Where,
Vt = final volume of the liquid.
Vo = introductory volume of the liquid.
K = Thermal expansion co-efficient of the liquid (constant).
T = temperature change.
Here the final volumetric expansion of the liquid on the immersion into the well, is linearly connected to Vo, introductory volume of the filled liquid. The temperature rane of measurement in the liquid filled theory 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 temperature of he gas under ideal condition.
Pv = Rt
Where,
P = absolute pressure.
V = exact volume of the gas.
R = gas constant.
T = absolute temperature.
The connection between the pressure and the temperature 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 theory operates on expanding or contracting according the changes in temperature.
Nitrogen and helium gases are used as filled gases. This theory works in the temperature 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 theory 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 quantum the temperature, the liquid in the filled theory 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 temperature is to be measured is low the vapour condenses resulting in reduce pressure in the filled theory the bourdons tube tightens and indicates the low temperature.
The generally used fluids are methyl alcohol, sulphur di-oxide, ether, toluene, butane, propane, and hexane.
The connection between vapour pressure and temperature 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 temperature and cramped graduation at lower temperature. The temperature 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 carport in operation.
This theory provides adequate power to drive operate mechanism.
Disadvantages:
Compensation vital for ambient (surrounding) temperature changes and long capillary tube.
For accuracy the pressure bulb should be large.
In case of error the entire theory has to be replaced.
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