The filled system 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 device 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 = Initial 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 related to Vo, initial volume of the filled liquid. The temperature rane of measurement in the liquid filled system 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 = specific volume of the gas.
R = gas constant.
T = absolute temperature.

The relationship between the pressure and the temperature is linear.

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

Nitrogen and helium gases are used as filled gases. This system 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 system 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 surface of the liquid. The pressure bulb is immersed to measure the temperature, the liquid in the filled system 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 system the bourdons tube tightens and indicates the low temperature.

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

The relationship 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 operation 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 stable in operation.
This system provides enough power to drive control mechanism.

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