Filled Systems Thermometer Or Pressure Gauge Thermometer

The filled system essentially consists of:The relationship between the pressure and the
Pressure tube:temperature is linear.
This is made up of copper, monel, or stainless steel.The filled gas expands on rise in temperature, the
It is filled with the liquid or gas or the vapourpressure is detected by receiving element. The
pressure and the liquid. Hence this thermometer ischange in the displacement of the bourdons tube
known as a filled system. The fluids are filled in atgives the indication of the temperature. The gas filled
absolute pressure and temperature.system operates on expanding or contracting
Capillary Tube:according the changes in temperature.
The capillary tube connects the filled tube with theNitrogen and helium gases are used as filled gases.
receiving element.This system works in the temperature range - 240'C
Receiving Element:to 850'C. The accuracy is plus or minus 0.5% to 1%.
The receiving element is pressure sensing device andLiquid vapour pressure filled system:
it can be bourdon's tube, bellows or diaphragm. TheThe liquid vapour filled system operates from the
free end of the bourdons tube is connected to avapour pressure of the liquid that partially fills the
pointer of temperature calibrated scale.system.
These are generally of 3 types:The vapour pressure is measured by receiving
1. Liquidelement with a scale calibrated in temperature.
2. Gas andThe vapour pressure depends on the free surface of
3. Liquid vapour pressure filled system.the liquid. The pressure bulb is immersed to measure
Liquid filled system:the temperature, the liquid in the filled system boils
The pressure bulb is filled with mercury, ethyl alcohol,and vaporizes. The liquid vapors expands and they fill
and toluene. The pressure is immersed in the wellthe capillary tube and the ourdons tube giving an
(open) where the temperature has to be measured.indication of the temperature. When the temperature
The filled liquid with a high expansion co-efficientis to be measured is low the vapour condenses
expands. The liquid fills the capillary tube and bourdonresulting in reduce pressure in the filled system the
tube. The pressure is converted into displacement,bourdons tube tightens and indicates the low
the displacement moves the pointer on thetemperature.
temperature scale.The commonly used fluids are methyl alcohol, sulphur
The equation:di-oxide, ether, toluene, butane, propane, and hexane.
Vt = Vo(1+KT)The relationship between vapour pressure and
Where,temperature is non-linear:
Vt = final volume of the liquid.P = loge(a-b/t)
Vo = Initial volume of the liquid.Where,
K = Thermal expansion co-efficient of the liquidA,b = constant.
(constant).P = vapour pressure.
T = Temperature change.T = temperature.
Here the final volumetric expansion of the liquid onThe scale used is non-linear with wider graduations at
the immersion into the well, is linearly related to Vo,higher temperature and cramped graduation at lower
initial volume of the filled liquid. The temperature ranetemperature. The temperature range of operation of
of measurement in the liquid filled system is -90'C tothis thermometer is -10'C to 300'C.
310'C.Advantages:
Gas filled system:It is more economical, versatile, widely used.
It basically works on the principle of the charle's law.Rugged in construction, low maintenance.
An ideal gas of a given weight at constant volumeIt can be used for remote indication.
produces a absolute pressure in direct proportion toIt is stable in operation.
the absolute temperature of he gas under idealThis system provides enough power to drive control
condition.mechanism.
Pv = RTDisadvantages:
Where,Compensation necessary for ambient (surrounding)
P = absolute pressure.temperature changes and long capillary tube.
V = specific volume of the gas.For accuracy the pressure bulb should be large.
R = gas constant.In case of error the entire system has to be
T = absolute temperature.replaced.