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TEMPERATURE MEASUREMENT - Coggle Diagram
TEMPERATURE MEASUREMENT
Temperature is most widely measured quantity in process industry.
The law that is used in temperature measurement is zeroth law of thermodynamic.
Common used temperature scales are : Fahrenheit and Celcius
Kelvin and Celcius are related by: T=t+273.15
Method of temperature measurement and sensing technique
non-electrical method
changes in physical dimension
bimetallic thermometers
used the principle of solid expansion and consist of a 'bimetal' strip usually in the form of cantilever beam
comprises of two metals which having different coefficient of thermal expansion,joint together (welded/riveted) to prevent the relative motion between it.
brass is used as high expansion material for the measurement of low temperatures
bimetallic thermometer usually function between -30 to 550 celcius
advantages:self generating type with low cost with no maintenance
disadvantages:Inability to measure rapidly changing temperatures.
A bimetallic strip. Brass expands more when heated than iron does and contracts more when cooled
A thermostat keeps temperature constant
found in ovens and refrigerators to keep the temperature hot and cold respectively
electrical method
thermocouples
A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots,
It produces a voltage when the temperature of one of the spot differs from the reference temperature at other parts of the circuit.
Any junction of dissimilar metals will produce an electrical potential related to temperature. Different alloys are used for different temperature ranges.
The characteristic of materials chooses:
ability to withstand the temperature at which the are used
immunity from corrosion
linearity characteristic
classified two categories :base metal and rare metal
quartz thermometer
Piezo-electrica crystal (quart) provides a highly accurate and sensitive method of temperature measurement based on the change in its resonant frequency which is directly proportional to the temperature change
to achieve below properties the crystal is cut in the form of shear type LC(liniear coefficient) cut
The detection of change in frequency of oscillation of 1 Hz gives a resolution of 0.002 celcius,thus temperature in the range of -40 to 230 celcius can be measured pricisely and accurately
advantages:-highly linear output and long term stability and reliability
limitations :-limited measuring range -40 to 230 celcius and piezo electric crystals have strong cross-sensitivity for pressure changes if they occur simultaneously in the temperature measuring system.
radiation method
infrared pyrometer