Thermal Conductivity Gauges and Pressure Measurement
Thermal conductivity gauges, such as Pirani and thermocouple gauges, measure pressure based on the principle of heat conduction in an enclosed gas. The change in temperature due to thermal conductivity is utilized to determine pressure, with variations in electrical resistance or voltage. Limiting cases and working principles are explained for a comprehensive understanding.
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Presentation Transcript
Thermal conductivity gauges: Thermal conductivity gauges:- - Partial pressure gauges: Reading of the gauge depends on nature of the gas in the enclosure Pressure range: 1-10^-4 Torr Principle: dependency of thermal conductivity on pressure. Main type of gauges we study under this category are: 1. Pirani gauge. 2. Thermocouple gauge.
Principle of working: Filament is heated by a constant current and maintained at a temperature T0. Now when the gas is allowed to enter in this enclosure, it takes away the heat from the filament(hot end) to the walls of the container(cold end). This conduction of heat Takes place mainly by three processes: Thermal conduction by enclosed gas molecules. Radiation loss. iii. Conduction through wire leads. in the pressure range, 1-10^-4 Torr, Fall in temperature due to thermal conductivity (minimising the other two effects) becomes directly proportional to pressure. i. ii. (A) Consider a heating element inside a vacuum enclosure Consider a heating element inside a vacuum enclosure which is fed to a system containing residual gases which is fed to a system containing residual gases whose pressure has to be determined, as shown in the whose pressure has to be determined, as shown in the above figure (A). above figure (A). Units of thermal conductivity : W/m-K or Cal/cm-s- 0C
Now due to Thermal conductivity of residual gas temperature falls from T0 to T. (T<T0) . Now change in temperature due to change in pressure is usually measured by change in electrical resistance (pirani gauge) or change in electrical voltage(thermocouple gauge). Change in pressure Change in temp of the filament Change bin electrical resistance of filament See-back effect: thermo-emf will be generated Measured as out of balance current in wheat-stone s bridge Read with a milli- voltmeter Pirani gauge Thermocouple gauge
Limiting cases of thermal conductivity gauges: At low pressure (P<10-4 T) : Radiation loss becomes more dominant. At high pressure(P>1T): mean free path becomes much smaller than mean distance between filament and the enclosure. Do T.C. becomes independent of pressure (Attains saturation).
Pirani gauge: Due to T.C. of gases filament temperature changes from T0 (R0 ) to T(R). Here, T<T0 and R<R0. Supply current to raise from T0 to T (out of balance current) is measured and can be calibrated as a function of pressure. R1 Rx R2 R3 Indirect, partial pressure gauge. Filament is made up of tungsten/nickel/ semiconductor thermistors. Measuring circuit is a simple Wheatstone's bridge. By the balancing condition of Wheatstone s bridge, ?1 ?2 =?? ?3,here ?2=?3 =fixed and R1 is variable.
Pirani gauge response curve: pressure in Torr(Y) versus out of balance current in A.
Applications and limitations: Major application in backing lines for continuous monitoring of pressure above 10-4 T. Available now in digital forms also. Oil vapours from pump has to be taken care of.
Thermocouple or Thermopile gauge: Instead of a single gauge, a series of thermocouples are used to increase the sensitivity . Such types are called thermopile gauges Indirect gauge(thermal conductivity gauge). Partial pressure gauge. Range is same as pirani gauge.
The gauge head is made up of glass/ metal housing. Most commonly used is Chromel-Alumel thermocouple Merits: continuous reading type, simple in design and can measure pressures of all gases and vapours. Unlimited service life. Demerits: contamination by oils/vapours have to be taken care of. An indirect gauge and needs calibration. Variation of filament current with time must be checked regularly (should be kept constant).
For more information on thermal conductivity gauges: https://youtu.be/RARjXXaFEQ0 http://www.idc- online.com/technical_references/pdfs/inst rumentation/Pirani_gauge.pdf https://youtu.be/GWfRuur0jrk https://www.duniway.com/images/_pg/th ermal-vacuum-gauges.pdf
