Pressure measurement Pirani gauges

A thermal conductivity gauge uses a constant electric current to heat an element whose temperature is a linear function of gas pressure over a limited range. The temperature is typically measured with a thermocouple. In the popular Pirani gauge, a single metal filament is substituted for a thermocouple, and filament resistance is monitored [19]. The range of pressures detected by thermal conductivity gauges is — lO -lO 4 torr, which makes them useful for... 

These measure the change in thermal conductivity of a gas due to variations in pressure—usually in the range 0.75 torr (100 N/m2) to 7.5 x 10"4 torr (0.1 N/m2). At low pressures the relation between pressure and thermal conductivity of a gas is linear and can be predicted from the kinetic theory of gases. A coiled wire filament is heated by a current and forms one arm of a Wheatstone bridge network (Fig. 6.21). Any increase in vacuum will reduce the conduction of heat away from the filament and thus the temperature of the filament will rise so altering its electrical resistance. Temperature variations in the filament are monitored by means of a thermocouple placed at the centre of the coil. A similar filament which is maintained at standard conditions is inserted in another arm of the bridge as a reference. This type of sensor is often termed a Pirani gauge. 

The method has been used to determine the thermal dissociation of chlorine and bromine. 3 The hot-wire manometer (Pirani gauge, 6.VII J) has also been used to measure small vapour pressures. ... 

This type of gauge, like the Pirani gauge, uses a filament with a high temperature coefficient of electrical resistance exposed to the gases in the vacuum system. The temperature of the filament, as measured by a small thermocouple connected to a sensitive millivoltmeter and attached to the heated filament, indicates the gas pressure. The range is 0.5 toiT to 0.001 torr. 

The thermal conductivity of a gas is the quantity which is measured in the Pirani gauge (page 125) and in the detector of a gas-phase chromatography column (page 171). The thermal conductivity is related to the heat capacity of the gas, which measures the amount of energy that can be absorbed per molecule to the velocity of the molecules, which is a measure of the number of collisions with the heated surface per unit time and pressure and to the pressure of the gas. 

The Pirani gauge was discussed as a device for measuring low pressures. Discuss another type of gauge that can be used. 

Thermal volatilization analysis EGA (51) Pirani gauge used for pressure measurement... 

Other more rehned methods that rely on pressure difference include the combination of different types of pressure probes, e.g. a combination of a capacitance pressure probe and a Pirani gauge. The former device measures the total pressure, whereas the Pirani reading is affected by the thermal conductivity of the gas, e.g. water vapour. Completion of sublimation is indicated when both pressure readings coincide. 

The concept of hot wire anemometry is similar to that of thermal mass flow meters as well as the Pirani gauge to measure pressure a fine wire is placed in a flow stream and then heated electrically. The heat transfer rate from the fluid to the wire equals the rate heat is generated by the wire. 

Pirani gauge An instrument used to measure low pressures (1-10 torr 100-0.01 Pa). It consists of an electrically heated filament, which is exposed to the gas whose pressure is to be measured. The extent to which heat is conducted away from the filament depends on the gas pressure, which thus controls its equUibrium temperature. Since the resistance of the filament is dependent on its temperature, the pressure is related to the resistance of the filament. The filament is arranged to be part of a Wheatstone bridge circuit and the pressure is read from a microammeter calibrated in pressure units. As the effect depends on the thermal conductivity of the gas, the calibration has to be made each time the pressure of a different gas is measured. 

The RC is a cylindrical tube with a lithium ion emitter centered along one side (Fig. 5.1). The chamber can be evacuated through a cone aperture, A, with a vacuum pump and can be maintained at high pressure by altering the pumping speed or the carrier gas flow rate. The pressure at the center of the RC zone is measured with a Pirani gauge. 

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