Pressure measurement thermal conductivity 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. 

Analysis of hydrogen mixtures with a thermal conductivity cell is well established. The most accurate measurements are obtained by use of a thermal conductivity gauge with the walls immersed in liquid nitrogen and the wire heated to 160° K. This is the teniperature when the difference in the rotational specific heats of orthpara-hydrogen is a maximum . Various modifications of thermal conductivity gauges have been made to improve their convenience in use . A room temperature flow analyser based on a thermal conductivity cell has been developed by Weitzel and White which is claimed to be as sensitive as low temperature units. Bridge current and temperature must be controlled very carefully, but the unit is relatively insensitive to changes in pressure and flow rate. 

The pressure measurements should always be done by capacitance manometers (CAs) and not by thermal conductive gauges. TM depend on the gas mixture (water vapor and permanent gases) and are not reproducible enough for BTM measurements details are given on pp 327, 328 in [2]. For leak testing no special equipment specifications are required. The leak rate for the chamber, e.g., <1 x 10 mbarL/s, and for the condenser, e.g., <1 X 10 mbarL/s, should be specified since it might be helpful to measure the chamber and condenser separately. The maximum tolerable leak rate in the two examples is <1 x 10 mbarL/s, if the DR data at 0.1%/h should have an error <10%. If a helium leak tester is not available in production, it should be specified for quotation. 

In a thermal conductivity gauge (Figure 7.5) a heated filament about 7-30 pm in diameter is axially suspended in a measurement tube with an inner diameter of 10-20 mm. At 10 mbar with a mean free path I of 0.65 mm and a distance (from filament to wall) of 5-10 mm the ratio oil tod is small enough so that the heat conductance by the gas from the heated filament to the wall (room temperature) is pressure dependent. This pressure-dependent heat conductance is the basis of the measurement principle. 

Vacuum gauges may be broadly classified as either direct or indirect (10). Direct gauges measure pressure as force pet unit area. Indirect gauges measure a physical property, such as thermal conductivity or ionisation potential, known to change in a predictable manner with the molecular density of the gas. 

Classical physics teaches and provides experimental confirmation that the thermal conductivity of a static gas is independent of the pressure at higher pressures (particle number density), p > 1 mbar. At lower pressures, p < 1 mbar, however, the thermal conductivity is pressure-dependent (approximately proportional 1 / iU). It decreases in the medium vacuum range starting from approx. 1 mbar proportionally to the pressure and reaches a value of zero in the high vacuum range. This pressure dependence is utilized in the thermal conductivity vacuum gauge and enables precise measurement (dependent on the type of gas) of pressures in the medium vacuum range. 

The tilting McLeod gauge (Fig. 7.8) is a simple, inexpensive, and portable gauge which may be used to measure pressures down to about 10 3 torr. These gauges are very useful for checking rough vacuum systems, Schlenk systems, and for the calibration of thermal conductivity vacuum gauges. 

The measurement of total pressure in a vacuum system is essential. Chapter 5 outlined the two general principles involved (direct and indirect). Direct methods included manometric measurements (Examples 5.1 and 5.3) and those involving the mechanical deformation of a sensing element. Indirect methods, which depend on the estimation of a physical property of the gas (e.g. thermal conductivity, ionisation) that depends on number density, were also discussed. Uncertainty of measurement is a parameter associated with the result of a measurement. It may influence the choice of a pressure gauge, and its practical expression was illustrated in Example 5.4. 

The measurement of pg requires a pressure transducer system that is not influenced by the electric power used for the plasma polymerization, particularly when a high-frequency radio frequeny power is employed. Some pressure transducers that give pressure readouts independent of the nature of a gas are ideally suited for plasma polymerization. Some electronic gauges the readout of which depends on the nature of the gas (e.g., thermal conductivity) do not provide accurate readings of Pg because in most cases the composition the gas mixture in the LCVD reactor is unknown and there is no way to calibrate the meter for an unknown gas mixture. 

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. 

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. 

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 IGC is a dynamic volumetric method that can be used to measure the activity coefficient at infinite dilution y of volatile liquids in IL. Figure 9.4 shows the setup of an IGC installation. The temperature of the GC column is kept constant by an air oven (1) with controlled temperature (TIC). The chromatographic column (2) usually is a simple stainless steel tube. It is packed with SILP particles including the IL but without the catalyst. The pressure drop over the column can be measured by two pressure gauges (PIR). The volatile sample can be inserted at the injection block (3). The sample is taken through the column by the carrier gas hehum (4). The volume flow rate of the carrier gas is measured at the oven outlet and controlled at the oven inlet flow meter (FIC). The sample is detected at the column outlet by a two-way thermal conductivity detector (HR). On the reference side, it is perfused by the pure carrier gas. The difference in the thermal conductivity is recorded. 

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