Vacuum heat conductivity gauge

With the rotary and diffusion pumps in tandem, aided by a liquid-nitrogen trap, a vacuum of 10 Torr became readily attainable between the wars by degrees, as oils and vacuum greases improved, this was inched up towards 10 Torr (a hundred-billionth of atmospheric pressure), but there it stuck. These low pressures were beyond the range of the McLeod gauge and even beyond the Pirani gauge based on heat conduction from a hot filament (limit Torr), and it was necessary to... 

The pressure range for DR measurements is normally one decade below the above data, and this has to be considered in the specification of the plant. All measurements discussed above have to be carried out by capacitance vacuum gauge, because these instruments measure pressure independently of the type of gas. All vacuum gauges based on the change of heat conductivity as a function of pressure show a result which depends... 

Fig. 1.81. Plot of the pressure measured by heat conductivity vacuum gauge (TM) during SD. In addition pressure rises in 30 s and related RM data are shown.

To insure an undisturbed water vapor transport (see Section 1.2.4) the leak rate of a freeze-drying plant must allow BTM with sufficient accuracy. This applies for vapor pressures with ice temperatures ranging between -50 and -10 °C corresponding to 0.04—2.5 mbar. The pressure range for DR measurements is normally one decade below the above data and this has to be considered in the specification of the plant. All measurements discussed above have to be carried out with a capacitance vacuum gauge, because these instruments measure pressure independently of the type of gas. All vacuum gauges based on the change of heat conductivity as a function of pressure show a result which depends not only on the pressure of the gas mixture but also on the type of gas. Leybold AG [1.67] indicate that for instruments based on heat con-... 

Fig. 1.81. Plot of the pressure measured by heat conductivity vacuum gauge (TM) during SD. In addition pressure rises in 30 s and related RM data are shown. 1, pch measured by TM 2, pressure rise in 30 s 3, RM in % of solids (Figure 5 from [1.62])...

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. 

Thermal Conductivity Vacuum Gauges. A very widely applied gauge of this type is the Pirani gauge. Such gauges consist of a wire (Pt, W or Ni, d = 5-20 pm / 5 cm) mounted axially in a cylindrical tube (d 2 cm). The wire is heated by an electric current to approximately 100°C above the ambient temperature and heat loss occurs by three mechanisms, as indicated in Figure 5.3. 

The Pirani gauge uses the principle that (usually) the hotter a wire gets the greater its electrical resistance. Therefore, if the resistance of a wire is going up, it must be getting hotter. This relationship implies that less air/gas is available to conduct heat away from the wire, and therefore a higher vacuum is being achieved. 

The thermocouple gauge is more straightforward than the Pirani gauge and less complicated electronically. The thermocouple gauge has a thermocouple attached to a filament under constant electrical load, and it measures the temperature at all times. If the filament becomes hotter, it means that there is less air/gas available to conduct heat away from the wire, and therefore there is greater vacuum within the system. 

As mentioned in Sec. 7.5.14, many gauges read an inferred pressure, not real pressure. Some vacuum gauges use the thermal conductivity of gases present in the system to infer the pressure of the system. These gauges are based on the concept that less gas will conduct less heat. Because different gases have different thermal conductivities,83 the user needs to make allowances if the gas in a system has a different thermal conductivity than the particular gas a gauge has been calibrated to use. 

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