Conductance, vacuum

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.

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 outside temperature must be taken into account and above all it is necessary to avoid hot kilns, furnaces or stoves or other sources of intense radiation which generate an ambient temperature around the measurement system which lies above the specific acceptable value. Excessive ambient temperatures will result in false pressure indications in thermal conductivity vacuum sensors. 

Fig. 6.21. Thermal conductivity vacuum-sensing (Pirani) gauge (a) Pirani transducer (6) typical bridge circuit for Pirani gauge...

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. 

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])...

Fig. 8.7. Schematic of a CEC—nanospray ESI-MS interface with conducting vacuum transfer conduit and heated capillary.

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. 

Figure 38 Electric energy needed to keep the temperature of the hot wire constant as a function of pressure in a thermal conductivity vacuum gauge. (From M. Wutz, H. Adam, and W. Walcher, Theorie und Praxis der Vakuumtechnik, 2nd ed. Copyright 1982 Friedr. Vieweg Sohn, Verlagsgesellschaft mbH, D-65048 Wiesbaden, Germany.)...

Contact/vacuum tray Conduction Vacuum oven, vacuum shelf... 

Contact/vacuum band Conduction Vacuum belt, vibrated tray... 

Slurries, powders, and granules overall heat transfer coefficient U = 0.03 to 0.23 kW/m °C. Drum dryer (conduction) overall heat transfer coefficient U = 0.001 to 0.002 kW/m °C. Jacketed screw (conduction) overall heat transfer coefficient U = 0.015 to 0.06 kW/m °C. Agitated pan (conduction) overall heat transfer coefficient U = 0.01 to 0.05 kW/m °C. Tray/pan shelf (conduction vacuum) overall heat transfer coefficient U = 0.02 to 0.5 kW/m2°C. 

Tray/shelf, (batch, conduction) vacuum, indudmg vacuum-tight box, shelves excluding vacuum equipment and trays. FOB 30 000 at shelf area = 30 m with n = 0.56 for the range 1-100. L-rM = 2.2-2.75. L/M = 0.36. Alloy cost factors c/s X 1.00 s/s, X 2.2. Factors excluding vacuum equipment, X 1.00 including vacuum equipment, X 1.5-1.9. 

Cone dryer (batch, conduction) vacuum tumble dryer, jacketted, s/s inside, including butterfly valve, stand, drive, motor, guard, vacuum equipment. FOB cost = 73 000 at working capacity = 0.28 m with n = 0.50 for the range 0.03-8.5 m. ... 

It is instructive to study the behavior of the passive current induced on the conducting vacuum vessel by plasma movement. Figures 11 and 12 show that the tilting modes are stabilized by a conductor parallel to the axis of toroidal symmetry and the sliding modes are stabilized by a horizontal plate. 

---