High vacuum range

A precision aneroid manometer is used for measurements in the 760— 1 torr range. Thermocouple gauges are used in the 1 — 1 x 10 3 range. A cold cathode ionization gauge is used in the high vacuum range down to 10-6 torr. 

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. 

For Dewar flasks, metal evaporation apparatus, and most research apparatus, a vacuum of 10 ° to 10 ° Torr is sufficient this is in the high vacuum range, while 10 ° Torr would be termed ultrahigh vacuum. However, for many routine purposes a utility vacuum or forepump vacuum of about 10 ° Torr will suffice, and for vacuum distillations only a partial vacuum of the order of 1 to 50 Torr is needed. 

Vacuum pressures of less than about 10 torr are generally considered to be in the ultra high vacuum range and are beyond the scope of this book. 

In the high-vacuum range, fractionating vapour jet pumps are employed as diffusion pumps after oil seal vacuum pumps. These vapour jet diffusion pumps are equipped with especially constmcted nozzles with a diffusion slot. The working range of this type only starts in medium high vacuum and leads to high vacuum or molecular distillation. 

Information on vacuum controllers for the medium and high vacuum ranges is scarce. Laporte [49] has described an instrument working on the thermal conductivity principle in which the Wheatstone bridge is connected to a signalling device which produces an acoustic signal when the pressure exceeds a given value. 

The vacuum processes which are utihsed in the area of chemical engineering are generally performed in the range of one to several hundred millibars (Figure 5.1). An exception to this is the process of short path distillation, respectively molecular distillation, which operates at low pressures in the medium and the beginning high vacuum range and which here is separately covered in chapter 15 of this book. 

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