High Vacuum and Exclusion of Air

The first device is the ordinary U-tube manometer. Its two arms should be of equal diameter (not less than 10 mm. for precision measurements, about 15-20 mm.) because of the meniscus depression. The calibration should preferably be etched directly on the tube and readings taken against a mirror mounted behind the tube so as to avoid parallax. In this way changes of 0.1 mm. may be estimated without difficulty. A cathetometer must be used for greater accuracy. 

When working with gases at varying pressures (high vacuum to slightly above atmospheric pressure), a manometer of the type shown in Fig. 24 should be used. One side of this manometer is connected to the atmosphere via a mercury spray trap which 

A very convenient addition is a barometer tube of the same diameter, moimted next to the manometer its reading then furnishes the zero mark for the manometer reading (see p. 67, Fig. 41). 

Excellent but expensive devices for locating leaks are commercially available they blow a halogen-containing gas (for example Freon or difluorodichloromethane) from the outside onto the suspect t when this gas enters the evacuated apparatus through the leak, it creates an ionic current in an attached ionization tube equipped with a Pt anode. This signal is amplified and triggers optical or acoustical devices. 

An important factor in the choice of stopcocks is a sufficiently large sealing surface. In vacuum equipment, three-way stopcocks are a constant source of trouble and should be replaced either with two-way stopcocks or three individual ones. Stopcocks with hollow plugs are usually preferred to those with solid ones because they are lighter. The most important types of stopcocks are shown in F. 29. Schlff stopcocks (Fig. 29 a, b) should be used wherever possible they stay more reliably leakproof, since no channels connecting the tubes can form. 

---