DEVICES FOR GAS DOSING

Devices for admitting small portions of gas into vacuum systems (controlled leaks) contain sections having extremely small cross-sectional dimensions, which therefore offer very large resistance to the gas flow and do not pass significant quantities of gases. Typical examples are: thin glass capillaries, flattened copper tubes, porosities in an unglazed earthenware body, and needle valves of various designs. All of them have two basic shortcomings: 1) they are suitable for purposes of regulating the amount of gas introduced only for one definite narrow range of pressures; 2) their dosing accuracy is considerably reduced, or the valves cease to operate altogether, when the narrowest parts of the system become clogged with the finest particles.

Fig. 1.

Bachman’s article*) describes devices for gas dosing that are free of the above shortcomings. Instead of introducing a jet of gas through a narrow opening with a small but constant rate of flow, the gas is introduced through a wide opening in short pulses. The quantity of gas introduced during each pulse, as well as the pulse frequency, is easily regulated. Clogging of the wide openings practically does not occur, and the dosing is independent of pressure over a wide range of pressure variation. Fig. 1 shows one of the types of valves used by Bachman.

Recesses 2 are drilled on the surface of shaft 1. Tubes 3, leading to the vacuum system, and 4, leading to the reservoir with gas, are inserted into rubber gland 5, fitted over shaft 1. The gland is pressed against the shaft sufficiently strongly to create a good seal in the spaces between the recesses when a vacuum grease is used. When the shaft rotates, gas is transported from tube 3 to tube 4. The quantity of gas transferred is determined by the volume of the recesses and by the pulse frequency. The latter is limited by the geometry of the tubes (the tube diameter used by the author was about 3 mm), by the dimensions of the recesses, and by the shaft and the speed of its rotation. At excessively high speeds of rotation, problems of lubrication and heating arise. For introducing extremely small portions of gas, the shaft with recesses may be replaced by a smooth shaft.

This type of dosing has the advantage that the volume of gas introduced can be calculated accurately if the dimensions of the recesses and the pulse frequency are known. As experience shows, errors do not exceed a few percent. When operating this valve, care must be taken that the volume of the recesses does not decrease during operation because of penetration of grease into them as the shaft rotates in the gland.

A disadvantage of such “rotating” systems is the presence in the system of vapors of the lubricants. A cam valve, based on

) C. H. Bachman, The Rev. Sci Instr. 20*, 3 (1949).

on the same principle, eliminates this shortcoming. The arrangement of a valve of this type is shown in Fig. 2.

When cam \(1\) rotates, rubber gasket \(2\) closes and opens the aperture of tube \(3\), which leads into the system.

In contrast to the rotational method of dosing, the amount of gas entering the system is not directly dependent on the rotational speed of the cam. The amount of gas admitted is characterized by the ratio of the time \(\tau_1\), during which the tube connecting the valve with the vacuum system is open, to the time \(\tau_2\), during which it is closed. The cam can be moved along the line indicated by the arrow. In the uppermost position the valve remains open during the entire revolution of the cam. In the lowermost position it remains closed.

Fig. 2.

Regulation of the quantities of gas introduced is effected by moving the cam along the line indicated by the arrow, and does not depend on the pressure range. Dosing is carried out over a pressure range from one atmosphere to \(10^{-4}\) mm Hg.

This second method of dosing eliminates the harmful effect of oil vapors, which cannot be achieved with the rotational method; however, in this case it is difficult to calculate in advance the amount of gas entering the system.

V. V. Fedorov