NEW IONIZATION PRESSURE GAUGES

One of the most widely used instruments for measuring low pressures is the ionization pressure gauge[^1]. The principal advantages of ionization pressure gauges are, first, the possibility of measuring very low pressures with their aid—down to \(10^{-8}\) mm Hg, and with pressure gauges of special design even lower—and, second, the proportionality of their ion current to the pressure. However, the most widespread types of these gauges are not free from certain shortcomings. Thus, an ionization gauge with a central heated cathode, a negative grid serving as an ion collector, and a positive anode intended to accelerate electrons emitted by the cathode, as a rule has a very low sensitivity. This is explained by the fact that at low pressures the probability of collision of an electron flying from the cathode to the anode with a gas molecule in the gauge, and consequently the probability of ion formation, is very small.

A considerable increase in sensitivity can be achieved if, in such a gauge, the grid is made positive and the anode negative. In this case the path length of the electrons, and consequently the probability of ionization, increases considerably as a result of oscillations of the electrons about the grid. However, this oscillatory mechanism often leads to the appearance of Barkhausen–Kurz ultrahigh-frequency oscillations in the collector circuit of the gauge, which make pressure measurement impossible, since in this case an electron current flows to the negative electrode of the gauge.

A preionization ionization gauge[^2] was proposed that has high sensitivity and in which the possibility of the occurrence of Barkhausen–Kurz oscillations is eliminated.

The design of this manometer is shown in Fig. 1. It consists of a glass tube \(K\), on the wall of which a thin layer of aquadag, serving as an ion collector, is deposited. Along the axis of the tube there is a tungsten rod \(A\), intended for accelerating electrons emitted by the cathode \(F\), which is placed to the side of the tube axis parallel to the rod. An increase in the path of the electrons is achieved by means of a magnetic field of strength 150 oersteds, directed parallel to the axis of the tube. This field is produced by a solenoid \(H\) fitted externally onto the manometer. If the sensitivity is defined as the magnitude of the ion current (in microamperes) at a pressure of 1 micron Hg \((10^{-3}\ \text{mm Hg})\) and an electron current of 1 mA, then in this manometer it reaches almost \(600\ \mu\text{A}/\text{mA}\cdot\mu\text{m Hg}\), whereas the sensitivity of ordinary ionization manometers, even with a positive grid, as a rule does not exceed \(20\ \mu\text{A}/\text{mA}\cdot\mu\text{m Hg}\). Such a high sensitivity makes it possible, when measuring pressures down to \(1\cdot 10^{-5}\ \text{mm Hg}\), to dispense with direct-current amplifiers, measuring the ion current of the manometer with an ordinary pointer-type microammeter.

Fig. 1.

A drawback of the ionization manometers usually employed is the dependence of their readings on the dimensions of the tube connecting the manometer with the volume in which the pressure is measured. This is explained by the fact that gas is absorbed in the manometer,3 while in some cases gas occluded by the electrodes and the walls of the manometer is released. Both effects create a gas flow in the connecting tube and, consequently, a pressure drop across it. The manometer whose design is shown in Fig. 24 is free from this drawback. Its electrodes are mounted in a tube connecting a high-vacuum pump with the evacuated volume. Thus the absorbing action of the manometer simply increases the total pumping speed of the volume, and the gases released by it are pumped out by the pump. Therefore, in steady-state operation such a manometer measures the true value of the pressure in the evacuated volume.

Labels in Fig. 2: Grid; Collector (platinum coating); To the pump; To the evacuated object; Cathode.

Fig. 2.

The wide spread of ionization manometers in industry is often hindered by their short service life, which is associated with chemical destruction of the incandescent cathode during operation (especially intense in the event of an accidental breakthrough of the external atmosphere into the manometer), and also with the fragility of the glass body. These draw—

articles is partially eliminated in the gauge⁵, the design of which is shown in Fig. 3. The grid and cathode in it are mounted on a removable flange. If the cathode burns out, the flange can be removed and the cathode easily replaced. Owing to its metal body, this gauge is not afraid of accidental impacts and shocks.

Fig. 3.

The service life of the gauge is increased very substantially if thoriated iridium⁶, which has come into ordinary use, is employed as the cathode. Such a cathode, even at an operating temperature of \(1450^\circ\)C, does not burn out in the event of an accidental inrush of atmosphere. In addition, this cathode does not change its emission properties in the presence of such an active gas as oxygen, at pressures as high as \(10^{-2}\) mm Hg. The service life of gauges with such a cathode is determined by the rate of sputtering of the thorium coating and is estimated by the author at 1000 hours, which is almost 10 times greater than the normal service life of gauges with a tungsten cathode.

L. Kh.

CITED LITERATURE

1. S. Dushman, Scientific Foundations of Vacuum Technique, I. L., Moscow–Leningrad (1950).
2. R. E. B. Makinson, P. B. Treacy, J. Sci. Instr. 25, 298 (1948).
3. J. Blears, Proc. Roy. Soc. A188, 62 (1946).
4. J. H. Burrow, E. M. J. Mitchell, J. Sci. Instr. 29, 27 (1952).
5. F. M. Kelly, Rev. Sci. Instr. 21, 673 (1950).
6. O. A. Weinreich, H. Blecher, Rev. Sci. Instr. 23, 56 (1952).