A NEW VACUUM MANOMETER

To measure pressures in the range from \(10^{-5}\) mm Hg up to atmospheric pressure, it is at present necessary to use at least three manometers of different types.

The authors of the paper under review*) propose a new measurement principle that makes it possible, with a single manometer, to cover the entire indicated range.

The model described by the authors (Fig. 1) has, as its sensitive element, electrically heated conductors (tungsten and platinum) placed in vibrating bellows (of volume \(6\ \text{cm}^3\)). A rotating electric motor with an eccentric shaft produces a periodic change in the volume of the bellows by 20%. Two bellows are used so that the external pressure does not impede the operation of the motor. The manometer is connected to the system in which the pressure is being measured by two narrow apertures (with diameters of 0.5 mm and 1 mm). Owing to the low rate of gas flow through these apertures, the vibration of the bellows produces a periodic change of pressure in them.

Fig. 1.

*) R. Havens, R. Koll and H. LaCow, Rev. Sci. Instr. 21, 596 (1950).

At low pressures a tungsten wire (with a resistance of 1000 ohms and a diameter of \(7\ \mu\)) operates as an ordinary thermoelectric resistance manometer (Pirani manometer). Pressure fluctuations cause an alternating voltage to appear on the filament of the manometer, the magnitude of which varies with pressure. Owing to the large amplification provided by the alternating-current amplifier, it is possible to measure pressures down to 50 mm Hg. In view of the fact that the quantity \(\dfrac{dQ}{dp}\) (where \(Q\) is the thermal conductivity of the gas, \(p\) is the pressure) decreases as the pressure increases, the alternating voltage on the manometer filament, as the pressure rises, first increases and then begins to decrease.

At pressures above 50 mm Hg the tungsten wire operates as a thermometer. At these pressures the temperature and, correspondingly,

Fig. 2.

the resistance of the wire increase when the Sylphon is compressed and, conversely, decrease when the gas expands. The change in temperature is then all the greater, the greater the amount of gas being compressed and, consequently, its pressure. Therefore, when the pressure is increased above 50 mm Hg, the alternating signal begins to increase again. The calibration curve for the tungsten wire is shown in Fig. 2.

Measurement of still lower pressures can be carried out by means of a thin platinum filament (0.2 μ thick, 0.5 mm wide, and 8 mm long) placed in another Sylphon. The calibration curve for this case is shown in Fig. 3.

Even lower pressures can be measured by means of the same system if an ionization manometer is used in it as the sensitive element instead of a heating wire or filament. The lower measurement limit of an ionization manometer is determined, as is known, by the photoemission current of electrons from its collector, caused by soft X-ray radiation arising when electrons are decelerated in the material of the accelerating electrode. For ordinary ionization manometers this current corresponds to a pressure of the order of \(10^{-8}\) mm Hg. If the manometer is used in the described system, the photoemission current will remain constant in time, whereas the ion current

will vary with pressure, and the alternating-current amplifier will respond only to the alternating component of the ion current. The lower

Fig. 3.
Axes: \(V\) (µV); \(P\) (mm Hg).
Curve labels: filament current \(2.7\) mA; filament current \(13\) mA.

limit of measurement in this case is determined by the sensitivity of the amplifier and, according to the authors’ estimate, is equal to \(4 \cdot 10^{-11}\) mm Hg.

L. Kh.