A NEW MANOMETER FOR MEASURING PRESSURES FROM 0.1 TO 190 mm Hg

Direct recording of low pressures (on the order of 100 mm Hg and below) by means of ordinary recording manometers presents considerable difficulties. Instruments of this kind, based on the mechanical action of the pressure of a rarefied gas, are characterized by a linear dependence between the displacement of the pen and the value of the pressure being measured, and they cease to operate if the latter is too small. At pressures close to 100 mm Hg, the mechanical errors of the recording attachment (friction, backlash) give an error on the order of 10%, and at 1.0–0.1 mm measurements by this method are altogether impossible.

Instead of such mechanical manometers, Vincent and Symons1 proposed an instrument, called by them a thermomanometer, based on measuring the temperature of the boiling vapors of mercury at the pressure being measured.

The manometer consists of a long (760 mm) vertical Pyrex tube sealed at the ends. In the upper part there is a branch connecting it with the volume in which the pressure is measured. Pure mercury is poured onto the bottom and heated to boiling by an external electric heater. Through the upper end a thermocouple is introduced into the manometer in a narrow sealed glass tube; the junction of the thermocouple is located approximately 8 cm above the surface of the mercury. The heating is regulated so that the mercury boils already at the greatest pressure being measured—100 mm Hg. The vapors condense on the walls of the tube above the thermal junction; at the level of the junction itself the tube is silvered on the outside and covered with a glass jacket.

The constancy of the vapor temperature at the established pressure is maintained, of course, “automatically” by an increase or decrease in heat transfer by convection currents, depending on the intensity of boiling. When the instrument was tested, it turned out that in the pressure interval from 1 to 100 mm Hg the vapor temperature at constant pressure does not depend on the current strength in the heating winding (from 1.7 A and higher). However, from 1.0 to 0.1 mm the constancy of the temperature is maintained less well, and below 0.1 mm the temperature depends on the current strength, and consequently measurements of such pressures are meaningless. This same property of the manometer is also expressed by its calibration curve—the dependence between the measured pressure and the vapor temperature. In the interval from 100 to 1 mm it coincides exactly with the corresponding experimental curve for the vapors of boiling mercury; below 1 mm it deviates from it, and below 0.1 mm the temperature readings of the instrument cease altogether to depend on the pressure.

Thus, the thermomanometer in the described form can serve for measuring pressures from 100 mm to 1.0–0.1 mm Hg. (Attempts to measure still lower pressures by the same method, replacing mercury with mineral oil, have so far proved unsuccessful.) The principal drawback of the instrument is its great inertia. Thus, after a sudden pressure jump from 1 to 9 mm, the new reading of the manometer was established only after 5 min, whereas the control McLeod manometer at the same installation responded to the same pressure jump after 2 min. Therefore the instrument is unsuitable for recording rapid pressure fluctuations.

E. Kofman, Moscow

LITERATURE

1. Proc. Phys. Soc., 51, 1003, 1939.

1. Proc. Phys. Soc., 51, 1003, 1939. ↩