Manometer

MANOMETER, an instrument for measuring the pressures exerted by gases or vapours (Gr. mavin, thin or loose; Airpov, a measure). A form of pressure gauge (q.v.). The manometer is simply a U tube (fig. 1), contain ing a liquid : if the pressures on the surfaces of the liquid be equal, then the surfaces will be at the same height. If, on the other hand, the pressure in one limb be greater than the pressure in the other, the surfaces will be at different heights, the differ ence being directly proportional to the difference of pressures and inversely as the specific gravity of the liquid used. One limb of such a tube (limb A) is con nected to the apparatus contain ing the gas or vapour whose pres sure it is desired to determine. The other limb (limb B) may be either open to the air (fig. 2), or closed. In the open-tube type the pressure in one limb is equal to the atmospheric pressure, and in the closed-tube type the experimental pressure is balanced against the liquid column and the pressure of the air compressed into the upper part of a closed limb of the tube (fig. 3). In the "open tube" form (fig. 2) the pressure on the surface a is equal to the pressure on the surface at b (one atmosphere) plus the hydrostatic pressure exerted by the liquid column of height ab. The liquid commonly used is mercury. If a scale be placed behind the limbs of the tube, so that the difference ab can be directly determined, then the pressure in a is at once expressible as P-Fab in milli metres or inches of mercury, where P is the atmospheric pressure, known from an ordinary barometric observation.

In the "closed tube" form (fig. 3) the calculation

is not so simple, since pressure on the mercury surface in the closed limb will vary with the position of that surface. Suppose the length of the air column in the closed limb be h and its pressure p when the mercury is at the same height in both tubes. Applying pressure at a the mercury column will rise and the air column diminish in the closed limb. Let the length of the air column be h', then its pressure is ph/h'. The difference in height of the mercury columns in the two limbs is 2(h—h'), hence the pressure in the open limb is equal to that of a column of mercury of length 2(h—h') pl-Fs and if p is known the pressure applied at a can be determined. A particular case occurs when p is zero—the pressure at a is then equal to the difference in height of the mer cury columns. Such instruments are serviceable for measuring pressures less than one atmosphere and are used in the laboratory to measure roughly the degree of vacuum obtained (see VACUUM).