Lec.4 Pressure Gauges

Two types of pressure gauge:
(a) The simplest pressure gauge is the open-tube manometer. The U-shaped tube contains a liquid of density ?, often mercury or water.
The left end of the tube is connected to the container where the pressure (p) is to be measured, and the right end is open to the atmosphere at pressure ( p0=patm).The pressure at the bottom of the tube due to the fluid in the left column is ( p+?gy1) and the pressure at the bottom due to the fluid in the right column is ( patm+?gy2) .These pressures are measured at the same level, so they must be equal:
(b)Another common pressure gauge is the mercury barometer. It consists of a long glass tube, closed at one end, that has been filled with mercury and then inverted in a dish of mercury .
The space above the mercury column contains only mercury vapor; its pressure is negligibly small, so the pressure at the top of the mercury column is practically zero. From (Eq. 6)
patm = p= 0+ ?g (y2-y1) = ?gh

Note:
1-Thus the mercury barometer reads the atmospheric pressure directly from the height of the mercury column.
2-A pressure of 1 mm Hg is called 1 torr, But these units depend on the density of mercury, which varies with temperature, and on the value of g, which varies with location, so the pascal is the preferred unit of pressure.