Manometers
Probably, the first vacuum test was done in 1641 by Gasparo Berti, and by water barometer. And possibly this test was starting for the use of vacuum, and the human walking in this vital field. At first, the pressure measurement was calculated based on the amount of movement of a liquid. Thus, for a fluid with density of ρ, and the displacement of h, the pressure of this movement came to be with the relationship p = ρgh. The most common used liquids were water and mercury.
But usually, instead of water was used of mercury. Because the density of mercury is more than water, and thus, had need to smaller manometer. As can be seen in Figure 1, if entered the atmospheric pressure into the water inside the container, the water rises up to 10.321 mm, while if instead of water, use of mercury, because mercury is 13.58 times heavier than water, its column height is also 13.58 times smaller, thus, according to the following equation, at atmospheric pressure, the mercury rises to 760 mm height
Figure 1: liquids manometer
In following, the U-shaped manometer was used, that according to Figure 2 includes a U-shaped tube, which is containing liquids (usually mercury or oil), which is inserted on it, on the one hand atmospheric pressure and on the other hand low pressure.
Figure 2: liquids manometer
In this case, by writing equilibrium conditions between the two sides of manometer, we have:
Pa = P0 +ρgh
In which ρ is the density of the liquid, and g is the gravitational acceleration of used liquid in the manometer. Thus, by determining the height h, is achieved the pressure at intended area.
However, Manometers were made and were used in various forms, but because density and the amount of liquids gravity, in various places, is different, as a result, this type of manometers were unable to offer exact definition of pressure. However, the manometry gauge are still being used in many cases such as taking blood pressure (mm Hg).