Page 391 - Mechatronics with Experiments

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JWST499-c06
JWST499-Cetinkunt
SENSORS 377 254mm×178mm
Vacuum

Glass tube

Height of mercury
Mercury
column 29.92 in

Atmospheric
pressure

Dish
FIGURE 6.47: Barometer to measure the local
atmospheric absolute pressure.

to another as a function of the height of the location from sea level. Average atmosheric
2
absolute pressure due to the weight of the air of the atmosphere is 14.7lb∕in = 14.7psi.
2
This means that the weight acting over an area of 1 in due to the weight of the air in the
atmosphere is 14.7 lb. Therefore, the absolute atmospheric pressure at sea level is nominally
2
14.7lb∕in (14.7psia).
Pascal’s law states that pressure in a contained fluid is transmitted equally in all
directions. Using this physical principle, a barometer is used to measure absolute pressure.
The pressure acting on the fluid due to the atmosphere at the surface is balanced by
the pressure due to the fluid (i.e., mercury) weight in the tube (Figure 6.47). Using this
measurement method it can be observed that the atmospheric pressure is equivalent to the
pressure applied by a 29.92 in(760 mm) column height of mercury. A column of mercury
2
with 29.92 in height and 1 in cross-section has 14.7 lbs of weight. If water is used in the
barometer instead of mercury, the height of the water in the tube to balance the atmospheric
2
pressure would be 33.95 in, which produces the same 14.7lb∕in pressure since the density
of water is lower than mercury. Notice that the top of the tube in the barometer must be a
vacuum. In order to establish a vacuum at the top of the tube, the tube filled with mercury or
water is placed upside down in the container filled with mercury. The height of the mercury
will drop until its height is 29.92 in if filled with mercury, or 33.95 in if filled with water,
which generates a pressure of 14.7 psi at the surface level of the container.
The pressure sensed by most pressure sensors is the relative pressure with respect to
the local atmospheric pressure. However, a sensor can be calibrated to measure the absolute
pressure as well (Figure 6.46). If a sensor measures pressure relative to the vacuum pressure,
it is referred to as the absolute pressure and the units denoted as psia. If a sensor measures
pressure relative to the local atmospheric pressure, it is referred to as the relative or gauge
pressure, and the unit used to indicate that is psig. The notation psi refers to psig by standard
convention in notation.