Just how do we and can we measure the atmospheric pressure? In this article, we will teach you how.

Two commonly used devices for measuring pressure are the manometer and the Bourdon tube. Manometers measure pressure differences in terms of the length of a column of liquid such as water, mercury, or oil. The manometer shown in Fig. 1.6 has one end open to the atmosphere and the other attached to a closed vessel containing a gas at uniform pressure. 

The difference between the gas pressure and that of the atmosphere is p - patm = rgL where r is the density of the manometer liquid, g the acceleration of gravity, and L the difference in the liquid levels. For short columns of liquid, and g may be taken as constant.

Because of this proportionality between pressure difference and manometer fluid length, pressures are often expressed in terms of millimeters of mercury, inches of water, and so on. It is left as an exercise to develop the equation using an elementary force balance.

A Bourdon tube gage is shown in Fig. 1.7. The figure shows a curved tube having an elliptical cross section with one end attached to the pressure to be measured and the other end connected to a pointer by a mechanism. 

When fluid under pressure fills the tube, the elliptical section tends to become circular, and the tube straightens. This motion is transmitted by the mechanism to the pointer. By calibrating the deflection of the pointer for known pressures, a graduated scale can be determined from which any applied pressure
can be read in suitable units. 

Because of its construction, the Bourdon tube measures the pressure relative to the pressure of the surroundings existing at the instrument. Accordingly, the dial reads zero when the inside and outside of the tube are at the same pressure.

Pressure can be measured by other means as well. An important class of sensors utilize the piezoelectric effect: A charge is generated within certain solid materials when they are deformed. 

This mechanical input electrical output provides the basis for pressure measurement as well as displacement and force measurements. Another important type of sensor employs a diaphragm that deflects when a force is applied, altering an inductance, resistance, or capacitance. 

Figure 1.8 shows a piezoelectric pressure sensor together with an automatic data acquisition system.

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