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It is important to understand the terms that are used to discuss pressure before you can begin to understand the theory of operation for different sensors. Pressure is defined as the amount of force applied to an area. This can be defined mathematically as pressure is equal to force per unit area:
P = F / A
The unit of pressure can be described in pounds per square inch (psi), which is the common standard for English units. To understand this concept, you need to understand all of the ways force can be used. In simplest terms, force may be defined as any push or pull. If the force is produced by the shaft of a motor in a circular motion, it's defined as torque. Other important terms that refer to pressure are the terms stress and strain. Stress can be defined as the ratio of force over the cross-sectional area. These are basically the same terms that we used earlier to define pressure, so this means that the terms stress and force may be used in similar examples. The term strain refers to the change an object goes through when force is applied to it. e.g., if four vertical support beams are used to hold up a tank of water, each of the beams will be deformed (compressed) slightly by the weight of the tank and water as the tank is filled. The deformation (change in length) can be divided by the original length to determine the amount of strain. In this section of the chapter, stress will be used to describe the force per unit area applied to a body, and strain will be used to describe the change in the length of the body. From the previous example, stress is the force that the weight of the water exerts on the beams, and strain is the amount each of the beams is compressed.
The next important area of pressure that must be understood is the units of measurement for pressure. Pressure measurement uses the amount of atmospheric pressure as a base value for most of the units of measure. e.g., the amount of air pressure caused by the atmosphere at sea level is 14.7 psi. This means that each square inch of land has 14.7 pounds of force pushing down on it. This pressure is coming from the weight of the air around us. If you climb a mountain, the amount of air is less dense so the amount of pressure that it exerts will be less than 14.7 psi. Since the atmosphere is always around us, it's used as the starting point (zero reading) for most pressure measurements. e.g., if you used a pressure gauge to measure the air in the tires of an automobile, the pressure gauge would read zero if you examined it before you placed it on the valve stem of the tire to measure the pressure. This means that the air pressure gauge is calibrated so that the 14.7 psi of atmospheric pressure that continually surrounds us is not added to the pressure of the air in the tire. If the atmospheric pressure was included in the pressure reading, it would make the value larger. You can now see why it will be important to indicate if the pressure reading includes atmospheric pressure or not. If atmospheric pressure is included in the reading, it's called absolute pressure and the units will be defined as psia. If the reading is from a gauge that does not include the original 14.7 psi from the atmosphere, it will be called gauge pressure and the units will be defined as psig or psi. In industry, if the term absolute is not specified with the pressure units, then the reading is designated as a gauge reading. it's also important to understand that since all readings are assumed to be gauge readings unless specified, the letter g is generally not used and the units are shortened to psi. This means that you are probably more familiar with the psig system than you realize because every time you used the term psi to describe the amount of air in a tire, or the amount of pressure in a hydraulic system, you were using the psig system. The table below shows absolute pressure and its equivalent gauge pressure.
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