Gauge Pressure, Absolute Pressure, and Pressure Measurement Quiz

PDF page 502: Gauge pressure is the pressure relative to atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure, and negative for pressures below it.

PDF page 502: Gauge pressure is the pressure relative to atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure, and negative for pressures below it.

PDF page 502: Absolute pressure is the sum of gauge pressure and atmospheric pressure.

PDF page 502: In fact, atmospheric pressure does add to the pressure in any fluid not enclosed in a rigid container. This happens because of Pascal’s principle. The total pressure, or absolute pressure, is thus the sum of gauge pressure and atmospheric pressure: where is absolute pressure, is gauge pressure, and is atmospheric pressure. For example, if your tire gauge reads 34 psi (pounds per square inch), then the absolute pressure is 34 psi plus 14.7 psi ( in psi), or 48.7 psi (equivalent to 336 kPa).

PDF page 502: If you limp into a gas station with a nearly flat tire, you will notice the tire gauge on the airline reads nearly zero when you begin to fill it. In fact, if there were a gaping hole in your tire, the gauge would read zero, even though atmospheric pressure exists in the tire. Why does the gauge read zero? There is no mystery here. Tire gauges are simply designed to read zero at atmospheric pressure and positive when pressure is greater than atmospheric. Similarly, atmospheric pressure adds to blood pressure in every part of the circulatory system. (As noted in Pascal’s Principle, the total pressure in a fluid is the sum of the pressures from different sources—here, the heart and the atmosphere.) But atmospheric pressure has no net effect on blood flow since it adds to the pressure coming out of the heart and going back into it, too. What is important is how much greater blood pressure is than atmospheric pressure. Blood pressure measurements, like tire pressures, are thus made relative to atmospheric pressure. In brief, it is very common for pressure gauges to ignore atmospheric pressure—that is, to read zero at atmospheric pressure. We therefore define gauge pressure to be the pressure relative to atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure, and negative for pressures below it.

PDF page 502: In fact, atmospheric pressure does add to the pressure in any fluid not enclosed in a rigid container. This happens because of Pascal’s principle. The total pressure, or absolute pressure, is thus the sum of gauge pressure and atmospheric pressure: where is absolute pressure, is gauge pressure, and is atmospheric pressure. For example, if your tire gauge reads 34 psi (pounds per square inch), then the absolute pressure is 34 psi plus 14.7 psi ( in psi), or 48.7 psi (equivalent to 336 kPa).

PDF page 502: For reasons we will explore later, in most cases the absolute pressure in fluids cannot be negative. Fluids push rather than pull, so the smallest absolute pressure is zero. (A negative absolute pressure is a pull.) Thus the smallest possible gauge pressure is (this makes zero). There is no theoretical limit to how large a gauge pressure can be. There are a host of devices for measuring pressure, ranging from tire gauges to blood pressure cuffs. Pascal’s principle is of major importance in these devices. The undiminished transmission of pressure through a fluid allows precise remote sensing of pressures. Remote sensing is often more convenient than putting a measuring device into a system, such as a person’s artery.

PDF page 502: For reasons we will explore later, in most cases the absolute pressure in fluids cannot be negative. Fluids push rather than pull, so the smallest absolute pressure is zero. (A negative absolute pressure is a pull.) Thus the smallest possible gauge pressure is (this makes zero). There is no theoretical limit to how large a gauge pressure can be. There are a host of devices for measuring pressure, ranging from tire gauges to blood pressure cuffs. Pascal’s principle is of major importance in these devices. The undiminished transmission of pressure through a fluid allows precise remote sensing of pressures. Remote sensing is often more convenient than putting a measuring device into a system, such as a person’s artery.

PDF page 503: gauges, pressure results in a force that is converted (or transduced) into some type of readout.

PDF page 503: FIGURE 11.13 This aneroid gauge utilizes flexible bellows connected to a mechanical indicator to measure pressure.

PDF page 503: An entire class of gauges uses the property that pressure due to the weight of a fluid is given by Consider the U-shaped tube shown in Figure 11.14, for example. This simple tube is called a manometer. In Figure 11.14(a), both sides of the tube are open to the atmosphere. Atmospheric pressure therefore pushes down on each side equally so its effect cancels. If the fluid is deeper on one side, there is a greater pressure on the deeper side, and the fluid flows away from that side until the depths are equal. Let us examine how a manometer is used to measure pressure. Suppose one side of the U-tube is connected to some source of pressure such as the toy balloon in Figure 11.14(b) or the vacuum-packed peanut jar shown in

PDF page 503: An entire class of gauges uses the property that pressure due to the weight of a fluid is given by Consider the U-shaped tube shown in Figure 11.14, for example. This simple tube is called a manometer. In Figure 11.14(a), both sides of the tube are open to the atmosphere. Atmospheric pressure therefore pushes down on each side equally so its effect cancels. If the fluid is deeper on one side, there is a greater pressure on the deeper side, and the fluid flows away from that side until the depths are equal. Let us examine how a manometer is used to measure pressure. Suppose one side of the U-tube is connected to some source of pressure such as the toy balloon in Figure 11.14(b) or the vacuum-packed peanut jar shown in

PDF page 503: is greater than atmospheric pressure, whereas in Figure 11.14(c), is less than atmospheric pressure. In both cases, differs from atmospheric pressure by an amount , where is the density of the fluid in the manometer. In Figure 11.14(b), can support a column of fluid of height , and so it must exert a pressure greater than atmospheric pressure (the gauge pressure is positive). In Figure 11.14(c), atmospheric pressure can support a column of fluid of height , and so is less than atmospheric pressure by an amount (the gauge pressure is negative). A manometer with one side open to the atmosphere is an ideal device for measuring gauge pressures. The gauge pressure is and is found by measuring .