Why gauge pressure

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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 P abs such as the toy balloon in Figure 2 b or the vacuum-packed peanut jar shown in Figure 2 c.

Pressure is transmitted undiminished to the manometer, and the fluid levels are no longer equal. In Figure 2 b , P abs is greater than atmospheric pressure, whereas in Figure 2 c , P abs is less than atmospheric pressure.

A manometer with one side open to the atmosphere is an ideal device for measuring gauge pressures. Figure 2. An open-tube manometer has one side open to the atmosphere.

Mercury manometers are often used to measure arterial blood pressure. An inflatable cuff is placed on the upper arm as shown in Figure 3. By squeezing the bulb, the person making the measurement exerts pressure, which is transmitted undiminished to both the main artery in the arm and the manometer. When this applied pressure exceeds blood pressure, blood flow below the cuff is cut off.

The person making the measurement then slowly lowers the applied pressure and listens for blood flow to resume. Blood pressure pulsates because of the pumping action of the heart, reaching a maximum, called systolic pressure , and a minimum, called diastolic pressure , with each heartbeat. Systolic pressure is measured by noting the value of h when blood flow first begins as cuff pressure is lowered.

Diastolic pressure is measured by noting h when blood flows without interruption. The typical blood pressure of a young adult raises the mercury to a height of mm at systolic and 80 mm at diastolic.

The first pressure is representative of the maximum output of the heart; the second is due to the elasticity of the arteries in maintaining the pressure between beats. The density of the mercury fluid in the manometer is This reduced height can make measurements difficult, so mercury manometers are used to measure larger pressures, such as blood pressure. The density of mercury is such that 1. Figure 3. In routine blood pressure measurements, an inflatable cuff is placed on the upper arm at the same level as the heart.

Blood flow is detected just below the cuff, and corresponding pressures are transmitted to a mercury-filled manometer. Army photo by Spc. An Intro to Pipeline Corrosion in Seawater. Follow Connect with us. Sign up. Thank you for subscribing to our newsletter! Connect with us. Gauge Pressure. What Does Gauge Pressure Mean? Corrosionpedia Explains Gauge Pressure A gauge is often used to measure the pressure difference between a system and its surrounding atmosphere.

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In the United States, the units used most commonly to measure pressure are pounds per square inch psi , bar, and millibar mbar. Pressure can also be measured in atmospheres atm , inches of mercury in. Hg , millimeters of mercury mm Hg , and torr about WIKA is a global leader in pressure instrumentation.

Our website uses cookies. By continuing to use it, you agree to their use. The gauge pressure is defined as the difference between an absolute pressure P abs and the prevailing atmospheric pressure P amb.

The difference between the two measurements is relatively easily clarified: in a gauge pressure measurement, it is always the difference from the current ambient pressure that is measured. However, this pressure changes with the weather and the height above sea level. An absolute pressure measurement measures the difference from the ideal or absolute vacuum. This is why this measurement is independent of environmental influences such as weather or altitude.