Machinery's Handbook, 31st Edition
2756 PRESSURE AND FLOW atmosphere or some known pressure reservoir. The height of the liquid in the tube is read against a calibrated scale and is related to pressure in the pipe or vessel. Barometers: Barometers are used to measure atmospheric pressure. They operate on the same principle as manometers, where the change in height of a column of gauge fluid (commonly mercury) is used to calculate a change in pressure. Commercial baro meters are calibrated and generally measure in the range between 25.5 and 32.5 inches (650–825 mm) of mercury. Corresponding pressures in pound per square inch and pound per square foot are given in Table 6. Table 6. Relation Between Barometric Pressure, and Pressures in Pounds per Square Inch and Square Foot
Pressure in Psi a
Pressure in Psi a
Pressure in Psi a
Pressure in Psf a
Pressure in Psf a
Pressure in Psf a
Barometer, Inches
Barometer, Inches
Barometer, Inches
28.00 13.75 1980 28.25 13.87 1997 28.50 13.99 2015 28.75 14.12 2033 29.00 14.24 2050
29.25 14.36 2068 30.50 14.98 2156 29.50 14.48 2086 30.75 15.10 2174 29.75 14.61 2103 31.00 15.22 2192 30.00 14.73 2121 31.25 15.34 2210
30.25 14.85 2139
… … …
a Psi is pound per square inch; Psf is pound per square foot Pressure Gauges: The most common method of measuring pressure in a fluid power cir cuit is by using a pressure gauge. There are gauges for liquids and gauges for gases. There are a variety of methods of construction of pressure gauges, including elastic diaphragm, dead weight, magnetic, and Bourdon tube gauges. Commercial gauges are normally cali brated to read zero at standard atmospheric pressure and temperature. Pressure gauges are available in a variety of ranges and units. Some are capable of reading vacuum, or negative gauge pressure. Liquid filled gauges provide more stable readouts when vibration or pulsation is present in the system. The use of a snubber is recommended when fluid pressure pulsates. Pressure Transducers: These are used when the display needs to be decoupled from the measurement device. Transducers convert pressures to electrical signals which can be transmitted to remote displays. Pressure transducers can be linear differential transform ers, piezoelectric, quartz resonators, or solid state devices. Pressure Regulation.— Fluids are often delivered at pressures higher than needed. Pres sure regulators are used where pressure needs to be lowered and controlled. There are regulators for liquids and others for gases. A pressure regulator is a device generally com- prised of three elements; a pressure reducing or restrictive element, a sensing element, and a force generator (usually a spring). In operation, the force generator opens the valve. Pressure introduced into the inlet port then flows through the valve and applies pressure to the sensing device (usually a diaphragm or piston). The pressure acts on the sensing ele ment to produce a force which opposes the opening force and closes the valve. Regulators may be manually or electronically adjusted, and may include a pressure gauge or sensor. In some cases, pressure may fluctuate due to variable supply and/or demand. This may be undesirable where work is being performed. A pressure reservoir with a regulator after it is a desirable source for most fluid circuits because the combination can buffer pressure variations and supply multiple circuits. When choosing a pressure regulator, there are many factors to consider. The allowable ranges of inlet and outlet pressures must be known when sizing a regulator. The maximum flow rate and any porting requirements are also factors. It is also important to consider the chemical properties of the fluid before determining the best materials for your applica - tion. There are a wide range of materials available to suit all applications. A non-relieving
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