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Manometers measure a pressure difference by balancing the weight of a fluid columnbetween the two pressures of interest. Large pressure differences are measured withheavy fluids, such as mercury (e.g. 760 mm Hg = 1 atmosphere). Small pressuredifferences, such as those experienced in experimental wind tunnels or venturi flowmeters , are measured by lighter fluids such as water (27.7 inch H 2O = 1 psi; 1cm H 2O = 98.1 Pa).
To calculate the pressure indicated by the manometer, enter the data below. (Thedefault calculation is for a water manometer with a 10 cm fluid column, with theanswer rounded to 3 significant figures.):
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ManometersWhen using a barometer, you calculate gas
pressure with the following equation:
Gas pressure = atmospheric pressure- h (height of the mercury)
The open-tube manometer is another device that can be used to measure pressure.The open-tube manometer is used tomeasure the pressure of a gas in a container.The pressure of the gas is given by h (thedifference in mercury levels) in units of torr or mmHg. Atmospheric pressure pushes onthe mercury from one direction, and the gasin the container pushes from the other direction. In a manometer, since the gas inthe bulb is pushing more than theatmospheric pressure, you add theatmospheric pressure to the heightdifference:
Pgas > P atm Gas pressure = atmospheric pressure + h (height of the mercury)
Pgas < P atm Gas pressure = atmospheric pressure - h (height of the mercury)
You could be asked about a closed-tubemanometer . Closed-tube manometers look similar to regular manometers except thatthe end that’s open to the atmospheric
pressure in a regular manometer is sealedand contains a vacuum. In these systems, the
difference in mercury levels (in mmHg) isequal to the pressure in torr.
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Bourdon Gauge:
The Bourdon gauge is shown in figure 9-4. It works on the same principle as that of thesnakelike, paper party whistle you get at a New Year party, which straightens when you
blow into it. With in the Bourdon gauge is a thin-walled metal tube, somewhat flattenedand bent into the form of a C. Attached to its free end is a lever system that magnifiesany motion of the free end of the tube. On the fixed end of the gauge is a fitting you
thread into a boiler system. As pressure increases within the boiler, it travels through thetube. Like the snakelike paper whistle, the metal tube begins to straighten as the pressureincreases inside of it. As the tube straightens, the pointer moves around a dial thatindicates the pressure in psi. The Bourdon gauge is a highly accurate but rather delicateinstrument. You can easily damage it. In addition, it malfunctions if pressure variesrapidly. This problem was overcome by the development of another type of gauge,the Schrader. The Schrader gauge is not as accurate as the Bourdon, but it is sturdyand suitable for ordinary hydraulic pressure measurements. It is
especially suitable for fluctuating loads. In the Schrader gauge, liquid pressure actuates a piston. The pressure moves up a cylinder against the resistance of a spring, carrying a bar or indicator with it over a calibrated scale. The operation of this gauge eliminates the
need for cams, gears, levers, and bearings.
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