The Manometer

A manometer is kind of like a barometer. The container of gas is attached to a U-shaped piece of glass tubing that s partially filled with mercury and sealed at the other end. Gravity pulls down the mercury column at the closed end. The mercury is then balanced by the pressure of the gas in the container. The difference in the two mercury levels represents the amount of gas pressure. 

Various scientific laws describe the relationships between four of the important physical properties of gases  

This section covers those various laws. Boyle s, Charles s, and Gay-Lussac s Laws each describe the relationship between two properties while keeping the other two properties constant. (In other words, you take two properties. 

Charles s, and Gay-Lussac s individual laws — enables you to vary more than one property at a time. 

But that combo law doesn t let you vary the physical property of amount. 

Second only to the mechanical gauge as the easiest device to measure and read a vacuum (and decidedly easiest in construction) is the liquid manometer (see Fig. 7.37). A well-made mercury manometer, kept very clean, can measure vacuums of up to 10 3 torr. This sensitivity can be increased by up to 15 times if a liquid with less density, such as diffusion pump oil, is used. However, diffusion pump oil is far more difficult to keep clean and can require either (a) a very tall (and thereby impractical) column or (b) a manometer of very limited range. In addition, because of the strong surface tension between diffusion pump oil and glass, long waiting periods between readings are required as the oil settles into place. 

The mechanics of a U-tube manometer are simple The difference between the levels of two interconnected columns of liquid is directly proportional to the difference between the pressures exerted upon them, assuming equal capillary effects on both tubes. 46 In practice, one end of a manometer is attached to an unknown pressure, and the other end is attached to a reference pressure that is known. For all practical purposes, the known value needs only to be a much smaller order of magnitude than the unknown value. 

The choice of mercury for manometers is often a matter of convenience, or rather the acceptance of the least amount of inconvenience. Mercury has a rather high vapor pressure (10 3 torr), but this vapor pressure is also at the upper ranges of what can be read by a manometer. Low-vapor-pressure oils (as used in diffusion pumps) can be used, but these oils wet the walls of a manometer and can take a long time to settle before reading can be made. Mercury is fairly nonreactive and retains a limited amounts of condensable vapors. 

One of the biggest reasons why mercury is used is because it is a very heavy liquid. Atmospheric pressure can only push mercury about 76 cm, while it pushes water some 30 feet. Manometers using lighter-density liquids can be so tall they become beyond inconvenient. Additionally, because mercury doesn t wet the glass walls of the manometer, there is never a time lag for reading as there is with oil. 

For very accurate manometer readings, the NIST (National Institute of Standards and Testing) has a compiled list of eight possible errors that can develop while reading manometers. These errors introduce extremely small variations and therefore are not included here. If you need to read a manometer to the sensi- 

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When constructing a manometer of the type shown in Fig. 12(c), it is impor tant to apply a very high vacuum (e.g., with a Hy-Vac pump) to the manometer while the mercury in the left-hand (sealed) limb is heated until it boils unless this is done, traces of air will remain in this limb and cause inaccurate readings. During a distillation, the tap I should be kept closed except when a pressure reading is being taken if it is left open indefinitely, a sudden default by the distillation apparatus or by the pump may cause the mercury in the sealed limb of G to fly back and fracture the top of the limb. 

The action of the manostat may be explained with the aid of Fig. II, 23, 6. Mercury is introduced into the container until the disc of the float just makes contact with the oriflce, when the pressure is equalised inside and outside the float. The device is connected to the pump and to the system by way of a large reservoir and a manometer. With the stopcock open, the pressure in the system is reduced by way of a by-pass between the pump and the system until the desired value as read on the manometer is reached, then both the stopcock and by-pass are... 

Example 3 Venturi Flowmeter An incompressible fluid flows through the venturi flowmeter in Fig. 6-7. An equation is needed to relate the flow rate Q to the pressure drop measured by the manometer. This problem can he solved using the mechanical energy balance. In a well-made venturi, viscous losses are neghgihle, the pressure drop is entirely the result of acceleration into the throat, and the flow rate predicted neglecting losses is quite accurate. The inlet area is A and the throat area is a. 

The continuity equation gives V2 = V AJa, and Vj = Q/A. The pressure drop measured by the manometer is pi —p2= (p — p)gA . Substituting these relations into the energy balance and rearranging, the desired expression for the flow rate is found. 

Differential pressures and subatmospherie pressures should be measured by manometers with a fluid that is ehemieally stable when in eontaet with the test gas. Mereury traps should be used where neeessary to prevent the manometer fluid from entering the proeess piping. Errors in these instruments should not exeeed 0.25%. 

If manometer has sealing fluid, make sure it is the proper type fluid, and that the manometer is properly filled. (This is an instrument technician s job.)... 

Fluid manometers are devices where the readout of the pressure differential is the length of a liquid column. The most fundamental implementation of this principle is the U-tube manometer. This is simply a tube of U shape filled with manometer fluid, as shown in Fig. 12.16. The pressure differential is applied at both ends of the tube, making the manometer fluid move downward in one limb and upward in the other, until the forces acting on the fluid are in balance. 

In ventilation applications, where the density of the manometer fluid is much higher than the density of air, the pressure difference Ap can be expressed using the equation... 

Several other variations of the fluid inanomerer provide a higher sensitivity, The inclined well-type manometer (Fig. 12.17) has a large-cross-section container for the manometer fluid connected ro an inclined rube with a scale. The pressure difference becomes... 

As well as measurement errors due to the pressure measurement instrument itself, other errors related to pressure measurements must be considered. In ventilation applications a frequently measured quantity is the duct static pressure. This is determined by drilling in the duct a hole or holes in which a metal tube is secured. The rubber tube of the manometer is attached to the metal tube, and the pressure difference between the hole and the environment or some other pressure is measured. 

From the calibration point of view, manometers can be divided into two groups. The first, fluid manometers, are fundamental instruments, where the indication of the measured quantity is based on a simple physical factor the hydrostatic pressure of a fluid column. In principle, such instruments do not require calibration. In practice they do, due to contamination of the manometer itself or the manometer fluid and different modifications from the basic principle, like the tilting of the manometer tube, which cause errors in the measurement result. The stability of high-quality fluid manometers is very good, and they tend to maintain their metrological properties for a long period. 

The principle of calibration is to compare the measurement result of the manometer to be calibrated to that of the measurement result of the reference... 

Figure 4-3 shows three one-liter bulbs at 25°C. The first bulb contains 0.0050 mole of air. The manometer shows that the pressure is 93 mm Hg... 

Gas is slowly added to the empty chamber of a closed-end manometer (see Figure 4-2B). Draw a picture of the manometer mercury levels, showing in millimeters the difference in heights of the two mercury levels ... 

According to Haber (loc. cit., p. 131) the results of Langen are probably correct to 3 per cent, even at 2,000°. Pier, in his recent explosion experiments, has shown, however, that the maximum pressures were not obtained by the previous observers, on account of the oscillations of their manometers, He used a steel plate with very high frequency of vibration, and registered the distortion by reflecting a beam of light from a mirror attached to the manometer disc on to a revolving drum of sensitised paper. The recorded curves show a well-defined maximum pressure, and his results are probably accurate to 1 per cent. Values of Gv ... 

A flow sheet of the process, as given by Ur banski (Ref 75, his Fig 53), is shown below According to Urbanski To start the nitration, current from the switch (18) is applied to the electromagnet (6), which closes the air inlet to the injector. Mixed acid is admitted by opening the valve under the acid rotameter (4). The injector now comes into operation. The manometer (13) must show full vacuum. The needle valve (7) is then opened and the vacuum adjusted to about 300mm Hg. The glycerine-glycol mixture is sucked in thru the rotameter (3) to the injector from (2)... 

Draft is usually measured in inches or centimeters of water using a U-tube manometer, with one side connected to the sample point (such as the furnace section, convective-pass section or the boiler stack) and the other side open to the atmosphere. The difference in the manometer column height indicates boiler draft pressure, which may be either higher (overpressure) or lower (underpressure) than atmospheric pressure. 

If fluid A is a gas, the density p will normally be small compared with the density of the manometer fluid pm so that ... 

If B is the angle of inclination of the manometer (typically about 10-20°) and L is the movement of the column of liquid along the limb, then ... 

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