Manometers mechanical

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. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Mechanical manometers are the oldest, simplest, and most reliable pressure measurement instruments. They have some disadvantages, which is one reason the use of electrical manometers is expanding. Their simplicity and fundamental nature can, however, be an advantage. 

The most frequently applied mechanical manometers in ventilation applications are fluid manometers, bur the following types are also used. The Bourdon tube is a small-voiume tube with an elliptic cross-section bent to the shape of a circular arc, the C-type. One end is open to the applied pressure while the other end is closed. The pressure inside the tube causes an elastic defonnation ot the tube and displaces the closed end, which is then converted, by means of a linkage mechanism, into the movement of a pointer. The Bourdon tube may be of a spiral or helical design as well. 

The second, mechanical and electrical manometers, require more frequent calibration. Changes in the elastic properties of the pressure transducer, wearing in mechanical parts, and electronic circuitry drift influence the properties of the instruments, giving rise to repeated calibration. 

A strange accident occurred with ammonia. A mercury manometer was in contact with ammonia in the presence of water traces. The grey-brown solid formed detonated when a technician attempted to take it away mechanically. Tests showed that this solid did not form in the absence of water. 

A mercury manometer used with ammonia became blocked by deposition of a grey-brown solid, which exploded dining attempts to remove it mechanically or on heating. The solid appeared to be a dehydration product of Millon s base and was freely soluble in sodium thiosulfate solution. This method of cleaning is probably safer than others, but the use of mercury manometers with ammonia should be avoided as intrinsically unsafe [1,2]. Although pure dry ammonia and mercury do not react even under pressure at 340 kbar and 200° C, the presence of traces of water leads to the formation of an explosive compound, which may explode during depressurisation of the system [3], Explosions in mercury-ammonia systems had been reported previously [4,5],... 

Figure 1. Diagram of apparatus (M) monomer reservoir (F) flow meter (VG) vacuum gage (mercury manometer) (E) electrode (T) liquid nitrogen trap (P) mechanical pump (V,) needle valve (Vt) stop valve (Vs) pressure control valve (OSC) discharge frequency oscillator (AMP) amplifier (1MC) impedance matching circuit...

If vapour pressure measurements are to be an essential part of the work to be undertaken, a cold cathode manometer is probably the best choice, despite the fact that it needs to be calibrated for each molecular species, and its use with mixtures of gases containing two or more species is correspondingly more difficult. If such mixtures are to be investigated, or if the chemicals concerned are corrosive, it is probably most efficient to use a mechanical gauge as a null-point instrument and to measure the pressure by means of a McLeod gauge. 

Spring or Mechanical Gauges (British), which include Petavel Manometer (See above) and High-Pressure Spring Gauge (p C338)... 

Teorell studied the system (6.1-1)—(6.1.5) graphically, by the isocline method, and also numerically. He recovered most of the features observed experimentally. This study was further elaborated by several investigators. Thus, Kobatake and Fujita [5], [6] criticized the original model for invoking the ad hoc equation (6.1.5). These authors assumed instead instantaneous relaxation of the resistance to its stationary value while preserving the overall order of the relevant ordinary differential equation (ODE) system by including consideration of the mechanical inertia of the liquid column in the manometer tube. 

Figure 1. Apparatus for the synthesis of borazine. (A) nitrogen supply from tank (B) oil bubbler (C) valve (D) 2-L, three-necked, round-bottomed flask (E) thermometer (F) distilling column (G) vacuum tubing (H) mercury manometer (I) four traps for the collection of the borazine product (J) valve to mechanical vacuum pump.

The mechanical fore pump is a heavy item which sets up considerable vibration. This vibration disrupts the menisci of mercury manometers and is otherwise undesirable. To minimize the transfer of vibrations to the vacuum line, the fore pump generally is mounted on the floor (rather than on the bench of the vacuum rack) and the connection between the fore pump and the vacuum system is made with heavy-walled vacuum tubing or flexible corrugated metal tubing. 

G. Bourdon Gauges. As an alternative to mercury manometers there is a variety of gauges based on mechanical or electrical pressure transducers. This section presents a description of purely mechanical gauges which still find use in this electronic age.4 The metal Bourdon gauge (Fig. 7.5) is fashioned around a semicircular thin-walled metal tube with mechanical linkage to a pointer. Fused-quartz spiral gauges are also available. In this case, a thin spiral is sensitive to a pressure differential, and the deflection is balanced with air pressure in the surrounding envelope. The air pressure is then measured with a manometer. 

Electronic manometers provide a convenient method of pressure measurement in a tensimeter, and the general arrangement may be very simple (Fig. 9.3). The one problem which must be anticipated is long-term zero pressure drift, which can be encountered with an electronic pressure gauge. Drift is minimized by maintaining a constant temperature on the pressure transducer and by avoiding mechanical vibration at the transducer. 

As mentioned in Section 5.1.B, the transmission of vibration from the mechanical pump to a vacuum rack decreases the accuracy of manometer readings. To minimize this problem, it is generally best to mount the pump on the floor below a vacuum rack and to connect the pump to the system with a short length of heavy-walled rubber tubing, Tygon tubing, or flexible metal tubing. 

Cold traps must be used if mercury is used in your system (such as manometers, diffusion pumps, bubblers, or McLeod gauges) and if your mechanical pump has cast aluminum parts. Mercury will amalgamate with aluminum and destroy a pump. Even if your mechanical pump does not have aluminum parts, the mercury may form a reservoir in the bottom of the mechanical pump, which may cause a noticeable decrease in pumping speed and effectiveness. Aside from a cold trap between the McLeod gauge and the system, place a film of low vapor pressure oil in the McLeod gauge storage bulb. This oil will limit the amount of mercury vapor entering the system that makes its way to the mechanical pump. In addition, an oil layer should be placed on the mercury surface in bubblers and other mercury-filled components. 

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. 

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