Manometer fluid

Due to the dirty environment, the ideal measurements, which should be by fluid manometer, cannot be used. 

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. 

The sensitivity of the well manometer can be adjusted by changing the angle or and can be some 30 times that of the U-tube. This type is also often called a micromanometer due to its ability to measure very small pressure differences. Several other types of micromanometers and fluid manometers are also available. 

A very obvious way to change die measurement range and sensitivity of a fluid manometer is by using fluids of different densities. There are only a few suitable liquids with specific gravit> between that of water and mercury. Ethylene bromide has a specific gravity of 2.2 and acetylene tetrabromide 3.0., but they are corrosive. 

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. 

Electrical manometers have developed during the last 30 years. Modern electrical manometers are well suited for ventilation applications, both in the laboratory and in the field. The advantage of this type of instrument is fhat they are sensitive enough to measure small pressure differences with electrical output, enabling monitoring. A convenient feature, especially in the field is that the instrument is hand-held and there is no need for leveling on a bench, as for fluid manometers. The conversion of the pressure difference into an electrical signal can be based on several different phenomena. 

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 three-fluid manometer illustrated in Fig. 4-P11 is used to measure a very small pressure difference (P1 — P2). The cross-sectional area of each of the reservoirs is A, and that of the manometer legs is a. The three fluids have densities pa, ph, and pc, and the difference in elevation of the interfaces in the reservoir is x. Derive the equation that relates the manometer reading h to the pressure difference P1 — P2. How would the relation be simplified if A al... 

Since all organic liquids, and also mercury, dissolve gases and especially the vapours of organic compounds, it is good practice to keep fluid manometers under vacuum and isolated from the system when they are not in use, so that they can discharge any dissolved material into an evacuated space while resting . 

Diaphragm pressure transmitters are most commonly used when an electrical signal is desired for data acquisition. These devices are also durable and reliable but require additional calibration and maintenance compared to a fluid manometer or spring gauge. 

It may seem that we went to a lot of trouble for such a simple problem. This is true working engineers use shorter calculation methods than that shown here. However, with complicated systems, such as two-fluid manometers, the shortcuts are confusing, and the method shown above is always reliable. 

Example 2.15. A two-fluid manometer is often used to make it unnecessary to read small differences in elevation. The one shown in Fig. 2.15 is measuring the pressure difference between two tanks. What is that pressure difference ... 

This reading corresponds to a pressure difference of 0.1 ft of water. The actual reading of this two-fluid manometer is 1ft. If [we assume that we can read with an accuracy of 0.005 ft, then a simple water manometer will have an uncertainty of 5 percent for this difference the twc -fluid manometer shown has an uncertainty of 0.5 percent. ... 

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 inverted differential U tube, in which the manometric fluid may be a gas or a hght liquid, can be used to measure hquid pressure differentials, especially for the flow of slurries where solids tend to settle out. Additional details on the use of this manometer can be obtained from Doolittle (op. cit., p. 18). 

Change of manometric fluid. In open manometers, choose a fluid of lower density In differential manometers, choose a fluid such that the difference between its density and that of the fluid being measured is as small as possible. 

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.)... 

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... 

The most common manometer fluids arc water, alcohol, and mercury. The density of water and alcohol arc quite close to each other, whereas the density of mercury is much higher. Many factors have to be considered when selecting a fluid for a manometer, including... 

By connecting manometer hoses to both output pressures given by the tube, the pressure difference Ap can be measured directly. The barometric pressure and the fluid temperature are required for the determination of the fluid density. The Pitot-static tube is not a suitable instrument for measuring low velocities. It can be applied in cases where the flow velocity is high... 

Manometer An instrument that measures pressure by fluid displacement in a U-shaped tube. 

Oil slant gauge An inclined manometer tube using oil as the measuring fluid to record the pressure. 

U-Tube manometer An instrument used for measuring pressure differences in a fluid or a gas by means of a LJ-shaped tube containing a fluid such as mercury or oil. 

Where fluid pressure is carried by a capillary tube, such as with the thermostatic expansion valve or pressure switches, the tube should be installed with due attention to the risk of it chafing against metal edges and wearing through. Tubes to manometers are usually in plastic, but may be copper. These must be carefully tested for leaks, as they are transmitting very low pressures. 

The pressure of a confined gas can be measured by a manometer of the type shown in Figure 5.2. Here again the fluid used is mercury. If the level in the inner tube (A) is lower than that in the outer tube (B), the pressure of the gas is greater than that of the atmosphere. If the reverse is true, the gas pressure is less 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 ... 

Example 4-1 Manometer. The pressure difference between two points in a fluid (flowing or static) can be measured by using a manometer. The manometer contains an incompressible liquid (density pm) that is immiscible with the fluid flowing in the pipe (density pf). The legs of the manometer are connected to taps on the pipe where the pressure difference is desired (see Fig. 4-2). By applying Eq. (4-7) to any two points within either one of the fluids within the manometer, we see that... 

Figure 4-2 Manometer attached to pressure taps on a pipe carrying a flowing fluid.

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