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General Primary Element The most typical category of primary element used in flow measurement is the head type, which basically is just some form of restriction in s flow line. Other common primary elements include the area type, such as the rotameter and the cylinder and piston methods; the head area type, such as wires and flumes; and the force type, including target meters and swinging vanes. All these techniques, and others not mentioned, are similar to some extent. Only the basic head type is discussed herein.
(Figure 1) Head Type Primary Element
^UP These elements all work on the theory (Bernoulli's) The Venturi tube produces a relatively large differential with a relatively small head loss. This element is often used where the process contains large amounts of suspended solids or if large head losses are unacceptable. Orifice plates are widely used in industrial applications. They are effectively utilized for "clean" fluid flow measurement and where line pressure losses or pumping costs are not critical. A flow nozzle is in a sense an orifice with a flared approach section. Line pressure loss is between that of an orifice and a Venturi, as is generally the cost. Often flow nozzles are used at the end of a pipe, discharging directly into the air, a tank, etc. Pitot tubes are used when fluid velocity is of prime concern. Very small pressure losses are incurred, and they are relatively inexpensive, but they are very susceptible to plugging with processes containing solids. Secondary Element
^UP Secondary elements in a general sense can be considered as wet meters or as dry meters. Wet meters, using this terminology, would be those elements with which the process fluid itself is in contact with liquid (commonly mercury) in the device. Dry meters use no liquid for contact with the process fluid.
(Figure 2) Wet Meters Where high pressures exist, mercury is often used as the liquid. Figure 3 also shows a typical mercury float-type manometer. Here the position of a float on the surface of the mercury defines the level of the mercury, which in turn defines the pressure required to give this level. There are many other related "wet meter" techniques to use in pressure measurements, such as the inverted bell meter, and the ring balance meter. Dry Meters
(Figure 3) Pressure Transmitters Figure 3 shows a typical pneumatic transmitter. Here, the differential pressure to be measured is applied across a pair of metal diaphragms welded to opposite sides of a capsule; the space between the diaphragms and core member is filled with liquid. The force developed on the diaphragm by differential pressure is brought out of the transmitters by a rigid rod passing through a metal seal diaphragm. This force is opposed by a balancing force developed by pneumatic bellows force is sensed by a pneumatic nozzle-baffle. A simple pneumatic servomechanism responsive to nozzle pressure re-established the balance. As a result, pneumatic pressure is maintained exactly proportional to differential pressure and is used as out put signal; a more or less standardized signals is 3 to 15 pounds per square inch.
(Figure 4) An electronic-type transmitter is shown in Figure 4. This particular type utilizes a two-wire capacitance technique. Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a sensing diaphragm in the center of the cell. The sensing diaphragm is a stretched spring element that deflects in response to differential pressure across it. The displacement of the sensing diaphragm is proportional to the differential pressure. The position of the sensing diaphragm and the capacitor plates is converted electronically to a 4-20 mA dc or 10-50 mA dc signal; these signals are standard in industry, with primary emphasis on 4-20 mA dc. The mechanical element techniques most generally used to convert applied pressures into displacement are diaphragms, bellows, Bourdon tubes, and straight tubes. These devices are depicted in Figure 5. Diaphragm types include flat, corrugated, and capsule designs, and the Bourdon types include circular and twisted tube designs.
(Figure 5) Theoretically, the flat diaphragm type shown in Figure 5, will exhibit the highest natural frequency value, defined by the following relationship:
Where
fn = natural frequency, This equation defines the natural frequency in a conventional
seismic systems processing one degree of freedom. The flat diaphragm type is characterized by having both, for a given diaphragm thickness, maximum stiffness (k) and minimum mass (M). The above analysis pertains to the natural frequency only. The frequency response of a transducer cannot always be estimated easily in relation to the preceding paragraph. Proper consideration must be given to the total volumetric displacement, dead volume, and constrictive orifices in the pressure port cavity. The amount of damping present also influences the frequency response. It should be kept in mind that in many types of transducers, the electrical elements contribute to the overall values of the spring constant and mass, which determine the natural frequency. If a constant range is assumed, the type of force summing member is generally dictated by the force and displacement necessary to actuate the transduction elements. The electrical principles applied to the measurement of pressure displacement are many employ one of the following:
No in-depth discussion of the above principles is presented herein as that would nearly require a text-book in itself. Each principle has certain advantages and disadvantages, however, but not necessarily with an equal mixture or ratio.
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