Flow meters rotameter

Instrument Society of America 400 Stanwix Street Pittsburgh, Pa. 15222 Standards l ibrary for Measurement and Control, 12th ed., 1994. Instmmentation standards and recommended practices abstracted from those of 19 societies, the U.S. Government, the Canadian Standards Association, and the British Standards Institute. Covers control instmments, including rotameters, aimunciators, transducers, thermocouples, flow meters, and pneumatic systems (see... 

The Prosser was calibrated by measuring the air flows using a laminar flow meter (1% accuracy) for the odorous sample and a pitot tube with a micromanometer for the fan-blown air (3). The pitot pressures were converted to air velocities (4) and hence, from the cross sectional area of the tube, to volumetric flow rates. Since flow near the tube wall was slower than the centre, the tube was traversed by the pitot head and the average value calculated. A rotameter was also tried but it induced a back-pressure of 250 N/m2 and, as the manufacturer states that the maximum permissible back-pressure is 60 N/m for calibration to be accurate, its use was not pursued. 

Figure 9. Configuration of the DS-IC system A, clean air input B, mass-flow controller C, permeation device chamber D and H, vents E, needle valve-rotameter F, needle valve G, mass-flow meter I, diffusion scrubber Jy scrubber liquid reservoir K, needle valve-rotameter L, suction pump M, injection valve Ny peristaltic pump O, eluent flow F, downstream chromatographic components and Q, sample loop. (Reproduced from reference 96.

A. Flow Control without Feedback. Plow can be controlled by means of a needle valve if the pressure drop across the valve is constant. The pressure on the upstream side often can be held constant with a single- or two-stage mechanical diaphragm regulator (Section 10.1. B). If the stream of gas does not experience a variable constriction after the needle valve, the above combination provides a simple and convenient means of providing a steady flow. Often an arrangement such as this is used in conjunction with a rotameter or electronic mass flow meter (Fig. 7.14). 

The rotameter, illustrated in Figure 6, is an area flow meter so named because a rotating float is the indicating element. 

The basic construction and theory of operation of rotameters, nutating disks, anemometers, electromagnetic flow meters, and ultrasonic flow equipment are summarized below. 

Both gas flow rates and liquid flow rates can be measured by a wide variety of devices such as bellow meters, Venturi nozzles, nutating disk meters, orifice meters, rotameters, weirs (for liquids), Pitot tubes, and magnetic meters among others. Some devices measure volumetric flow directly as with meters in which the space between rotating paddles incorporates small volumetric displacements of fluid. Other device measure the flows indirectly by measuring the pressure drop caused by an orifice between two different sites in the pipe, or the change in voltage of a heated wire. 

This chapter discusses the unit operations of flow measurements and flow and quality equalizations. Flow meters discussed include rectangular weirs, triangular weirs, trapezoidal weirs, venturi meters, and one of the critical-flow flumes, the Parshall flume. Miscellaneous flow meters including the magnetic flow meter, turbine flow meter, nutating disk meter, and the rotameter are also discussed. These meters are classified as miscellaneous, because they will not be neated analytically but simply described. In addition, liquid level recorders are also briefly discussed. 

The last flow meter that we will address is the rotameter. This meter is relatively inexpensive and its method of measurement is based on the variation of the area through which the liquid flows. The area is varied by means of a float mounted inside the cylinder of the meter. The bore of this cylinder is tapered. With the unit mounted upright, the smaller portion of the bore is at the bottom and the larger is at the top. When there is no flow through the unit, the float is at the bottom. As liquid is admitted to the unit through the bottom, the float is forced upward and, because the bore is tapered in increasing cross section toward the top, the area through which the liquid flows is increased as the flow rate is increased. The calibration in rates of flow is etched directly on the side of the cylinder. Because the method of measurement is based on the variation of the area, this meter is called a variable-area meter. In addition, because the float obstructs the flow of the liquid, the meter is an intrusive meter. 

In addition to a proper choice of collecting material (filter paper), a reliable measurement of flow rate is required. Flow meters are classified into rotameters and integrating flow meters. The latter are further classified into wet-gas meters and dry-gas meters. A rotameter has a specially graduated vertical tube, whose diameter increases in the ascending direction, containing a spinning top-shaped or spherical float. A gas-stream is admitted into the bottom of the tube and the float is held at a vertical position which varies in proportion to the flow rate of gas. 

The carrier gas used was argon or nitrogen. The flow rate was adjusted by a pressure reduction valve and kept constant, as indicated by a rotameter. The carrier gas was then passed through adsorption towers to remove H2O and CO2. The volume of the carrier gas was measured by a gas-flow meter. The reproducibility in the measured gas volume was of the order of 0.1%. 

AREA METERS ROTAMETERS. In the orifice, nozzle, or venturi, the variation of flow rate through a constant area generates a variable pressure drop, which is related to the flow rate. Another class of meters, called area meters, consists of devices in which the pressure drop is constant, or nearly so, and the area through which the fluid flows varies with flow rate. The area is related, through proper calibration, to the flow rate. 

The measurement of the air flow rate is the most common method in practice, by using rotameters and cross-sectional or bubble flow meters. 

Figure 13-6 Apparatus for RESS A = SCF reservoir PV = high pressure volumetric pump HI, H2 = heat exchangers V, VI, V2, V3 = on-off valves S = saturation vessel EN = expansion nozzle E = expansion vessel F = flow meter G = rotameter PI = pressure indicator TI = thermometer.

However, also this calorimetric principle is not without weaknesses. Equ.(4-226) shows that the exact knowledge of the coolant mass flow is pivotal to the overall accuracy of the measurement. Today s commercially available calorimeters are equipped with pumps for the coolant like those known in type from thermostats. They are not able to provide a frilly constant mass flow rate, especially not if the experiment lasts several hours. Furthermore, the preinstalled mass flow meters usually are rotameters of quite a simple type. 

The types of instruments used in an industrial setting are often different from those used in the laboratory. The most common high precision laboratory instrument is the mass flow controller and rotameters are frequent for applications for both gases and liquids but are much less accurate. In industry, obstruction flow meters, Coriolis meters, and vortex shedders are more standard. 

In continuous reactors, the concentration varies with space time, which is a variable equivalent to time but measured as a function of reactor volume and inlet flow of the fluid (or inlet velocity of the fluid). The flow is measured experimentally by using a rotameter or mass flow meters (MFM) through conductor signals. They are concrete measurements. We can define the new variables as follows ... 

---