Area meter rotameters

In orifice and Venturi meters, tite area of constriction is constant, and tite pressure drop increases with the increase in flow rates. In area the meter, the pressure drop is constant, but the area of constriction increases witii increase in flow rate. The rotameter consists of a gradually tapered glass tube moimted vertically in a frame with the large end up. The fluid flows upward and with increase in flow, the float moves upward. 

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For a given flow rate, the equilibrium position of the float depends upon three forces (1) the weight of the float, (2) the bouncy force of the fluid on the float, and (3) the drag force on the float. 

If tile volume of the float is replaced by nif/pf, Equation 2.27 can be written as 

The right-hand side of the equation means Fd is constant for any flow rate. A constant value of fj, is mainfained by changing the flow area with volumetric flow rate (i.e., keeping constant velocity). 

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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. 

RP16.4 Nomenclature and Terminology for Extension Type Variable Area Meters (Rotameters). 

Variable-Area Meters Variable-area meters, which are also called rotameters, offer popular and inexpensive flow measurement devices. These meters employ a float inside a tube that has an internal cross-sectional area that increases with distance upward in the flow path through the tube. As the flow rate increases, the float rises in the tube to provide a larger area for the flowing fluid to pass. 

Other sensors which are described in Volume 1 (Sections 6.3.7-6.3.9) are the variable area meter, the notch or weir, the hot wire anemometer, the electromagnetic flowmeter and the positive displacement meter. Some of these flowmeters are relatively less suitable for producing signals which can be transmitted to the control room for display (e.g. weir, rotameter) and others are used in more specialist or limited applications (e.g. magnetic flowmeter, hot wire anemometer). The major characteristics of different types of flow sensor are summarised in Table 6.1. Brief descriptions follow of the principles underlying the more important types of flowmeter not described in Volume 1. In many instances such flow sensors are taking the place of those more traditional meters which rely upon pressure drop measurement. This is for reasons of versatility, energy conservation and convenience. 

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. 

Full-bore meters include variable-head meters such as venturi and orifice meters and variable-area meters such as rotameters. These will be described in some detail. Briefer descriptions are given of other full-bore measuring devices V-element, magnetic, vortex shedding, turbine and positive-displacement meters, ultrasonic meters, and mass flow devices such as Coriolis and thermal flowmeters. 

A typical meter of this kind, which is commonly known as a rotameter (Figure 6.21). consists of a tapered tube with the smallest diameter at the bottom. The tube contains a freely moving float which rests on a stop at the base of the tube. When the fluid is flowing the float rises until its weight is balanced by the upthrust of the fluid, its position then indicating the rate of flow. The pressure difference across the float is equal to its weight divided by its maximum cross-sectional area in a horizontal plane. The area for flow is the annulus formed between the float and the wall of the tube. 

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. 

Rotameter A rotameter consists of a vertical tube with a tapered bore in which a float changes position with the flow rate through the tube. For a given flow rate, the float remains stationary because the vertical forces of differential pressure, gravity, viscosity, and buoyancy are balanced. The float position is the output of the meter and can be made essentially linear with flow rate by making the tube area vary linearly with the vertical distance. 

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

A specimen of 100 mm square is fastened horizontally and then ignited from below by a Maker burner with a flame burning a controlled amount of gas metered through a rotameter. Temperature rise is measured above the ignited point at the surface of the specimen by a copper calorimeter with 3 sensors. The thermal protective performance rating is calculated from the amount of energy transmitted per unit area (F) determined by preliminary calibration and from the exposure time (time to the thermal end-point, t) ... 

Rotameter A registered name for a type of variable area flow meter used to measure the rate of flow of fluids. It consists of a tapered tube and contains a float. The elevation of the float in the tube gives a measure of the rate of flow and is read from a calibrated scale on the tube. 

Flow transmitters. Flow measurements are made in high-pressure lines by sensing the pressure drop across a calibrated orifice or venturi, or by the transmitting variable-area type of flowmeter. The latter meter resembles a Rotameter with float position transmitted electrically. It has the advantage of being an in-line element but is not readily applicable to large flows. 

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