Float meter

Instruments which measure the rate of flow (velocity) of liquids and gases are called flowmeters they may be broadly defined as being mechanical or electronic in operation. Examples of mechanical flowmeters are orifice plate and float meters (Fig. 5.8), venturi meters, and pitot tube meters, all of which depend on a constriction being introduced into the flow stream in order to produce a difference in pressure across the constriction. The rate of flow can then be obtained from the difference in pressure. 

Figure 2.33 On-line coupled simultaneous DTA/DSC-EGD apparatus. 1, Gas cylinder 2, reducer valve 3, gas drying 4, governor valve 5, buffer 6, float meter 7, preheater 8, controlled temperature unit 9, bridge 10, fourway valve 11, gas conduit 12, crucible 13, furnace 14, infrared lamp 15, controlled temperature programmer 16, power compensation unit 17, DTA amplifier unit 18, three-way valve 19, soap film flow meter 20, rrxrorder for DTA/DSC 21, recorder for EGD 22, selective switch 23, tail gas...

Figure 2.37 Schematic diagram of the DTA-EGD-GC online coupled simultaneous apparatus (reproduced by permission of Tian Ping Instrumental Factory, China). 1, Gas cylinder for DTA system 2, carrier gas (or GC system 3, reducer valve 4, gas drying 5, governor valve 6, float meter 7, six-way valve 8, gasifier, 9, 5A molecular sieve 10, 401 organic support 11, soap film flow meter 12, sampling valve 13, infrared lamp 14, recorder for DTA/T 15, recorder for EGD-GC 16, tail gas...

Float metering The level of the float in the flow helps to identify the volume flow. Preferably used for on-site measurements. Suitable for pure liquids only. 

Variable-Area Flow Meters. In variable-head flow meters, the pressure differential varies with flow rate across a constant restriction. In variable-area meters, the differential is maintained constant and the restriction area allowed to change in proportion to the flow rate. A variable-area meter is thus essentially a form of variable orifice. In its most common form, a variable-area meter consists of a tapered tube mounted vertically and containing a float that is free to move in the tube. When flow is introduced into the small diameter bottom end, the float rises to a point of dynamic equiHbrium at which the pressure differential across the float balances the weight of the float less its buoyancy. The shape and weight of the float, the relative diameters of tube and float, and the variation of the tube diameter with elevation all determine the performance characteristics of the meter for a specific set of fluid conditions. A ball float in a conical constant-taper glass tube is the most common design it is widely used in the measurement of low flow rates at essentially constant viscosity. The flow rate is normally deterrnined visually by float position relative to an etched scale on the side of the tube. Such a meter is simple and inexpensive but, with care in manufacture and caHbration, can provide rea dings accurate to within several percent of full-scale flow for either Hquid or gas. 

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 since 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 makiug the tube areavaiy hn-early with the vertical distance. 

A simple example of an area meter is a gate valve of the rising-stem type provided with static-pressure taps before and after the gate and a means for measuring the stem position. In most common types of area meters, the variation of the opening is automatically brought about by the motion of a weighted piston or float supported by the fluid. Two different cyhnder- and piston-type area meters are described in the ASME Research Committee on Fluid Meters Report, op. cit., pp. 82-83. 

A measure of self-compensation, with respect to weight rate of flow, for fluid-density changes can be introduced through the use of a float with a density twice that of the fluid being metered, in which case an increase of 10 percent in p will produce a decrease of only 0.5 percent in w for the same reading. The extent of immunity to changes in fluid viscosity depends upon the shape of the float. 

Head meters with density compensation. Head meters such as orifices, venturis, or nozzles can be used with one of a variety of densitometers [e.g., based on (a) buoyant force on a float, (b) hydrauhc couphug, (c) voltage output from a piezoelectric ciystal, or (d) radiation absolution]. The signal from the head meter, which is proportional to pV" (where p = fluid density aud V = fluid velocity), is multiphed by p given by the densitometer. The square root of the produc t is proportional to the mass flow rate. 

Be sure that meter has proper tube and float installed. 

The volume rate of air flow, F, in cubic centi-1 meters per minute required to balance the settling velocity of the largest particle which it is desired to float is given by the following equation ... 

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. 

This meter may thus be considered as an orifice meter with a variable aperture, and the formulae already derived are therefore applicable with only minor changes. Both in the orifice-type meter and in the rotameter the pressure drop arises from the conversion of pressure energy to kinetic energy and from frictional losses which are accounted for in the coefficient of discharge. The pressure difference over the float —AP. is given by ... 

The constant coefficient for float C arises from turbulence promotion, and for this reason the coefficient is also substantially independent of the fluid viscosity. The meter can be made relatively insensitive to changes in the density of the fluid by selection of the density of the float, pf. Thus the flowrate for a given meter will be independent of p when dG/dp = 0. 

The range of a meter can be increased by the use of floats of different densities, a given float covering a flowrate range of about 10 1. For high pressure work the glass tube is replaced by a metal tube. When a metal tube is used or when the liquid is very dark or dirty an external indicator is required. 

FIGURE 7.6. DO measurement in a manhole of a sewer. A bag (1) contains a waterproof box (2) with electrical equipment, data logger and batteries for a DO-meter (4). The float (5) is anchored by a steel rod (3). 

Most fires on floating roof tanks are small rim fires caused by vapors leaking through the seals. The source of ignition is normally lightning strikes. With proper seal maintenance and inspection, coupled with adequate shunt straps across the seal at every meter or so will reduce the probability of a tank fire. 

As we have seen, toxic materials are commonly found as aerosols, that is, floating minute particles within the breathing air, either as gases and vapors or liquids and solids. As we have seen also, gases and vapors are measured in volume units, namely, parts per million (ppm), while liquids and solids are measured in weight units, namely, milligrams per cubic meter. 

The mass flow rates of air and fuel were measured with a set of float-type flow meters to a precision of 1%, and the overall equivalence ratio was determined with an uncertainty of less than 2%. The swirl number of the second-flow arrangement was defined at the exit of the swirler as... 

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. 

---