According to Faraday's law, when a conductor passes through an electromagnetic field, an electromotive force is induced in the conductor that is proportional to the velocity of the conductor. In the actual application of this law in the measurement of the flow of water or wastewater, the salts contained in the stream flow serve as the conductor. The meter is inserted into the pipe containing the flow just as any coupling would be inserted. This meter contains a coil of wire placed around and outside it.
The flowing liquid containing the salts induces the electromotive force in the coil. The induced electromotive force is then sensed by electrodes placed on both sides of the pipe producing a signal that is proportional to the rate of flow. This signal is then sent to a readout that can be calibrated directly in rates of flow. The meter measures the rate of flow by producing a magnetic field, so it is called a magnetic flow meter. Magnetic flow meters are nonintrusive, because they do not have any element that obstructs the flow, except for the small head loss as a result of the coupling.
Another flow meter is the nutating disk meter. This is widely used to measure the amount of water used in domestic as well as commercial consumption. It has only one moving element and is relatively inexpensive but accurate. This element is a disk. As the water enters the meter, the disk nutates (wobbles). A complete cycle of nutation corresponds to a volume of flow that passes through the disk. Thus, so much of this cycle corresponds to so much volume of flow which can be directly calibrated into a volume readout. A cycle of nutation corresponds to a definite volume of flow, so this flow meter is called a volume flow meter. Nutating disk meters are intrusive meters, because they obstruct the flow of the liquid.
Another type of flow meter is the turbine flow meter. This meter consists of a wheel with a set of curved blades (turbine blades) mounted inside a duct. The curved blades cause the wheel to rotate as liquid passes through them. The rate at which the wheel rotates is proportional to the rate of flow of the liquid. This rate of rotation is measured magnetically using a blade passing under a magnetic pickup mounted on the outside of the meter. The correlation between the pickup and the liquid rate of flow is calibrated into a readout. Turbine flow meters are also intrusive flow meters; however, because rotation is facilitated by the curved blades, the head loss through the unit is small, despite its being intrusive.
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.
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