![]() Because of the intermittent use patterns of residential users, this corresponds to recalibrating 5/8 x 3/4 in residential water meters after they have metered 5 million gallons. The AWWA requires that residential water meters be re-calibrated every 10 years. Higher viscosity can produce higher accuracy, while lower viscosity and wear over time will reduce accuracy. The accuracy of these meters is required to be ☒% of actual flow rate. These meters must meet American Water Works Association (AWWA) standards for accuracy. Hot water units are available up to 250☏. ![]() Cold water service units are temperature-limited to 120☏. They are suited for 150-psig operating pressures with overpressure to a maximum of 300 psig. Nutating disc meters are available in 5/8-in to 2-in sizes. Meters with rubber discs give better accuracy than metal discs due to the better sealing they provide. Nutating disc meters are designed for water service and the materials of which they are made must be checked for compatibility with other fluids. The wetted parts such as the disc and spindle are usually bronze, rubber, aluminum, neoprene, Buna-N, or a fluoroelastomer such as FKM. Because the flow meter entraps a fixed quantity of fluid each time the spindle is rotated, the rate of flow is proportional to the rotational velocity of the spindle (Figure 3-1A).īecause it must be nonmagnetic, the meter housing is usually made of bronze but can be made from plastic for corrosion resistance or cost savings. This magnet is coupled to a mechanical register or a pulse transmitter. As water flows through the metering chamber, it causes a disc to wobble (nutate), turning a spindle, which rotates a magnet. They are used as residential water meters around the world. Nutating disc meters are the most common PD meters. If you want to learn more about positive displacement flow meter, read this article. PD meters do not require a power supply for their operation and do not require straight upstream and downstream pipe runs for their installation. Accuracy is not affected by pulsating flow unless it entrains air or gas in the fluid. Positive displacement meters provide high accuracy (☐.1% of actual flow rate in some cases) and good repeatability (as high as 0.05% of reading). Other types of rotary element flow meters include the propeller (impeller), shunt, and paddlewheel designs.įigure 3-1: Click on figure to enlarge. Turbine rotation can be detected by solid state devices or by mechanical sensors. The diameter of the rotor is very close to the inside diameter of the metering chamber, and its speed of rotation is proportional to the volumetric flow rate. The turbine flow meter consists of a multi-bladed rotor mounted at right angles to the flow, suspended in the fluid stream on a free-running bearing. While PD meters are operated by the kinetic energy of the flowing fluid, metering pumps (described only briefly in this article) determine the flow rate while also adding kinetic energy to the fluid. The frequency of the resulting pulse train is a measure of flow rate, while the total number of pulses gives the size of the batch. Each PD design uses a different means of isolating and counting these volumes. By counting the number of passed isolated volumes, a flow measurement is obtained. Discussed in this chapter are various types of mechanical flow meters that measure flow using an arrangement of moving parts, either by passing isolated, known volumes of a fluid through a series of gears or chambers (positive displacement, or PD) or by means of a spinning turbine or rotor.Īll positive displacement flow meters operate by isolating and counting known volumes of a fluid (gas or liquid) while feeding it through the meter.
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