Water Well Journal
Issue link: https://read.dmtmag.com/i/655200
manufacturer to another, but the basic concept is fairly univer- sal between all brands and types of flowmeters. In the water and wastewater fields, there are two distinct types of output signals produced from flowmeters, whether an electronic type or a propeller meter with an output module. 1. Instantaneous flow—the value of the volume (or rate) of the liquid passing through the flowmeter per unit of time. This can be configured to match various units of flow as well as periods of time. This rate of flow measurement is often used on a pump station's discharge piping to gauge the performance of the entire facility, a single well, or booster pump and to ensure the pump or pumps are operating at or near their design point. The most common unit of instantaneous flow used in the United States is in gallons per minute (gpm). This is a fairly obvious type of flow rate as it simply means the meter examines the rate of gallons passing through the meter for each minute of time. Other common units are cubic feet per minute (cfm) or cubic feet per second (cfs), acre-feet per day (ac-ft/d), liters per second (L/s), cubic meters per second (m 3 /s), or cubic meters per minute (m 3 /m). When used in SCADA or remote readout applications, the instantaneous flow is usually transmitted through an analog signal—typically a varying DC electronic current or voltage output proportional to the rate of flow. The most common analog outputs used are 4-20 mA (milliamps) or 0-20 mA (at 24 VDC) for current and 0-10 or 0-5 VDC for voltage. The analog output can typically be scaled for any de- sired available flow range of the meter. For example, even though a flowmeter's full range may be 0-1000 gpm, the analog range could conceivably be calibrated for 4-20 mA DC or 0-10 VDC output at 0-500 gpm. 2. Totalized flow—created from the meter's instantaneous readout and transmitted through a magnetic drive to a set of gears to create a total flow output, much the same as a car's odometer works from a speedometer. For water, the total flow is typically registered on the totalizer in gallons, cubic feet, acre-feet, or cubic meters. Cubic feet is the one primarily used on residential and commercial water meters for billing purposes, and are often billed in multiple units of 100 cubic feet (which equals 748 gallons). The SCADA output of a totalizer can be from the meter itself or through a counter in the SCADA system. A total- izer output from a flowmeter is often accomplished through a pulse signal sent from a magnetic or optical sen- sor in the meter that provides a pulse for every partial or full rotation of a disc mounted on the meter element. In other words, a pulse output might correspond to one pulse from the sensor for every 10 gallons passing through the meter. The pulse-counting method for flow measurement in- volves converting the kinetic energy from the flowing fluid into rotation of a disc, detecting this rotation, and convert- ing it into electrical energy in the form of digital pulses ENGINEERING continues on page 38 Figure 1. Differential pressure flowmeter Figure 2. Electromagnetic flowmeters Figure 4. Vortex meters Figure 3. Ultrasonic meters WWJ April 2016 37 Twitter @WaterWellJournl Bluff body Alternating vortices Sensor wing Bluff body