Michael Faraday, with his Law of Electromagnetic Induction, could scarcely have envisioned the extent to which magnetic flowmeters (magmeters) are used today on both water and wastewater applications.
One excellent example is current practices reported at Detroit’s water and sewerage department. There, 8- to 24-in. magmeters are steadily replacing other types of meters on a vast network of wholesale distribution lines for drinking water.
These meters are the basis for wholesale billings to communities and need to retain high accuracy. Equally impressive is the fact that each magmeter sensor can remain online in an underground vault while a trained technician uses a portable electronic system to quickly verify and certify its factory-calibrated accuracy. Also notable is the remote transmission system that sends magmeter readings back to several of the city’s departments and customers.
For many reasons, applications for magmeters have mushroomed in a number of key markets, including those in water and wastewater. Such growth has naturally attracted many suppliers. From a handful 50 years ago, it is estimated that today there is some 40 different manufacturers, plus another 20 that private label selected models.
This brief overview includes basic facts that should help in the understanding of magmeter technology with its specialized terminology and products.
What magmeters offer
A magmeter measures volumetric flow rate of conductive liquids or slurries. In summary, the following are advantages magmeters offer prospective user versus other methods of measuring flow rate in water and wastewater applications.
• One distinct benefit is the flow path through the meter’s inline sensor, as it has no obstructions to flow. It acts like a short section of pipe and causes virtually no pressure loss. For raw sewage and similar slurry-like fluids, this is an advantage.
• No moving parts, therefore no loss of accuracy due to bearing wear, for example.
• A variety of models offer a wide range of sizes from 1?25-in. to 10 ft in diameter, allowing the meter to handle very low to very high flow rates.
• Measurement is unaffected by changes in viscosity, density or temperature of the measured liquid.
• Available with a wide choice of construction materials for wetted parts. A magmeter can measure highly corrosive, abrasive and/or solids-bearing liquids such as raw sewage.
• Requires only a short run if straight pipe upstream—e.g., three pipe diameters, compared to 10–20 for turbine meters and at least eight for orifice meters.
• A much greater turndown—i.e., the ratio of maximum to minimum operating capacity while maintaining guaranteed accuracy. Value can be as high as 75:1 and commonly exceeds 50:1 as compared to 3:1 to 20:1 using other methods.
• A linear output signal, which requires no characterization and is beneficial in control applications.
• Read-outs of measured flow rates can be digitally indicated as well as chart recorded and totaled at or near the sensor. Such readings can also be transmitted to remote locations by various means, including hard-wire, fieldbus, serial and wireless communication.
• As installed in the pipeline, the magmeters can measure flow in either direction. Such bi-directional capability can be set up to use such readings to total only the net flow in one direction.
•As a proprietary option, they can have factory-calibrated accuracy of entire system verified and recertified while the sensor remains installed and operating online.
Two component system
All magmeters have two main components that make up the flowmetering system: the primary (or sensor), which installs in the flow line, and the transmitter (or signal converter). The latter can be a separate electronic instrument located up to several hundred feet from the sensor and connected to it by shielded cable. Alternately, the transmitter functions can be integrally contained in the sensor’s housing.
There are many different sensor models that can be paired with one or several types of transmitters. Once married, the combination forms a unique system with unique performance characteristics.
Suitably equipped with magnet coils that are energized by a power source, the sensor uses Faraday’s Law to develop a small voltage directly related to the flow rate of fluid passing through it. The transmitter’s basic job is to amplify and condition the sensor’s small voltage signal and convert it into desired engineering units to represent flow rate.
Today, the transmitter often is a highly-capable, microprocessor-based instrument that serves a number of diagnostic, data processing and communications functions.
Key magmeter advances
Over the past 50 years, to meet specific application needs, there have been a number of design innovations in both magmeter sensors and transmitters. The significant innovations involving water-related applications are briefly summarized in the following overview.
Buriable Magmeter Sensor: Where process piping runs underground and the magmeter is not installed in a protective vault or pit, flow sensors can be ordered to be buriable. This means that they can be installed merely by excavating to the underground pipeline and fitting the sensor into the line. The cable connects to the aboveground transmitter.
Electrodeless Sensor: This refers to a magmeter with non-wetted electrodes (i.e., electrodes embedded in or located behind the liner). Introduced in the 1980s, one design utilizes a capacitance measuring technique to sense the amplitude of the voltage induced by the flowing fluid using large plate electrodes. It also made the meter immune to effects of coatings when measuring sludge and slurries. As a bonus, it extended the applicability of the meter to more abrasive fluids.
Inline Verification by Portable Instrument: A portable electronic instrument that a trained technician can use in the field to verify and recertify the factory-calibrated accuracy of a magmeter while it is installed online.
Insertion Type Magmeter: Generally classified as any device that infers an overall flow rate based on single or multiple measurement of flow velocity, taken with a probe element at one or more strategic locations inside the pipe.
Liners for Sensors: As magmeters became the flowmeter of choice, flowtube liners were made available in a wide assortment of materials. The correct liner must be selected based on the temperature and pressure of the flowing fluid, as well as abrasiveness and corrosiveness. If the meter is to be cleaned in place as with sanitary applications, the liner must also be compatible with the cleaning fluids.
Low Flow Magmeters: Sizes of magmeter sensors below about 1?2 in. have been classified as low flow. In one flangeless wafer style, sensors come with five fitting sizes from 1?10 to 1?2 in. Such types can provide accurate measurement flows as low as 0.003 gpm. Lower flow rates can be obtained with reduced accuracy. Such low-flow meters can handle addition of fluorine in the form of hydrofluorsilicic acid to drinking water.
Noise, Magmeters for Handling: Using a magmeter to measure flow rates of certain fluids, particularly slurries, can result in a flow signal that contains error-producing noise (i.e., a spurious signal that falsely alters the true flow signal). A good example is the effect of particles in a slurry of sludge that impinge on the electrode. The electrodeless magmeter is one method of eliminating such a problem. Pulsed DC systems can also minimize the effect of noise.
Partially-filled Pipe Magmeters: The first magmeter for measuring the flow of liquids in a partially-filled pipe or large conduit was introduced in 1992. Such designs measure both flow velocity and level of liquid in the pipe to calculate flow rate.
Transmitter Classifications: Transmitters have been classified into three design categories: analog, digital and fieldbus. As covered earlier, the basic job of any transmitter is to amplify and convert the sensor’s low-level signal into a usable form to represent flow rate. It is also known as a signal converter—a name widely used today, even for advanced designs.
Two-wire Transmitter: The input power and output signals share the same two wires that connect the sensor and transmitter. Before this technology was invented, magmeters with remote transmitters required separate pairs of wires for power supply and sensor output signal—possibly in separate conduits. The design offers reduced installation cost and low power consumption.
Wafer-style Sensor: This style has a flowtube with no flanges, rather it bolts between two flanges in the adjacent piping. Overall, it lowers the installation cost and has become quite popular for many water and wastewater applications.