Where flow measurement is required but the media is harsh (such as in sewage, sludge, or slurry) the electromagnetic ?mag? meter is preferred because of its non-intrusive design. In these environments, where suspended solids plug up conventional flowmeters, the magmeter offers reliability, low maintenance, and high accuracy.
Essentially a flow tube that measures an induced voltage created by liquid passing through an electromagnetic field (based on Faraday?s Law), the magmeter offers no flow restrictions so nothing plugs it up. With no moving parts, the magmeter is very low maintenance, and measures flow more accurately (0.5 percent) than ultrasonic or Venturi meters (1 to 2 percent). The magmeter also has a larger range of flow operation than conventional meters, as well as bi-directional measuring ability.
Despite the magmeter?s rugged utility, proper planning and installation are essential to its overall performance. Here are 10 simple tips to help operators prepare, install, and use a magmeter.
1) Make sure the flow media is above the minimum conductivity level, so electromotive force can drive the induced voltage and current to the microprocessor-based signal converter. The flow media must have a minimum conductivity of 5 microseimens per centimeter.
2) Make sure the meter tube is full at all times, or the magmeter?s readings may be inaccurate or erratic. Most magmeters assume that the flow tube is full and cannot sense the difference between a full or partially-full pipe. Methods to ensure full liquid flow include:
- positive head pressure from pumps
- uphill grades
- vertical installation
- positioning the meter in the lowest point of a plumbing trap
3) Maintain straight piping before and after the meter for accurate measurement. Liquids traveling through obstructions and rapid changes in fluid conductivity (such as that caused by chemical injecting and by blending) may create process noise, leading to inaccuracy and erratic behavior. Chemical injecting should be done early enough so the flow media and chemicals are thoroughly mixed before entering the measurement area.
4) Make sure that provisions have been made for proper grounding and electrical connections, or the magmeter may act erratically or not register at all. The magmeter relies on a potential to earth ground to operate, and it is imperative that the magmeter be well grounded, especially on pipelines that have cathodic protection. Stainless steel ground rings are typically used to achieve this. Taking a few minutes to read the installation manual and to properly ground the meter will save hours of troubleshooting.
5) Know the metering application helps in choosing the proper converter orientation. When the signal converter is integrally mounted on the flow sensor, the assembly is NEMA (National Electrical Manufacturer?s Association) 4X rated. If the converter is remotely mounted, the sensor end is rated NEMA 6P, which is intended for limited submergence. If installation must exceed the NEMA 6P rating, then waterproof remote cabling should be used.
6) If using a remote configuration, be precise on the remote cable?s length. Many manufacturers use a unique cable that contains delicate inner conductors. Extending this cable with wire nuts or electrical tape is unacceptable. Use a special splice kit, offered by most manufacturers, to extend the cable length with no risk of lost accuracy or performance life. Also available are retrofit kits that convert a directly mounted signal converter to a remote mount configuration.
7) Avoid heavy vibration, because the signal converter?s electronics may be damaged, causing meter failure. Gas or diesel-driven pump motors and liquid traveling through obstructions are two things that can create resonance vibration. To minimize vibration, create concrete or structural steel supports. Flex joints and special vibration suppressing fittings are also useful.
8) Avoid wide temperature variations. The magmeter?s sensor is robust, but its electronics are more sensitive. The magmeter works well from 14 to 140 degrees Fahrenheit. But even within this wide temperature range it is a good idea to shade the signal converter with an awning or covering. If outdoor conditions are extreme, remotely mount the signal converter in a better environment such as a NEMA enclosure, pump house, or indoor facility. A signal converter can be mounted up to 300 feet away from the flow sensor, which helps when dealing with pit applications and confined-space laws.
9) Avoid welding or extreme heat near the magmeter, so the liner and outer coatings do not get damaged. Weld before installing the flow sensor, or use compression and transition couplings to avoid welding altogether.
10) DC powered signal converters can be used when AC power is not available. Magmeters have successfully been used with solar powered systems and with uninterruptible power supplies (UPS) to avoid unmeasured water flow.
About the Author:
David Vidrio is the Technical Support Manager at Water Specialties Corporation in Porterville, Calif. Water Specialties manufactures and markets flow measurement technologies.
