Magmeter transmitters can be supplied with either ac or dc power. One new dc design uses significantly more power than the earlier generations and thereby creates a stronger flowtube signal.Īnother new design uses a unique dual excitation scheme that pulses the coils at 7 Hz for zero stability and also at 70 Hz to obtain a stronger signal. In addition to being more accurate and able to measure lower flows, dc meters are less bulky, easier to install, use less energy, and are most cost-effective than ac meters. This provides a stable zero and eliminates zero drift. Therefore, the noise can be continuously eliminated after each cycle. In between pulses, the transmitter sees only the noise signal. When the coils are pulsed on (Figure 2), the transmitter reads both the flow and noise signals. In dc excitation designs, a low frequency (7-30 Hz) dc pulse is used to excite the magnetic coils. The main problem with ac magmeter designs is that noise can vary with process conditions and frequent re-zeroing is required to maintain accuracy. Out-of-phase noise is easily filtered, but in-phase noise requires that the flow be stopped (with the pipe full) and the transmitter output set to zero. In addition to the flow signal, noise voltages can be induced in the electrode loop. The amplitude of the wave is proportional to velocity. As a result, the flow signal (at constant flow) will also look like a sine wave. When ac excitation is used, line voltage is applied to the magnetic coils. The magmeter's coils can be powered by either alternating or direct current (Figure 2). Because the magmeter signal is a weak one, the lead wire should be shielded and twisted if the transmitter is remote. ![]() Intelligent magnetic transmitters with digital outputs allow direct connection to a distributed control system. This signal is typically converted into a standard current (4-20 mA) or frequency output (0-10,000 Hz) at or near the flowtube. The voltage that develops at the electrodes is a millivolt signal. In-line calibration and special compensating designs should be considered for both of these fluids. ![]() These types of fluids can affect the density of the magnetic field in the tube. The K value obtained by water testing might not be valid for non-Newtonian fluids (with velocity-dependent viscosity) or magnetic slurries (those containing magnetic particles). Magmeters can measure flow in both directions, as reversing direction will change the polarity but not the magnitude of the signal. For this reason, flow tubes are usually calibrated at only one velocity. The K value thus obtained is valid for any other conductive liquid and is linear over the entire flowmeter range. Manufacturers determine each magmeter's K factor by water calibration of each flow tube. Compensation is also provided by shaping the magnetic coils such that the magnetic flux will be greatest where the signal weighing factor is lowest, and vice versa. The velocity differences at different points of the flow profile are compensated for by a signal-weighing factor. ![]() Because the magnetic field density and the pipe diameter are fixed values, they can be combined into a calibration factor (K) and the equation reduces to: If a conductive fluid flows through a pipe of diameter (D) through a magnetic field density (B) generated by the coils, the amount of voltage (E) developed across the electrodes-as predicted by Faraday's law-will be proportional to the velocity (V) of the liquid. A pair of magnetic coils is situated as shown in Figure 1, and a pair of electrodes penetrates the pipe and its lining. The newer designs have reduced that requirement a hundredfold to between 0.05 and 0.1.Īn electromagnetic flowmeter consists of a non-magnetic pipe that is lined with an insulating material. Early magmeter designs required a minimum fluidic conductivity of 1-5 microsiemens per centimeter for their operation. Magmeters can detect the flow rate of conductive fluids only. Electromagnetic flow meters (or magmeters) are a type of velocity or volumetric flow meter that operate pursuant to Faraday’s law of electromagnetic induction – which states that a voltage will be induced when a conductor moves through a magnetic field.
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