High performance two-wire magnetic flowmeter
09 July 2010
The Yokogawa ADMAG AXR is a two-wire magnetic flowmeter that uses dual-frequency excitation technology. To date, most magnetic flowmeters have been of the four-wire type, where high accuracy and performance are achieved by using a dedicated power-supply cable that is separate from the signal cable used for the measurement current output.
To date, most magnetic flowmeters have been of the four-wire type, where high accuracy and performance are achieved by using a dedicated power-supply cable that is separate from the signal cable used for the measurement current output.
The two-wire type, on the other hand, uses just one pair of wires for current output and power supply, offering significantly lower installation costs by requiring neither a dedicated power supply cable nor a power-supply unit.
Despite these benefits, conventional two-wire magnetic flowmeters do have limitations in terms of measurement accuracy and stability, and as a result have not been well accepted in the process automation sector.
Now, however, Yokogawa has adapted its dual-frequency excitation technology to the two-wire configuration, resulting in minimal electrical and flow noise in the output signals. The performance in upper flow ranges is comparable to that of conventional magnetic flowmeters, thereby achieving the high levels of measurement accuracy and stability needed for use in control loops.
The Yokogawa dual-frequency excitation method uses high-frequency switching of the magnetic field combined with high-speed digital processing to execute the complex algorithms required to enhance stability and performance, while at the same time minimising power consumption.
A further performance enhancement is provided by a new coil configuration. Because a two-wire magnetic flowmeter has a limited supply power available, the excitation current supplied to the coils is normally too weak to generate flow signals that can be differentiated from electrical noise. To resolve this problem, Yokogawa has adopted a new high-density coil construction method that enables a stronger magnetic field to be generated with a low current.
Yokogawa has also found ways to minimise the noise level by smoothing the liner surface of the measuring tube and developing an electrode system that is immune to noise pickup. As a result, the Admag AXR makes it easy to differentiate signals from electrical noise, thereby achieving a measurement accuracy comparable to that of conventional magnetic flowmeters.
User-friendliness in the new Admag AXR has been enhanced through the addition of a full dot-matrix LCD display capable of displaying messages in six different languages, enabling customisation of the device – by pushbuttons inside or magnetic switches through the display window – without opening the cover, the addition of an electrode adhesion diagnosis function, and a design change that allows the orientation of the converter to be changed on site.
The elimination of the need for a dedicated power-supply unit and power-supply cable has allowed a sharp reduction in instrumentation cost. The lower power consumption of the two-wire instrument also reduces the total cost of ownership.
The Admag AXR is ideally suited to fluid flow measurement in a variety of industries including chemicals, iron & steel, and food & beverages.
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