Normal tap water has an electrical conductivity of several hundred µS/cm. That of distilled water is less than 5 µS/cm. However, many scientific and industrial applications such as the manufacture of drugs and injectable serums, medical research, analytical chemistry, or the fabrication of computer chips and solar cells, require pure water having electrical conductivities much less than that. For example, water used for generating steam at power plants has electrical conductivities of 0,5 µS/cm or less.
The advantages of Admag AXF
Most manufacturers of inductive flowmeters specify a minimum electrical conductivity falling within the range 5 µS/cm – 20 µS/cm for demineralised water. The Admag AXF has a minimum tolerated electrical conductivity of just 1 µS/cm for lines with inner diameters of 15 to 200 mm. Practical experience has shown that the AXF is even capable of reliably measuring ow rates of demineralised water having an electrical conductivity as low as 0,5 µS/cm, while remaining within its specified accuracy range, thanks to its special features. The key is the unique “enhanced dual-frequency method” developed by Yokogawa. Unlike conventional inductive flowmeters that employ a low-frequency, alternating, magnetic field, the Admag AXF employs a pulsed magnetic field with superimposed repetition rates of 6,15 Hz and 160 Hz, which explains its short response time, extraordinarily high noise suppression, and low zero drift. The meter also features superior filtering that yields output signals having better signal/noise ratios in problematical media, particularly water having a low electrical conductivity or media whose electrical conductivity is subject to rapid fluctuations, termed ‘slurry noise’.
Pure water is dipolar. Fluctuations in the number densities of the few solvated ions present in water cause sudden changes in its electrical conductivity, which, in the case of inductive flowmeters, causes continual, rapid, fluctuations in the internal resistance of the voltage source supplying the voltage to their electrodes, which, in turn, causes rather noisy output signals. In an application at a facility operated by a natural-gas supplier, the Admag AXF was incorporated into the feedback loop controlling the metering of water to the system for generating steam. Accurate measurements of the flow rate of the pure water employed, which has an electrical conductivity of 0,5 µS/cm, allows accurate control of the operation of the entire system. The instrument involved is an AXF050 set up to accommodate volume flow rates of about 14 m3/h, which corresponds to flow velocities ranging up to 2 m/s.
Yokogawa recommends the following in the case of applications involving demineralised water:
• Use an AXF equipped with the enhanced dual-frequency option, option HF2.
• Use the integral version of the AXF, or a very short sensor cable, in the case of the remote type installation.
• Use removable electrodes, to make it possible to inspect them periodically.
• The electrode material employed should be 316L, or even better, Hastelloy.
• Use grounding rings to ensure that the instrument is properly grounded.
• Limit the maximum flow velocities to less than 2 m/s.
• Eliminate turbulence by providing long upstream and downstream lines.
• Avoid formation of vapour bubbles due to, eg, cavitation.
• Set signal-damping parameters to 3 to 5 seconds.
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