Our CEO Charles Wemyss looks at the benefits and advantages of an electro magnetic flow meter (also known as a magmeter).
At first sight, the primary benefit of an electro magnetic flow meter (magmeter/magnetic flow meter) is that there is no obstruction to flow. For good measurement, the internal diameter of the pipe should match that of the meter. Not only does that ensure no pressure drop and no turbulence, there is also excellent measurement. Not only is there no obstruction there is no intrusion. Beyond the slight profile of the electrodes, the pipe will appear to be full bore. One of the benefits of this is that pumping capacity is not compromised as it might be by an intrusive sensor or a moving part, for example.
There are, of course, more benefits to the magnetic flow meter. There are no moving parts, nothing that rotates in the flow, just an electronic circuit and electrodes, which aids longevity. With good electromag design good linearity is a given along with good repeatability. The simple wetted part design of an insulating liner and conductive electrodes enables good chemical compatibility. For example, a PTFE liner and 316 stainless steel electrodes is going to work with 90% of all chemicals. More exotic electrode materials such as Hastelloy™ or titanium further increases the compatibility.
Another benefit of the magnetic flow meter is derived from the simple obstructionless design in that small amounts of particles and entrained air can pass through the meter causing no harm and only a small change to the accuracy. The measurement basis is velocity. Combined with area this produces a volumetric reading. The area of the air or particle would therefore need subtracting from the assumed velocity/area calculation. Therefore, it is best these are just small amounts. Larger amounts, particularly where air forms into slugs which might leave the electrodes exposed, will cause undesirable sudden spikes in readings. Specialist versions can measure slurries and pulp. These require different sensing frequencies and may have less accuracy.
Finally, ‘intelligent’ versions can have built in diagnostics and calibration although, arguably, that relies on the user knowing that the
electrodes are in clean and perfect condition, and that the pipe hasn’t got coated. So how do you specify the best electromagnetic meter for your application? The flow rate and flow rate range will be an important factor. It’s unusual to have electromags running at less than 3mm pipe diameters, and at larger sizes they become uneconomic and insertion electromags may be used. Next, and most important factor is the conductivity of the liquid. This must be greater than 3 µS/cm in most cases. Pressure and temperature can be limited but, for most applications, the answer can be an electromag.
Things to note with a magnetic flow meter:
What could go wrong? There are a few other factors to watch out for:
- The flow range is derived from the velocity range. Whilst the maximum may be quoted as 10 metres per second or the equivalent volume flow rate. The 100:1 turndown becomes 30:1 if the max is 3m/s. 10 m/s is unrealistic in the real world as it leads to high piping pressure drop and high energy costs etc.
- At zero flow or no zero flow the sensing electronics have to determine if it’s meant to be zero or not. A cut-off is programmed at a certain value to show zero. That cutoff is critical to low flow determination.
- The meter must be correctly grounded, otherwise electrical noise can confuse the electronics. Earthing to adjacent pipe flange is mandatory. For plastic pipe a third electrode or separate grounding ring is required.
- The meter needs to be full of fluid at all times.
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