Magmeter – Magnetic Flow Metres
How do you measure flow through a pipe? It’s a trickeier problem than it initially sounds. It’s also a question with many answers. Many approaches centre around pressure transducers measuring total pressure and static pressure to then infer dynamic pressure and therefore fluid velocity. Although this means that the density of the liquid must either be measured, or assumed to be invariant. It also requires multiple sensors. Other approaches utilise a pressure drop after a pipe constriction to infer velocity. The problem is, if there are solids in the flow, these can get deposited on pressure transducers or build up in or around constrictions which can eventually alter or impinge the fluid flow.
In some circumstances disturbing the flow is not an option, or exposing sensitive pressure transducers to corrosive fluids is not feasible. So is there a way to measure fluid flow without having to place anything protruding into the pipe? And this is where we now return to electro magnetism a phenomenom often utilised in instrumentation. One of the electro-magnetic effects that Faraday discovered is that a conductor moving through a magnetic field generates a voltage. This is the effect that’s utilised in generators where rotating a copper coil through a magnetic field generates power.
The conductor doesn’t need to be rotating, as in a generator, it just needs to be moving through the field. To generate a continuous voltage we’d need a continuous conductor… like a conductive fluid flow. This effect has not taken on as a means of generating power, it’s far more efficient to pass fluid through a turbine to extract energy from a flow. However, by setting up a magnetic field across a pipe with fluid flowing through it we can create such a situation. With a pair of electrodes to measure the resultant potential difference across the fluid we have the beginings or a sensor – a magnetic flow meter.
The magnetic flow meter or magmeter has some excellent advantages. The conductivity of the material moving through the magnetic field does not have a direct effect on the voltage generated rather effecting the current. So long as some charge carriers are available to move and produce the resultant electric field the potential difference can be measured.
This means that the density of material, and solids in the flow and to an extent voids, do not affect the measurement, as they only effect conductivity, not velocity. This holds true so long as the electrodes contact with the fluid is not impinged. This means bubbles smaller than the electrodes are no issue, and that it can often be possible to orientate the pipe such that larger voids do not contact the electrodes either, thus mitigating their effect also. Magmeters therefore become the go to choice for flows with solids in them e.g. slurries. Magmeters don’t obstruct the pipe so there is nothing for solids to deposit on. Electrodes can be designed such that solids are unlikely to desposit upon them to preserve continuity and minimise maintenance. This is a huge advantage.
The disadvantage is that the fluid flow does need to be conductive. In practice, for many applications, this is not important. Fluids, particularly water based fluids, tend to be conductive due to the presence of contaminants that form ions. This is why water is generally considered conductive even though, when pure, it is not. However in the domain of chemical engineering, many flows are very pure and not conductive, and this does need to be considered.
Part of the complexity of magmeters comes in the choice of how to generate the magnetic field. Permanent magnets decay in field strength with time, and with no consistent way to measure the field strength this means that the magmeter would slowly drift out of calibration. A very early attempt at creating a magmeter actually attempted to use the earths magnetic field to measure flow under a bridge of the Thames. However now we near exclusively use electro-magnets. There are two types of current that can be used to drive an electromagnet, DC and AC, and both kinds have been used for magmeter use.
In the AC case an alternating magnetic field is induced. This has a couple of benefits, noise not related to the current can be filtered out, as it is invariant and largely a different frequency to that of the excitation frequency, however in phase noise can not be discarded this way. It also means that zero drift can be studied by looking at the offset in the signal at 0 excitation. A second large advantage in the past was that power transformations were more practical with AC power, this is no longer the case and has significantly contributed to the rise in the alternative: DC excitation.
DC excitation version utilises pulsed DC, in that a DC current is switched on and off. This allows noise to be filtered as you can see the noise level when no current is flowing and contrast this with the measurement when there is DC excitation. In the same way it allows for elimination of zero drift. Due to modern power electronics and the reduced cost associated with them, DC excitation is the most common type for magmeters or magnetic flow meters today. As the response signal should be essentially steady state (unless the velocity of the fluid flow varies very quickly) noise can be filtered by discarding high frequency components of the signal.
Another point of complexity is that whilst magmeters core function is unaffected by voids as mentioned (so long as the electrodes are unaffected – also mentioned). If volume flow is the desired characteristic, as it often is in pipe flow, then the pipe must be assumed to be full to get accurate estimations of volume flow from velocity through the pipe, voids would violate this assumption. Therefore placement of the magmeter can be very very important to ensure that the data it produces is relevant and can be used to measure what actually needs to be measured.
This is not a rare case in the field of measurement engineering. Part of the expertise of an instrumentation engineer is knowing how to get the information needed from the sensors and measurements available given the constraints of a particular problem or project. They need to know what assumptions are valid, and what proxies are viable, in this case velocity for volume if a pipe is full.
Do you have such a task? Flintmore are experienced multi-discipline instrumentation engineers ready to tackle your challenge. Get in contact now
Further Resources
The best resource I have concerning Magmeters is from P.I. . I highly recommend if you found this article interesting.
Another great resource I found for understanding Magmeters in application is this from the isa:
https://blog.isa.org/tip-12-the-good-the-bad-and-the-ugly-of-magmeters
Omega continue to have very good articles about sensors. Have a read for a different perspective solidly grounded in engineering and scientific principles.
https://www.omega.com/en-us/resources/magmeter
The wikipedia article on Magnetic Flow Metres is below the standard that is often found for sensors on wikipedia, but linked here just for consistency with past transducer thursdays.
