Are Earthing Rings Always Necessary in Magnetic Inductive Flow Meters?
To use earthing rings or not to use earthing rings? That is the question. Some time ago, we published the article “How Does a Magnetic Inductive Flow Sensor Work?”, where we explained the physical operating principle of a magnetic inductive flow meter (or MID for short). Earthing rings are used in some applications, but not in others. What is the reason for this? What do earthing rings actually do?
As we explained in the previous article, a magnetic field acting on a flowing, electrically conductive medium generates a voltage transverse to the direction of flow. Electrical disturbances are superimposed onto this voltage, for example, electrostatic charges carried by the medium. If the medium is sufficiently earthed, such disturbances are minimised. Earthing also ensures a defined reference potential, as is customary in measurement technology.
For metal pipes, the earthing generally occurs “naturally”. But what about pipelines made from non-conductive materials such as plastic or concrete, or with non-conductive linings?
In these cases, the medium may be insufficiently earthed, resulting in electrical disturbances which can have a negative effect on measuring accuracy. Earthing rings can be used to minimise this effect or, ideally, to prevent it altogether. Earthing rings are metal rings which are in contact with the medium and are connected to the earthing system of the installation (e.g. an equipotential bonding rail) to ensure an electrical connection between the medium and the earth.
Whether earthing rings are required therefore depends on the installation situation. Alternatively, an earthing electrode may be installed in the measuring pipe, depending on the design of the magnetic inductive flow meter. This also establishes a connection between the medium and the earth, eliminating the need for earthing rings. In this case, it is necessary to ensure that the device is properly earthed.
Since earthing rings must be in continuous contact with the medium in order to function, their chemical resistance must be taken into account when selecting the material. This is particularly true for corrosive media. Earthing rings are usually made of commercially available stainless steel, such as 1.4404 (AISI 316L). For more aggressive media that is not safe for stainless steel, Hastelloy C© is often used.
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