Over the last 60 years, the electromagnetic flow meter has come into it’s own since it was introduced into process and water infrastructure industries. The key design factor is the conductivity of the fluid which, briefly put, provides a conducting path between two electrodes in contact with the flow in the presence of a magnetic field. For this to work, the water-based fluid or fluid must have free electrically charged ions within it. The minimum electrical conductivity should be between 5 to 20 micro-siemens/cm).
Whilst the electromagnetic flow meter (“magmeter” or “magflow meter”) is an elegant solution to flow measurement, it is clear that the fluid conductivity is insufficient in a whole range of process fluids such as most hydrocarbons and many non-aqueous fluids. Strange as it may seem, the actual conductivity is not important as long as it doesn’t vary too much and it stays above the practical threshold values as above. It is important, however, for the fluid flow to be reasonably homogeneous across the flow. The pipe layout both upstream and downstream of the magflow meter is a factor in obtaining a reasonably homogeneous flow.
The process designer needs to consider the following when seeking to specify the magflow meter and the accompanying pipe layout with respect to the conductivity of the fluid:
1. Chemical and pharmaceutical companies often use de-ionized or distilled water, or other solutions which are not conductive enough for use with magnetic flow meters. In this case, other meters such as vortex meters ,venturi meters or other flow meters should be considered.
2. Water used for drinking supplies does have a wide variation in conductivity at the low end of the conductivity scale. Extraction sources in one part of the country may well have a different conductivity from another.
3. Temperature of the conducting fluid does change the electrical conductivity, not always in a regular fashion. Therefore, the supplier should always be given the temperature range of the fluid to be measured.
4. Electrical conductivity does generally increase with concretion of solution for most conducting fluids. However, some fluid solutions’s conductivity relationship is counter-intuitive. For example, aqueous solutions of acetic acid (vinegar) increase with concentration up to 20% and then fall to a point at some 99% to below the threshold concentration.
We will be covering other specification considerations in future articles so keep visiting.