When discussing the sizing of magneric flow meters to the application, there is only one rule to follow:

1. Never size the meter to the pipe - Always flow it to the flow rates.

This can be where most of the problems come from. But how can you prevent this from happening? The first step is to understand the right way to size a meter to prevent the over sizing of the meter. A good rule of thumb is to size the meter so the average flow rate is around 15 to 25% of the maximum flow capability of the meter.

For example, on a meter with a maximum flow rate of 4000 gpm, the average flow should not be less than 500 to 1000 gpm. This flow rate will maintain sufficient velocity through the meter and still give the customer room for expansion. This, of course, is not always possible. Many installations are designed for expansion in the future, so large size pipes are installed. In this case, you must look at the minimum flow expected, and ensure that the average flow should never fall below 2 ft/s, or in this case, 300 gpm.

If you cannot reduce the overall size of the pipe to accommodate the proper size meter, you should install a reducer in the line located about 3 diameters upstream of the flow meter. You can then install an expander downstream of the meter and return to the original pipe size. This will ensure proper flow measurement now, and still allow you to remove the smaller meter in the future and replace it with a larger meter. This may sound extreme, but in the long run, you will be happier with an accurate meter.

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