Differential Pressure (DP) flow meters have long been the common method for measuring flow in a process.  Any obstruction in a flow path creates a differential pressure which can be measured.  Using the relationship of DP and flow a fairly accurate and reliable flow measurement can be realized.

With the advancement of measurement technology multi-varible instruments are becoming more and more common.  The relationship between DP and flow assumes a couple of constants which are anything but constant.  Measuring both temperature and static pressure the relationship becomes more accurately calculated.

There are a number of multivariable transmitters available on the market.  These all use the same methodology for calculating the flow.  The biggest different is how accurate the measurements are and how quickly the “flow computer” reports the flow.  Naturally, there various price positions held by their respective manufacturers.

The upshot is that with a modest investment in a multivariable instrument, you can improve the accuracy of your process.  Since you already have the primary element, manifold, impulse piping, etc., all you need to do is replace the instrument.  Oddly enough, these all have standard mounting configuration.  Of course, you will need to add an RTD, but these are inexpensive and you likely already have one (bringing temperature into the instrument, you will also relieve some I/O).  Not too shabby!

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