Orifice Plate

First, there is the question of standard calibration values for ease of maintenance and reasonably sized orifices. Usually they are 50", 100", 150 and 200" H2O. These come out to 1250, 2500, 3750 and 5000 mm H2O. "

Secondly, there's a question of the optimum region for uncertainty of discharge coefficient "C" versus the smallest possible permanent delta-P loss for pumping or energy saving reasons. Although AGA 3 accepts Beta ranges from 0.25 to 0.75, the optimal region falls around a Beta of 0.6.

When one combines these two factors with the typical flows in process pipes, one finds that calibration calculations fall around the four values listed above. Given the fact that orifice boring is a machine operation irrelevant of round values (A machinist gives a hoot whether he has to bore a 54.3 mm or a 61.2 mm hole), it seems reasonable to ease the pain of the transmitter calibration technician by giving him round mm WC values.

Of course, there are other reasons why one would choose one or the other of the four "standard" calibration values. For example, in an existing installation, one may be restricted for space (upstream pipe diameters), thus, one may choose a higher calibration, giving a smaller Beta, resulting in shorter required upstream pipe lengths from the AGA 3 tables, solving your space limitation problem.