A “smart” DP transmitter with built-in square root characterization is used to measure flow through a pipe.

The orifice plate range is 0 to 150 inches WC at 0 to 480 gallons per minute:

Operations personnel have strong reason to believe that the actual flow rate through this pipe is 160 GPM, yet the DCS registers a flow rate of 182.4 GPM. Based on this information, determine the likely source of calibration error in this system. Also determine whether this transmitter has square-root characterization enabled or not.

Additionally, suggest a good “next step” to perform to either pinpoint the location of this problem or correct it.

Answer :

The calibration error lies either with the transmitter, the impulse lines (unequal fluid heights inside), or with the orifice plate itself. The transmitter does have square-root characterization enabled.

A good “next step” would be to block and equalize the transmitter manifold to check what its PV and AO parameters register with no applied differential pressure.

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