A multivariable DP transmitter reads more than differential pressure — static pressure and temperature ride along — and the family question is what happens to those numbers. The Foxboro IMV30 computes on board: density, mass flow and standard-volume flow leave the transmitter as finished answers. The IMV25 measures and reports: the raw variables travel to the control system, and the computation happens elsewhere. Same sensing heritage, two philosophies about where the math lives.
What on-board computation buys
The IMV30 (5627-class in the catalog) turns three measurements into custody-grade mass or standard-volume flow at the pipe — density compensated, live, and identical at the transmitter, the panel and the historian. Gas and steam flows whose density moves with pressure and temperature stop depending on the DCS doing the math right; the answer travels finished. For fiscal and balance points, that single-source arithmetic is the audit story.
What the measure-only tier keeps
Flexibility and cost discipline: the IMV25 delivers the compensated raw variables for systems that compute in the control layer — plants with established flow computers, standard metering runtimes, and preferences about where their truth is calculated. Fewer onboard functions, simpler configuration, and the same sensing accuracy — the math just lives somewhere else.
The decision rule
Flow is money: IMV30, and let the transmitter own the answer. Flow is process input with a plant-standard computation layer: IMV25. Both integrate into the same flow computing and measurement landscape — the difference is which device signs the number.
Does on-board computation slow the transmitter?
The computation runs at the device's cycle and adds negligible transport delay for process purposes — the trade is configuration discipline, not speed. The honest question is who maintains the flow equation when conditions change: the instrument's firmware or the control system's blocks.
Can IMV25 upgrade to IMV30 later?
They are different functional tiers of the family — the migration is a device exchange, not a firmware push. Plan the tier by the measurement's consequence at purchase; the sensing platform is shared, the math tier is not.
