Foxboro's S-series differential pressure transmitters come in three named tiers — IDP50S, IDP10S and IDP05S — and the naming is honest marketing for once: the digits track the accuracy class — with a reference nuance that matters: the 50S carries ±0.025% of reading, the 10S ±0.050% of reading, and the 05S ±0.075% of span. All three share the 400:1 turndown design goal; what differs is both the tightness of the figure and the reference it is measured against.
What the tiers actually buy
Accuracy referenced to reading is the spec that matters at turndown, and this is where the tiers separate honestly: the 50S and 10S rate their error against the reading (±0.025% and ±0.050% respectively), so the error stays proportional as the flow or pressure drops, while the 05S’s ±0.075% is referenced to span — an error that matters most when the working point sits far below the configured span. The 50S exists for custody transfer, fiscal metering and rare process points where error is money. The 10S covers advanced process control and balanced monitoring. The 05S is the honest workhorse for monitoring duty near the top of its range, where span-referenced and reading-referenced error practically coincide.
Turndown: the quiet differentiator
All three tiers share the 400:1 turndown design goal, but turndown without accuracy is a trap: a transmitter that ranges 400:1 and degrades at the bottom is a 10:1 instrument wearing a bigger number. The 10S and 50S answer it with percentage-of-reading accuracy and FoxCal's 11 embedded calibration curves; the 05S uses traditional span-referenced calibration — which is why deep-turndown duty belongs on the higher tiers, not because the 05S is bad, but because its error reference was never designed for that job.
The decision rule
Grade the measurement point by consequence: money flows and contract boundaries — 50S; control loops and balance monitoring — 10S; monitoring, alarms and everything else — 05S, and spend the savings on more measurement points. All three integrate into the same flow computing and DCS layer, and the multivariable step-up lives in the IMV family covered separately.
Is the 05S accurate enough for filter monitoring?
Filter drop is a DP question where the delta itself is the signal — the 05S's class is comfortably inside what filter differentiation needs. Reserve the 50S for the points where auditors and contracts read the number.
Does the 11 FoxCal sensor range matter to me?
On the 10S and 50S, the 11 embedded FoxCal curves mean one sensor covers the ranges you might re-range to — plant standardization buys spare-part simplicity. The 05S uses traditional calibration; respect its span reference when re-ranging wide.
