Two pressure-transmitter datasheets can both print "±0.1%" and still describe very different errors. The percentage is only meaningful once you know what it is a percentage of.
A pressure transmitter is the field instrument that converts process pressure into a standardized output, usually 4-20 mA (sometimes with HART) or a digital fieldbus, for a PLC, DCS, or recorder. It is not a bare sensing cell. The electronics add temperature compensation, damping, engineering units, and the range you actually calibrate.
URL, span, and turndown
The Upper Range Limit (URL) is the model's maximum settable range. The calibrated span is the slice of that URL you order and use. Turndown (rangedown) is URL divided by calibrated span: 10:1, 100:1, and on some models 200:1.
Turning a wide-URL model down to a small span is convenient for spares, but it is not free. Datasheets state that a higher turndown raises the measured error. A model used near its URL and the same model turned down 100:1 are not the same installation.
What %span and %URL actually reference
- %span (also written %FS, percent of full scale) is referenced to the full scale of the selected, calibrated range.
- %URL is referenced to the model's maximum settable range, not to the range you turned it down to.
At the same printed percentage, %URL produces a larger real error on a small turned-down range, because the percentage is taken against a much bigger number.
Sketch, in engineering units, not a vendor claim: URL = 100 bar, calibrated span = 10 bar (turndown 10:1).
- ±0.1 %span is 0.01 bar at that span.
- ±0.1 %URL is 0.10 bar on the same installation.
The datasheet percentages looked identical. The loop error did not. Comparing a tight %span figure with a %URL figure, or with a "% of reading" flow specification, without converting them first will rank the wrong instrument.
Long-term stability lines have the same trap. One vendor may quote drift as a percent of span; another quotes percent of URL per year. Those lines are not interchangeable with the reference-accuracy line, and BFSL (best-fit straight line) is not the same method as a terminal-based accuracy statement.
A check before the RFQ
- Write down the URL and the calibrated span you will actually buy.
- Note whether each accuracy line is %span, %URL, or % of reading, and which method (BFSL or terminal) it uses.
- Convert every line into engineering units at your span before you rank vendors.
- Treat turndown as part of the error budget, not as a footnote.
- Confirm the output the PLC expects (4-20 mA, HART, or a fieldbus) and the fault behavior (for example NAMUR NE 43 currents outside 3.8-20.5 mA) separately from the accuracy reference.
This note is selection research, not a product offer. SpecForge is an industrial product specification encyclopedia for procurement engineers, automation integrators, and metrology readers. It compares datasheet-level parameters for instruments such as pressure transmitters, PLCs, VFDs, sensors, pumps, and valves. It is not a manufacturer or a distributor, and it does not sell the devices discussed here.
The longer decoding of the %span versus %URL trap, with the selection order and the NAMUR NE 43 fault-current behavior, is in the SpecForge pressure transmitter guide:
https://www.sourcebyspec.com/encyclopedia/pressure-transmitter.html
Verify every value against the manufacturer's current PDF before procurement. A directory figure is a reading aid, not a substitute for the datasheet you will attach to the purchase order.
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