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.…