Pressure is always measured against something. Get the reference wrong and the number is not inaccurate — it is meaningless. The three references define the three transmitter families: absolute (referenced to vacuum), gauge (referenced to local atmosphere) and differential (one process pressure referenced to another). The Foxboro families cover all three, and picking the reference is the first decision in any measurement specification.
Absolute: when atmosphere is the noise
An absolute transmitter seals a vacuum reference inside the sensor, so it reads total pressure regardless of weather or altitude. Vacuum systems, distillation columns and boiling-point calculations need it, because atmospheric pressure is a variable they cannot afford in the math. A few millibar of barometric drift is a real error when your setpoint sits near vacuum.
Gauge: when atmosphere is the baseline
A gauge transmitter vents its reference to atmosphere and reports pressure above it. Tanks, lines and hydraulics live here — the plant cares about pressure relative to the world it leaks into. It is the default family, and the one where a Foxboro field-mounted pressure transmitter earns its keep on thousands of points.
Differential: when the delta is the process
A differential transmitter measures one pressure against another — filter drop, level head, orifice plate. The delta is the process signal; the absolute magnitudes are trivia. The multivariable variant goes further: a Foxboro IMV30 takes static pressure, differential and temperature and computes compensated mass or standard-volume flow on board — three transmitters and a flow computer in one device.
Level is the classic crossover case: a Foxboro 244LD displacer transmitter measures interface and level mechanically, while a DP transmitter infers level from head pressure — reference choice and density assumption decide which is honest in foam and vapor.
Can I use a gauge transmitter for filter drop?
Only by buying two and subtracting — which doubles the error budget. A true DP transmitter measures the delta directly at matched accuracy; that is why filter monitoring lives on DP.
Does vented gauge still work indoors?
Yes, via a vented cable or filter — but the reference must actually see atmosphere. A blocked vent turns your gauge transmitter into a barometer, and your tank reading into fiction.
