A 0.5-4.5 V ratiometric output spans 10% to 90% of a 5 VDC excitation, giving a 4.0 V span with a 0.5 V live-zero offset that flags a broken wire or lost excitation [S1][S3].
4-20 mA current loop output sits on a 16 mA span (4 mA = zero, 20 mA = full scale) with its own 4 mA live-zero and is the default signalling for hard-wired pressure transmitter installations in process plants [S2][S4].
Transfer functions: how each output maps pressure to signal
Ratiometric voltage is dimensionless: with 5.00 VDC excitation applied, the sensor returns Vout = 0.5 V at 0% pressure and 4.5 V at 100% pressure, and the receiver ADC uses the same 5 V rail as its reference so supply drift cancels by definition [S1][S2]. Because no internal reference IC is needed, the device draws less current, which is exactly why WIKA's know-how article tags ratiometric as the right pick for low-power, vehicle, and battery-operated loops [S2].
4-20 mA is an absolute current, not a ratio: the transmitter regulates loop current independent of supply (within compliance limits, typically 12-36 VDC), so the receiving PLC's ADC reference is irrelevant and the 4 mA live-zero survives any cable resistance up to the loop's voltage budget [S4]. For differential-pressure transmitter loops in flow and level service this absolute signalling is one of the reasons 4-20 mA has stayed dominant since the 1950s [S4][S7].
Decision criteria: cost, noise, cable length, power, diagnostics
On noise immunity, the 4-20 mA current loop wins because the loop current is set by the transmitter and rejected by the receiver's sense resistor; induced voltages on long runs cancel in the loop and do not modulate the signal [S4][S7]. Ratiometric voltage, by contrast, sits in the 0-5 V domain and is vulnerable to conducted and radiated noise picked up on the cable, which is why Ashcroft and Core Sensors both warn that ratiometric is intended for short OEM harnesses, not trunk runs across a plant [S7][S8].
On cable length and supply: ratiometric requires a regulated, low-noise 5 VDC at the sensor end because the output is referenced to that rail, and most OEMs cap practical runs in the 1-3 m range inside an equipment enclosure [S7][S10]. 4-20 mA tolerates hundreds of metres of twisted pair at 24 VDC loop supply, which is the standard pattern in chemical and oil-and-gas plants [S4].
On power: omitting the internal voltage reference lets ratiometric sensors run at single-digit mA, whereas 4-20 mA transmitters typically need 12-36 VDC loop compliance and draw 3.5-22 mA of signal current plus quiescent overhead [S2][S7].
On diagnostics: a healthy ratiometric sensor sits at 0.5 VDC (10% of 5 V) at zero pressure; a reading of 0.0 V indicates a short or lost excitation, a reading at 5.0 V indicates a short to supply or sensor failure [S6][S10]. The 4-20 mA live-zero is 4 mA; readings below 3.6 mA or above 21 mA are out-of-range faults per Namur NE43, which is widely adopted on level transmitter loops in tank farms [S4][S6].
Side-by-side comparison: ratiometric vs 4-20 mA on five criteria
Output type: ratiometric is a voltage ratio (0.5-4.5 V on 5 VDC rail, 10-90%); 4-20 mA is an absolute current (4 mA zero, 20 mA full scale) on a 16 mA span [S1][S3][S5].
Power budget: ratiometric operates from a regulated 5 VDC rail at low mA, no internal reference [S2]; 4-20 mA needs 12-36 VDC loop compliance with 3.5-22 mA signal current plus quiescent draw [S4][S7].
Cable length: ratiometric is limited to short OEM harnesses, typically under a few metres [S7][S10]; 4-20 mA routinely runs several hundred metres of twisted pair in process plants [S4].
Noise rejection: ratiometric voltage is exposed to EMI on the 0-5 V domain [S4][S8]; 4-20 mA current is largely immune to induced voltage on the loop and is the standard in electrically noisy plants [S4][S7].
Live-zero diagnostics: ratiometric uses a 0.5 VDC floor (broken wire or lost supply reads 0 V) [S6][S10]; 4-20 mA uses a 4 mA floor (Namur NE43 fault band roughly 3.6-21 mA on most modern devices) [S4][S6].
Who should pick ratiometric, who should pick 4-20 mA
Ratiometric 0.5-4.5 VDC is the right call for OEM equipment builders: mobile hydraulics, HVAC/refrigeration controls, battery-powered monitors, engine test cells, and any short-cable application where a regulated 5 VDC rail already exists on the customer's PCB [S2][S7][S8]. Johnson Controls' P599 transducer datasheet, for example, offers 0.5-4.5 VDC ratiometric specifically as a drop-in for building-automation controllers with 5 V supply rails [S5].
4-20 mA is the right call for process instrumentation, hazardous-area loops, long cable runs, and any installation where a PLC or DCS is the receiver. For absolute-pressure transmitter service in chemical reactors, 4-20 mA also pairs cleanly with HART for digital overlay on the same two wires [S4][S7].
The wrong pick is also easy to name: do not run a ratiometric output across a plant floor, and do not waste a 4-20 mA channel on a 30 cm harness inside a battery-powered data logger where a 5 V rail already exists [S4][S7].
Engineering pitfalls and sourcing
Two recurring errors to avoid on the ratiometric side: do not power a ratiometric sensor from an unregulated rail and then digitise it with a 3.3 V ADC reference, because the ratiometric cancellation breaks the moment the ADC reference and the sensor supply drift apart [S1][S8]. And do not assume the sensor body is a 5 V device; many so-called ratiometric parts accept 4.5-5.5 VDC and will scale the output proportionally, which is the whole point of the ratio but is sometimes missed by integrators [S2][S10].
On the 4-20 mA side: check loop compliance at the worst-case cable run, because a 24 VDC supply minus transmitter drop (typically 12 V) and sense resistor drop leaves the budget for cable IR, and 250 ohm at 20 mA alone burns 5 V [S4]. For hazardous areas, confirm the transmitter carries the appropriate rating for the zone; the analog signalling does not change with the protection method, but the entity parameters and barrier selection do.
For a vendor-agnostic spec walkthrough of how a pressure transmitter output is defined, the Honeywell SPS support note [S1] and the WIKA analogue-signals know-how series [S2] are the two cleanest public references. Sourcing checklist items worth tracking in 2026: confirm whether the device supports HART on top of 4-20 mA, whether the ratiometric output is true 10-90% of supply or a clamped 0.5-4.5 V regardless of supply drift, and whether the live-zero floor is hardware-detected or only visible in digital diagnostics.
Closing signal: when evaluating a new sensor spec, lock down the excitation voltage tolerance (ratiometric) and the loop compliance window (4-20 mA) before quoting lead-time, and verify the live-zero behaviour against Namur NE43 in writing for any 4-20 mA device going into a SIS or alarm path. If the application is mobile or battery-powered, default to ratiometric on a regulated 5 V rail; if the application is a process plant, default to 4-20 mA.
This topic is covered further in PVC-U Pipe Selection for Schools: Pressure Class, Temperature, and Code Fit.