Every loop powered transmitter running today owes its design to a choice engineers made decades before transistors existed.
Tracing the mA signal back to its pneumatic ancestor explains why the numbers 4 and 20 were never arbitrary.
The 4 and the 20 were not picked at random. Both numbers trace back to a pneumatic standard that came decades earlier.
We will cover the original 3 to 15 psi pneumatic standard, the short lived 10 to 50 mA standard, why 4 to 20 mA replaced it, and the live zero idea that runs through all three.
What Is the mA Signal?
An mA signal represents a process value, like flow, level or pressure, as a proportional electrical current traveling through a two wire loop.
Instead of a raw voltage that can drop over long cable runs, current stays constant around the loop regardless of distance.
That single property is exactly why the format survived as long as it has, through several generations of hardware and multiple attempts at replacing it entirely.
Our why 4 to 20 mA is used instead of voltage guide goes deeper into that specific advantage.
Understanding where the format came from also makes today's signal conversion calculations easier to reason about.
The same 1 to 5 ratio still governs how a reading translates into a percentage of range, whether the underlying value is expressed in psi, mA or a scaled engineering unit.
3 to 15 psi: The Original Pneumatic Standard
Long before electronics reached the plant floor, instruments communicated through air pressure, using flapper nozzle mechanisms to convert a measurement into a pressure signal.
Engineers settled on 3 to 15 psi because that specific band sat in the most linear portion of the curve relating flapper movement to nozzle backpressure.
Outside that range, the relationship bent and became harder to read accurately, so instrument designers deliberately stayed inside the section of the curve that behaved predictably.
That choice set a pattern nobody had planned to repeat. Both electronic standards that followed kept the same 1 to 5 ratio between minimum and maximum.
Neither of the later engineering teams derived that ratio independently. They simply inherited it from this pneumatic original, because it already worked and there was no reason to reinvent it.
10 to 50 mA: The First mA Signal Standard
When electronics began replacing pneumatics, the earliest transmitters relied on magnetic amplifiers rather than solid state components.
Those magnetic amplifiers could not reliably operate below about 10 mA. Engineers set the live zero at that floor and kept the same 1 to 5 ratio used in the pneumatic world.
The math worked out to 10 to 50 mA, and for a while, that was the best electronic instrumentation could offer.
This standard was formalized in ISA 50.00.01 1975, and it dominated new installations for years before a better option arrived.
4 to 20 mA: The Modern mA Signal Standard
The transistor changed the math completely. Once solid state circuits could operate reliably at much lower currents, a live zero of just 4 mA became achievable.
Keeping the familiar 1 to 5 ratio again gave engineers 4 to 20 mA, and this new mA signal quickly displaced its predecessor across the industry.
Our 4 to 20mA best signal for industrial automation guide covers why the format still leads new installations today.
Why Every mA Signal Uses a Live Zero
None of these three standards start at true zero, and that decision was deliberate rather than accidental.
The 1 to 5 Ratio Behind Every mA Signal
The same proportion shows up in all three standards, even though the technology underneath changed twice.
Once a working formula proved itself in the field, nobody had a reason to change it, only to shift the numbers to match whatever the current generation of hardware could reliably produce.
3 Signal Standards Side by Side
| Standard | Technology | Live Zero | Status Today |
|---|---|---|---|
| 3 to 15 psi | Flapper nozzle pneumatics | 3 psi | Legacy, still found on older plants |
| 10 to 50 mA | Magnetic amplifiers | 10 mA | Obsolete for new designs |
| 4 to 20 mA | Transistor circuits | 4 mA | Industry standard today |
HART: Riding on Top of the mA Signal
The 4 to 20 mA format eventually gained a digital companion rather than being replaced outright.
The HART protocol superimposes a small digital signal over the same 4 to 20 mA wiring, letting a transmitter send diagnostic and configuration data without disturbing the analog reading.
This is why so many transmitters installed decades apart can still share the same two wires without any rewiring at all.
Our loop impedance and accuracy guide covers what limits how far a modern loop can actually run, and HART loop resistor requirements add one more constraint worth checking during design.
Why This Signal Still Dominates New Designs
Digital fieldbus alternatives have existed for decades, yet plenty of new installations still specify the same analog current loop this history traces.
None of this means digital fieldbus and DCS integrated networks are going away. It means the analog loop earned its long service life honestly, one design generation at a time.
Plenty of plants run a mix of both worlds today. Fieldbus handles complex multivariable devices, while the older current loop still covers straightforward single variable measurements.
A full digital network would be overkill for many of those simpler points, which is part of why that coexistence is unlikely to end soon.
A wiring standard that survived a transition from pneumatics, then a transition from magnetic amplifiers to transistors, has already earned its durability.
Add the arrival of an entire digital protocol riding on top of it, still working today, and the case for its staying power gets even stronger.
Reading a Live Zero in Practice
A quick example shows why the fault detection benefit is not just theoretical.
Broken wire or dead transmitter: 0 mA, clearly abnormal
Result: Control system flags the 0 mA reading as a fault instantly
A dead zero standard could never offer that same instant distinction, since a genuine zero reading and a broken wire would look identical on the display.
That single design decision, made decades before any of today's diagnostic software existed, still shapes how alarm logic is configured on modern control systems.
A DCS or PLC input card checking for a signal under about 3.6 mA will typically flag a wire fault well before the value drops all the way to zero.
That catches the failure a little earlier than waiting for a true zero reading would allow, giving operators a head start before the loss of signal becomes a loss of visibility.
Watch: 4 to 20 mA Current Loop History
mA Signal History Questions Engineers Ask
Related Articles on This Site
- 4 to 20mA: The Best Signal for Industrial Automation
- HART Protocol: How It Works
- Instrument Loop Impedance and 4 to 20mA Accuracy
- 4 to 20 mA Signal Conversion Calculator
- HART Loop Resistor in HART Communication
External References
What We Learn Today
- Today's mA signal descends directly from a 1 to 5 ratio first set by the 3 to 15 psi pneumatic standard.
- 10 to 50 mA came first, limited by magnetic amplifiers, before transistors made 4 to 20 mA possible.
- A live zero on every one of these standards exists specifically so a broken wire never looks like a real reading.