Instrumentation reference · reviewed 2026-07-25
I/P Converter: 4–20 mA to 3–15 PSI Explained
An I/P converter—current-to-pressure converter or electro-pneumatic transducer—turns an electrical current command into proportional pneumatic pressure. A common direct-acting range maps 4–20 mA to 3–15 psig (0.2–1.0 bar), but the installed device may use another range or reverse action.
1. Complete signal path
PLC / DCS AO
4–20 mA command
I/P input
Measured loop current
Pneumatic stage
Clean regulated air
Output / positioner
3–15 psi nominal
Valve and process
Travel + process feedback
How the conversion is calculated
For a direct-acting 4–20 mA to 3–15 psi calibration, percent span is(% = (mA − 4) ÷ 16 × 100)and pressure is(psi = 3 + % × 12 ÷ 100). This is a calibration relationship, not a guarantee that the connected valve position is linear.
2. Five-point calibration map
| Span | Input current | Output pressure | Metric pressure |
|---|---|---|---|
| 0% | 4.0 mA | 3.0 psi | 0.2 bar |
| 25% | 8.0 mA | 6.0 psi | ≈0.4 bar |
| 50% | 12.0 mA | 9.0 psi | ≈0.6 bar |
| 75% | 16.0 mA | 12.0 psi | ≈0.8 bar |
| 100% | 20.0 mA | 15.0 psi | 1.0 bar |
3. Ideal direct-acting response
4. Fault isolation
| Symptom | Likely cause | Measure next |
|---|---|---|
| Output stays near zero | No supply air, blocked filter, no loop current or failed transducer | Measure supply pressure, input current and output pressure separately |
| Correct at 4 mA, wrong at 20 mA | Span error, insufficient supply pressure or downstream loading | Isolate the output, verify supply margin, then adjust span per manual |
| Offset wrong across the range | Zero calibration drift or gauge/reference error | Verify the pressure standard and adjust zero before span |
| Output oscillates | Dirty air, restriction, vibration, pneumatic volume or valve/positioner interaction | Test the I/P unloaded, inspect air quality and trend the input current |
| PLC output is correct but valve position is wrong | Fault is downstream of the I/P or feedback is missing | Measure I/P pressure, positioner input and actual valve travel in sequence |
5. Loop-document boundary
Electrical
AO channel, loop supply, barrier/isolator where specified, cable, shield and measured mA.
Pneumatic
Clean dry supply air, regulator/filter, I/P output, tubing volume, leaks and measured pressure.
Mechanical / process
Positioner, actuator action, valve travel, feedback, process response and engineered failure position.
6. Commissioning test matrix
| Test | Stimulus | Expected | Record |
|---|---|---|---|
| Zero | Command 4.0 mA | 3.0 psi nominal | Input current and output pressure |
| Midpoint | Command 12.0 mA | 9.0 psi nominal | As-found error before adjustment |
| Span | Command 20.0 mA | 15.0 psi nominal | As-left error after zero/span iteration |
| Signal loss | Open or drive loop to defined fault state | Response matches device, actuator and process design | Pressure and final valve position |
| Air loss | Remove supply using approved procedure | Final element reaches its engineered response | Pressure decay, travel and alarm |
Primary technical sources
- Fisher 646 electro-pneumatic transducer
Emerson — official 4–20 mA to 0.2–1.0 bar (3–15 psig) product behavior
- Fisher 846 instruction manual, June 2025
Emerson — input/output ranges, supply-air requirements, calibration and performance boundaries