The bridge between electronic and pneumatic
An I/P (current-to-pressure) converter turns the control system's 4–20 mA output into the 3–15 psi air signal that pneumatic actuators and positioners still use across much of industry. Both are linear live-zero signals, so the mapping is a straight line:
The percentages line up exactly — 0% is 4 mA and 3 psi, 100% is 20 mA and 15 psi — because both standards share the same live-zero design philosophy. The SI equivalent of 3–15 psi is 20–100 kPa (0.2–1.0 bar).
Worked example
The controller outputs 12 mA to an I/P converter:
- Percent: (12 − 4) ÷ 16 = 50%
- psi = 3 + 0.5 × 12 = 9 psi
- = 60 kPa = 0.6 bar → a standard 3–15 psi actuator sits mid-stroke
Reference points
| % | mA | psi | kPa | bar |
|---|---|---|---|---|
| 0 | 4 | 3 | 20 | 0.2 |
| 25 | 8 | 6 | 40 | 0.4 |
| 50 | 12 | 9 | 60 | 0.6 |
| 75 | 16 | 12 | 80 | 0.8 |
| 100 | 20 | 15 | 100 | 1.0 |
Field notes
- Calibrate at 3 and 15 psi (the 4 and 20 mA endpoints); the linear span takes care of the middle. Verify 9 psi at 12 mA as the mid-point check.
- Supply air must exceed 15 psi with margin — usually ~20 psi regulated, clean and dry. A sagging supply clips the top of the range and the valve never fully strokes.
- Split-range signals reassign the mapping — e.g. 4–12 mA to one valve (3–15 psi) and 12–20 mA to another. Adjust the endpoints accordingly.
- Reverse-acting positioners invert the relationship (4 mA → 15 psi); confirm the action before trusting the numbers.
Frequently asked questions
What is the standard I/P converter signal relationship?
4-20 mA maps linearly to 3-15 psi: 4 mA = 3 psi, 12 mA = 9 psi, 20 mA = 15 psi. In SI that is 20-100 kPa or 0.2-1.0 bar.
Why is the pneumatic standard 3-15 psi and not 0-15?
Like the 4 mA live zero, the 3 psi offset provides a live zero: a valve reading 3 psi is confirmed at 0% while 0 psi indicates a dead air supply or broken line.
What pressure corresponds to 12 mA?
12 mA is 50% of the signal, giving 9 psi (60 kPa, 0.6 bar) — mid-stroke on a standard 3-15 psi actuator.
What supply pressure does an I/P converter need?
Typically a clean, dry instrument air supply around 20-25 psi (about 1.4-1.7 bar) to deliver a full 15 psi output with margin. The supply must exceed the maximum output.
Provided for reference and education. See our disclaimer.