The 4-20mA and 3-15 psi ranges are preferred over 0-20mA and 0-15 psi because they allow for easy detection of signal faults. A 0mA or 0 psi signal can indicate a broken wire, a disconnected sensor, or a power failure, while a 4mA or 3 psi signal provides a “live zero” that indicates the system is functioning correctly, even at its lowest measurement point.
✅ 1. Live Zero (Fault Detection)
- 4 mA or 3 psi is called a “live zero”, meaning the system is powered and functioning.
- If the signal drops to 0, it indicates a circuit break, wire failure, or instrument fault, not a valid measurement.
- If you use 0-20 mA or 0-15 psi, a 0 reading could be either a real value or a fault, making troubleshooting difficult.
✅ 2. Linear Signal Range
- Both 4-20 mA and 3-15 psi are linear, which simplifies signal processing and scaling.
- The ranges represent 0% to 100% of the measured variable:
- 4 mA → 0%
- 20 mA → 100%
- 3 psi → 0%
- 15 psi → 100%
✅ 3. Powering Loop Instruments (Only for 4-20 mA)
- The 4-20 mA loop can provide power to the field device (e.g., transmitter).
- This is called loop-powered instrumentation, reducing wiring complexity and cost.
- A 0-20 mA loop can’t power devices effectively, especially when current is 0.
✅ 4. Noise Immunity
- Analog signals like current (mA) are less susceptible to electrical noise over long distances than voltage.
- The 4-20 mA range ensures signal strength is always above the noise threshold.
✅ 5. Standardization & Interoperability
- 4-20 mA and 3-15 psi are industry standards (used since the mid-20th century).
- Widespread adoption means compatibility across manufacturers and systems.
✅ Summary Table
| Feature | 4-20 mA / 3-15 psi | 0-20 mA / 0-15 psi |
|---|---|---|
| Fault Detection (Live Zero) | ✅ Yes | ❌ No |
| Loop Power Support | ✅ Yes (4-20 mA) | ❌ No |
| Noise Immunity | ✅ High | ✅ High |
| Standard Compliance | ✅ Widely used | ❌ Rarely used |
| Easier Troubleshooting | ✅ Yes | ❌ Harder |
Advantages of 4-20 mA vs 0-20 mA
- Using 2 instead of 3/4 wires reduces cost of wiring
- Power supply and signal circuit is combined into one
- Distinguish between zero signal and a fault
- Zero offset can be adjusted without need for polarity change of signal
Advantages of 3-15 psi vs 0-15 psi
- Easier to regulate pressure from 3 psi
- No need to provide vacuum when regulating zero signal at 3 psi
- Distinguish between zero pneumatic signal and a pressure failure
