Why we mainly use 4-20 mA current and 3-15 psig pressure and not use from zero to 20 mA or 15 psig?
For 4-20mA devices there is a single loop which is used to provide the signal and power to the device. If the signal was zero then there would be nothing powering the device. 0-4mA is used to power the device.
If you have an offset signal at zero reading it is good for failure mode indication: If the signal is 0 mA or 0 psi then there is fault with the equipment.
For a pneumatic 3-15 psi signal it is difficult to control very low pressures accurately with a simple regulator, plus you have to provide vacuum to allow for calibration and measurement hysteresis & repeatability errors.
- Natural Gas Pressure Transmitters
- Water Level Transmitters with 4-20mA Output
- 4-20mA Output Hydrostatic Level Transmitters & Sensors
- SIL 2 Approved Safety Rated Pressure Transmitters
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