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Why use 4-20mA and 3-15 psi rather than 0-20mA & 0-15psi

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.

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Advantages of 4-20 mA vs 0-20 mA

  1. Using 2 instead of 3/4 wires reduces cost of wiring
  2. Power supply and signal circuit is combined into one
  3. Distinguish between zero signal and a fault
  4. Zero offset can be adjusted without need for polarity change of signal

Advantages of 3-15 psi vs 0-15 psi

  1. Easier to regulate pressure from 3 psi
  2. No need to provide vacuum when regulating zero signal at 3 psi
  3. Distinguish between zero pneumatic signal and a pressure failure

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