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.

• High Temperature Pressure Transmitters
• DPT200 Pressurised Tank Level Differential Pressure Transmitter
• Differential Pressure Transmitters
• LMP305 Borehole Level Transmitter

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

• DPS200 HVAC Differential Pressure Transmitter
• DPS300 User Switchable Pressure Range, Volts or Current Output Low DP Sensor
• 0-20mA Output Pressure Transmitters
• DPS Low Differential Pressure Transmitter