Why 4-20ma signal used not 0-20ma

Question

Accepted Answer

4-20mA signal is used instead of 0-20mA because it allows for a reliable and accurate measurement of the process variable.

4-20mA signal provides a clear distinction between zero and full-scale measurement.
It allows for easy detection of faults or breaks in the circuit.
The 4mA minimum value ensures that the signal is always present, even if the measurement is at its lowest possible value.
The 20mA maximum value provides a standardized upper limit for the signal.
This signal range is commonly used in industrial applications for its robustness and noise immunity.
Example: In a temperature measurement system, a 4-20mA signal can represent a temperature range of 0-100 degrees Celsius.
Example: If a 0-20mA signal was used, it would be difficult to differentiate between a zero measurement and a fault in the circuit.

Accepted Answer

The primary reason that 4-20mA is used instead of 0-20mA is because an elevated 'zero' allows for powering the field device on the same two wires that the signal uses. Passive, loop powered field transmitters use the DC electrical currrent below 3.5mA to power the transmitter to make the measurement.

Why 4-20ma signal used not 0-20ma

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