Explain why the transmitter has 4-20 ma output
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The real reason the 4-20 mA scale is the standard for process control devices is that 4mA is the live zero. if a transmitter faults or loses power you will see 0 and know the transmitter failed.
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4ma through you can see the live zero and dead zero also between 4 to 20ma input against output is linear
Historically, the the primary reason for 4mA as a ‘range zero’ is not its live-zero error detection functionality; the reason is to get 2-wire loop-powered functionality using only 2 wires from the control room to each transmitter, not 3 or wires. A 4-20mA range with a 4.0mA ‘range zero’ allows the transmitter to use 3.5 to 3.6mA of electrical “loop power” to power the field instrument itself. Using the same 2 wires for power and signal is what 2-wire loop power functionality is and that was the driving factor for a 4mA range zero. The live-zero aspect was a nice feature, but it was and is the 2-wire loop power function that maintains 4-20mA as the primary field signal in the process world.
4-wire 4-20mA signals, like a controller output that drives a valve positioner, is not loop powered. But the device it drives, the positioner, uses up to 4mA to power or drive its internal electronics.
If the range zero were truly zero mA, then there would be no loop current or too little loop current at 1mA or 2mA to power a field instrument. Then field instruments would have to 3-wire or 4-wire devices, with all the attendant cost of additional wiring.