In 2 wire transmitter 1 wire is used for power and other is
used for signal( 4-20)ma
In 4 wire transmitter 2 wires are used for power and 2 wires
are used for signal (4-20)ma .

but my question is "what is the criteria or parameters we
have to consider while selecting these transmitters for eg(
distance, safety zones etc.

Answer Posted / omer baosman

A 2 wire xmitter is powered by the mother instrument. The
output signal(4-20ma) is returned on the same wires that
powers it. the main purpose of this design is to reduce
wiring cost, standrdisation of circuits, and above all, to
kep all the transmitters at same ground potential.

A 4 wire inst. has design flexibility to utilize any power
like, 230vac, 24 vdc or 110 vac. It can feed the signal to
any mother instrument. Wherever the basic sensor of an
instrment needs power other than 24 vdc (like chemical
analyser) u have no choice but to use a 4 wire xmitter.

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