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1,why we use 4-20 ma output of all instrument qhy not use 0-
20,
2,what is the different between PLC & DCS.
3,how can we meaure maximum span of new flowmeter if not
given any information.
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Answer Posted / himanshu apte
4-20 mA current has the highest linearity with the field
instruments. Same linearity cannot be found in other
current options like 0-20mA or any others. In addition to
that, a cable failure in case of 0-20mA will not be
detected if the signal is already at 0 mA.
PLC have a very small scan time i.e. @ 50-100 milliseconds,
which is useful in Emergency shutdown systems..Controlling
algorithms are available in modern PLCs, but fine tuning of
controllers can be better achieved with DCS. DCS has a scan
time in the range @ 500ms, which is why it cannot be used
in ESD systems. DCS can also handle more no. of I/Os than
PLC.
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