why we use 4-20 mA to do calibration of the instruments.why
we cant use 0-20 mA?
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Answer Posted / zainuddin
1.We use 4-20mA is enough weak to ignite spark to prevent
fire. so these method is designed for safety.
2.There are several types to send the signal that
standardized in current(4-20mA), voltage(1-5v) and in
pneumatic (3-15psi 0r 0.2-1 bar). That means, u determine
how u were sending the signal to operate your process. but
here the best to sent the signal is in current(4-20mA).
3.24V is min. supply that transmitter to operate. if u put
supply more than that as long as not over the transmitter
supply range, u still get the same output 4-20mA, but the
voltage is changed. the change of voltage not effect your
process because you used current signal to operate your
process. Usually engineering will use current
rather than voltage because current not reduce at the long
distance to sent the signal. if u use voltage (1-5v) you
are facing with reducing signal(voltage) in certain
distance. however voltage that better to use is only 1-
5v(enough weak to ignite spark).
so here u should have a calculation how to produce signal
1-5V.(calculation can see at previous answers).
it more complicated than current not depend to supply(
either 24V 30V or 40V) & resistance( either 250ohm, 500ohm
or 700ohm) as long as you not reach over than transmitter
manufacturer range, because the transmitter is converter,
that convert standard signal to 4-20mA.
4.so why transmitter has supply range? this is because when
surging happens that the supply is going to transmitter
is still under transmitter supply range to reduce or
minimize transmitter going burned. so that means,
transmitter can operate at any supply voltage as long as
under supply range. it also can protect your process
immediately going shutdown.
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