Answer / kashif mahmood

1) This is called the live zero or elevated zero concept.
It makes us differentiate between the signal failure and
the zero reading of the transmitter. Let's say, LRV=0 mA
and the instrument cable is cut. How can somebody ascertain
that this is process zero(LRV) and not the cable fault.
whereas if LRV = 4mA and the cable is cut, the current
through the loop would be 0mA and one can find out that
this is the cable fault.

2) its the lowest linear possible range with minimum power
consumption.

Answer / 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.

Answer / rathina

Normally we say Live Zero concept. means transmitter shows
0 means due to powerloss or process value loss.

Answer / ramu

For electronic Instruments operates with 24V(DC),will give
output 4-20MA and pnuematic Instruments operates with 20PSI
will output 3-15PSI.These instruments circuits are arranged
in such a manner that the out put should be as mentioned
above because you will get LENIERITY only in these
ranges.If you take 5-22 MA, you will not get lenier output
( 0%-4MA,25%-8MA,50%-12MA,75%-16MA and100%-20MA)

Answer / seshu

if we use 0 to 20ma, if any case cable was damaged then it
gives 0ma signal. so we r using 4 to20ma

Answer / kenechi

Yes

Answer / sathish r

Module require 1-5 v for further processing thats why we
use 4 - 20 mA because with 250 ohms resistance we can
easily convert 4 - 20 mA into 1 - 5 V

.004 A x 250 ohms = 1 V
.02 A x 250 ohms = 5 V

if we use 0 - 20 mA it converts 0 - 5 v but we required 1 -
5 v for further processing in module.

Answer / naresh yadav

Module require 1-5 v for further processing thats why we
use 4 - 20 mA because with 250 ohms resistance we can
easily convert 4 - 20 mA into 1 - 5 V

.04 A x 250 ohms = 1 V
.2 A x 250 ohms = 5 V

if we use 0 - 20 mA it converts 0 - 5 v but we required 1 -
5 v for further processing in module.

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