Answer / nagarajan.p

First the question is why can't we use 3~15mA or other
range, here also we can check live zero and also we can go
step current like 3,6,9,12,15(i.e 0%,25%,50%,75%,100%).ok
The reason is the transmitter voltage is 24Vdc , here we
can load max of 600 ohms, for this voltage & load better
range is 4~20.
Why 4 ~ 20 is better since we can convert easily 1~5Vdc by
put 250 ohms.All the controllers and DCS , PLCs are operated
by the input of 1~5Vdc (external or Internal convertion)

Answer / nilesh upadhyay

There are basically 2 reasons of using 4-20ma
1. we can know whether the instrument is working properly
or not. we can distinguish "live zero" and "dead band". if
there is problem in instrument then also it is going to
show zero and if there is no supply then also it will show
zero so we can differentiate this two.
2. 4-20ma is linear with 3-15psi .

Nilesh Upadhyay...

Answer / richard marks

the 4mA zero is termed a "live zero". If 0mA was the zero
point there would be no way of telling if the transducer
was reading zero or in a fault condition eg. an open loop.
with a 4mA zero an open loop would drive the indicator eg.
a recorder into a minus figure (-25%). The 4 - 20mA scale
is to give a calibration points for 0-25-50-75-100%.
Because the start point is 4 the steps are 4mA.

Answer / samrat kar

Answers 5 & 6 are totally correct.. just adding another point.
20 mA as upper value is used to keep electrical components
intrinsically safe at hazardous area.

Answer / dhaval joshi

for any active ckt make it standard 1 volt dc

actually formula is 20-4= 16 then

16/1000*250=4 ma
32/1000*250=8 ma
48/1000*250=12 ma
64/1000*250=16 ma
80/1000*250=20 ma
by using 250 ohm resistance and standard 1000 ma it give
standard voltage.

Answer / rajesh.a

STANDARD VOLTAGE IS 1-5V DC,250OHM RESISTANCE

AT V=I*R
AT 1V=I*250OHM = 4MA
AT 5V=I*250 OHM = 20MA

SIMPLE CALCULATION

Answer / anup ramteke

Answers 5, 6 & 7 are correct. just want 2 add that why cant
we use 3 mA current as lower value. The reason is that there
are many small noise signals that can be picked up by our
signal wire which has the potential of providing upto 3 mA
current. So to be on the safe side to distinguish between
our proper field signal and the noise pick-ups we use 4 mA
as our lower value

Answer / gopi

the 6th answer is very pakka

Answer / abdus sattar mollah

ALL ANSWERS ARE WRONG.....
4-20 is a standard because transistor which is the basic building block of any transmitter has a linear output charecteristics in 4-20ma range.

Answer / mayur

ans. 5,6,7 & 8 r correct.only two points add.
1.in ancient time,niddle type galvanometers are used,only if
we pass 4mA current it shows deflection.therefore 4mA.
2.if 21mA current will pass through our body,man can goes
dead.therefore 20mA. & 4-20 mA can divided into 5 equal %.

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