Answer / vimal

Whether our instrument is in good condition or not, in order
to check that we are using 4ma as a low value. If we use 0
ma, We can't say that whether the problem is in instrument
or in supply panel.Normally, when we not give supply to the
instrument it will show 0 ma. for that, we use 4 ma as a low
value. Moreover we use 4 to 20 ma as a standard output
because to get linear graph. So, we prefer 4 to 20 ma
instead of 0 to 16 ma..

Answer / ninad gaikwad

Please give you attention below:

4 mA 0 %
8 mA 25 %
12 mA 50 %
16 mA 75 %
20 mA 100 %

at the time of no voltage (zero voltage) we get 0 mA
at the time of 24v dc supply we get 4 mA.
IF we keep the range starting from 0 mA we will get 0 mA at
the time of zero voltage & 24 V dc voltage supply which is
technically not good.
If your observation is too good that time you will
find that from above every gap between each other of 'mA'
& '%' the differrences are equally same.
see for mA : 0-4-8-12-16-20 (gap of 4 mA equally)
and see for % : 0-25-50-75-100 (gap of 25 % equally)
This things easily helpful to sort out the problems.

Therfore this STANDARDISATION is Universal.

Answer / faiyaz ahmad

4 to 20 mA is a analog signal and its continuous signal its allow to 1 to 5 volts in this signal because its drop of voltage its not drop of current that's why its called standard signal its electrical out put of the transmitter.
4 mA is zero
20 mA is hundred

Answer / farooq ahmad

Because we want to check the tx is damaged or not.If it
will be normal & no process variable applied then we can
get o/p 4mA...if it will damaged then o/p will not show 4
or near about..

Answer / sandeep varma

the instruments starts responding from 4mA

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