Answer / b_saravanakumar

We can use 0-10 volt also for signal. But while signal
transmission voltage drop will occur on lead wires. Also
voltage will have noise due to this we may loose signals at
receiving end. To overcome this we are using current
(4-20mA) for signals.

Answer / priyabrata samanta

The reason is - if we use 0 to 10 volt range then even if
an error occur in the transmission line we will not be able
to identify it. As there will not be any way to compare this
state with zero signal.
Whereas in 4to 20 ma current range we will be able to
compare the fault in the transmission line with zero
signal(4 ma).
Another reason is that using voltage as a transmission
signal is unfaithful and erroneous. As there will be voltage
drop in the transmission line , and therefore true signal
will not be received at the receiving side. But if we use
current as the transmission signal then noise can effect the
transmission, but "current drop" will never happen.

Answer / dev

Answer related to 0-10V is voltage drop as all frnds said
above but why 4-20mA preferred is as below:

Actually, you can use any range you want! But if you do, you
will have to design and manufacture both the transmitter and
receiving controller to match.
The 4-20 ma range is a "standard" adopted by tradition and
also ANSI 50.1, and most process instruments and controllers
use it. It was developed during a time when teletypewriters
used a 20 ma current loop for communications, so parts for
that type of circuit were readily available. There once was
a 10-50 ma current loop, but it has pretty much faded out.
It does have several advantages:

* The 4 ma "bottom of span" signal allows the receiver
to detect a broken wire or failed instrument.
* Since the loop current never falls to zero, the
instrument can be loop powered, allowing for two-wire devices.
* 20 ma is low enough to be made intrinsically safe,
making it easier to apply in hazardous locations.
* The constant-current feature of a current loop cancels
out voltage drop errors due to long wiring runs (of course
this would also be true if you selected different current
values for zero and span).
* The 4-20 ma signal dropped across a 250 or 500 ohm
resistor creates a very convenient 1-5V and 2-10V,
respectively, also fairly standard ranges in the industry.


So, while technically a manufacturer could use 7-17.3 ma or
6.2-21.1V as their range, who would buy their pressure
transmitter? No one! That's both the beauty and trap of a
"standard". There is a huge investment out there in process
controls that use 4-20 ma. That's the real reason, Im
afraid. You will not change that until you find a different
range that has some huge benefit over the current (pun
intended) standard.

In short bottom span is 4mA which is standard bcoz if any
instrument is dead and have 0 o/p then we won't detect what
actually is because both gives 0 o/p....... and
standardization of 4-20mA is as above....

Hope Got It....

Reference:copy and paste this link u will get what i
got>...::::


http://wiki.answers.com/Q/Why_does_the_process_industry_use_a_4-20ma_range_for_signal_processing_why_cant_you_use_different_ranges

Answer / eid elatawy

L
The resistance R= Resistance const * ---
A

Where L the length of the conductor
and A section area of the conductor

form the above equation we can note that the resistance change
the length of cable so the length of cable affect on signal (V) according to Ohm's Law
V=I*R
so, in case of long distance will affect on the accuracy of the signal.


Regards
Eid Elatawy
I&C Engineer
TCI Sanmar chemicals CO
Contact no 002 01000470262

Answer / vijay kumar

Generally in a transistor some amount of voltage is
required for turning it on. This voltage is the cut in
voltage. Up to this voltage the exists a nonlinearity in
its characteristics. Beyond this cut in voltage the char.
of transistor is linear. This nonlinear region lays between
0-4ma, beyond 4ma it linear. This is why 4-20 ma range is
used.

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