Answer Posted / 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

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