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Why 4-20mA SIGNAL IS PREFERRED OVEA A 0-10v signal?
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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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Q1: Consider part of a control loop, which excludes the transmitter, consisting of a process, a controller and a control valve which may be represented by two dead times of 0.5 min each and three exponential lags of 0.8 min., 1.0 min. and 1.5 min. respectively. We wish to express this system as an overall first order plus dead time (FOPLD) model ie gain, time constant and process dead time. (We will see later that this is often done, to simplify controller tuning). For this exercise, gain is considered to be 1.0. (A) If the transmitter is a flow transmitter whose behaviour can be described by a dead time of 0.2 min. and an exponential lag of 0.5 min. in terms of the overall dead time and overall first order lag how can the system behaviour be approximated ? Overall dead time = Overall time constant = (B) If the transmitter is a temperature transmitter with a temperature sensor in a protecting well whose behaviour can be described by a dead time of 0.7 min. and an exponential lag of 15 min. how can the overall system behaviour be approximated now? Overall dead time = Overall time constant =
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