Interested to Buy Any Domain ? << Click Here >> for more details...
why do we use 4-20mA as o/p and why don't use 0-20mA or 5-
21mA?
- 3 Answers
- 9088 Views
- I also Faced
- E-Mail Answers
Answer Posted / carl_ellis
Question:
why do we use 4-20mA as o/p and why don't use 0-20mA or 5- 21mA?
There are systems that use 0-20mA as an output from a
controller. Those are active', powered outputs, not
signals from 2 wire, loop powered field transmitters. 2
wire, loop powered instruments use about 3.5mA to power the
transmitter's electronics, so 2 wire, loop powered
instruments can never go below 3.5mA without losing the
current needed to power the instrument.
An active analog output from a controller that uses the
controller's power from the controller to drive the current
signal can be a 4-20mA signal or a 0-20mA signal.
People in the process industries use the 4-20mA output
because it is a 'live-zero' that immediately indicates a
broken wire or an open circuit if the loop current falls to
0.0mA. Those using 0-20mA probably have a 0-10Vdc or 0-5Vdc
analog input and want to use the entire span with a 500 ohm
or 250 ohm resistor.
As to why 5-21mA is not used, there are conventions used
that make systems maintainable over time. If every analog
output used oddball ranges or spans, it would be near
impossible to maintain the system over time. Given that
many control systems operate in excess of 10 years, it is
important to use standard analog signals in order to keep
the system maintainable.
The convention is 4-20mA, established by the ISA SP-50
standard in 1967. Deviations are punishable. Be warned.
Carl Ellis
Measure First
| Is This Answer Correct ? | 1 Yes | 0 No |
Post New Answer View All Answers
what is moisture probe? What are there types? What is there working principle and how they works? guys this is one of technical question I had ask in interview. If you know its answer please let me know. The interviewers were more interested in microwave moisture probe, so if you find its any link please just forward me on my mail id.avinash1200@gmail.com Thanks in advance.
is motorized valve closed when power shutoff?
what is difference between fork type and capacitance type level switch. where it is using
How is work 5/2 sov?
what is the reason while warning lamp is on while the engine is cranking?
How does a single acting, spring return ball valve actually work? Does it require continuous air supply?
Describe the functions of the Operator Interface including the considerations related to alarming and display layout/configuration/layers.
what are procedure of sox-nox calibration
give some question relative instrument fitter?
Dear Sir/Madam,im going to attend Hpcl written exams on feb 17th in instrumentation stream..can anyone send me the HPCL model papers and pattern of HPCL ....i ve not yet attended anything in HPCL...so i dont know how it comes...so just i need some model test papers and pattern ..pls help me friends...
full process of gas turbin?
what is function of sensor, and how is it work , explain with difination
our kks tagging is crossing more than 12 character please suggest how to take tags in the DCS
Why humidity limit is 30 to 55 % for raw material storage?
how many types of preheaters are in cement industry & what r they can any body explain?
