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Join Date: Oct 2012
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4 to 20mA

10/09/2012 12:21 PM

Hi frnds, More r less many of the transmitters are meant with rating of 4 to 20mA.with minimum and maximum rating,but why not with 0 to 20mA. Regards, Joganand

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#1

Re: 4 to 20mA

10/09/2012 12:25 PM

<4mA can be used as an equipment fault alarm.

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#2

Re: 4 to 20mA

10/09/2012 12:29 PM

great information on 4-20mA current loops..

as PWSLACK said, < 4mA is hard to measure and can be used an alarm

http://www.ni.com/white-paper/6940/en

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#3

Re: 4 to 20mA

10/09/2012 3:50 PM

2 wire loop powered field instruments use part of the signal current as power to operate the instrument itself. Going to down to zero mA for a signal does not allow the field instrument to power itself. 0-20mA works fine as an output from a controller to a 4 wire powered device, but any current signal starting at zero eliminates the possibility of using a loop powered field instrument.

For 2 wire loop powered devices, generally ~3.5mA is used to power the instrument, the 3.6 - 3.8mA range is used for alarm/fault indication, 4.0 - 20.0mA range is used for the 'signal' and the range of 21.0 - 22.5mA is used for alarm/fault indication.

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#4

Re: 4 to 20mA

10/12/2012 3:17 PM

I agree with Iris and the idea that part of the current signal is used to power the instrument.

I have always thought that the 4 ma bias is a carry over from the 3 psi bias in pneumatics.

Its main purpose is that if you use a zero based signal you cannot ever tell how far below zero you are. On a zero based system a reading of zero could be a good zero a slight under zero or a broken wire open circuit zero all would show as zero.

On a biased system 4.0 ma is a good zero, 3.8 ma is approx. -5 % but still working and an open circuit is easily detected and can be reported as -25%.

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#5
In reply to #4

Re: 4 to 20mA

10/19/2012 5:23 AM

Zero offset of the base current (ie. 4mA) makes cable break detection simple.
if the current suddenly drops to 3.68ma or exceeds above zero 20.32 indicates that the device out of service.

0 ma base signal can get affected due to any electrical interference which may result into wrong measurement.

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