DoctorFrogman

HART current ratio - What is this?

3 posts in this topic

Hello fellas.

I'm trying to learn to use the HART functions of a FLEX i/o 1794 input card, and I have most of it figured out.

There is this setting though, called HART current ratio, which allows a selection of disabled, or 5% to 31%. I can't figure out what it does though. The "HELP" guide explains that it is a threshold percentage for forcing HART read back indication (Input signal deviation HART to analog) but I am quite unsure what this means.

Is it the allowed deviation that the analog signal can have, compared to the transmitted HART data?

Hope the question makes sense,

cheerio.

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I've been mulling your question for a couple weeks now because I'd never heard of current ratio in relation to HART.

If I interpret your conclusion correctly, the HART digital variable is compared to the analog value and if the analog value wanders or drifts beyond the percentage limit, then current ratio alarm is triggered.

The analysis function would have to be smart enough to know what the 4-20mA signal represents, in eng. units, in order to make a valid evaluation and the card is probably smart enough to know that.  

But what if the HART variable is the secondary or tertiary variable, not the primary variable?  Then there's no direct correlation between the analog value and the HART value.

And, what's the purpose?   Detecting drift between the digital and analog variable?   
When I started this field in the late 1970's, the analog (non-smart, zero and span pot) pressure transmitters would drift several percent in 6 months, necessitating periodic re-calibration.  But the electronics have improved so much over 40 years that today's' 4-20mA's don't drift much, if at all;  they're remarkably stable, and they don't typically drift to the over 5% range.  

There is a possibility of ground loops occurring and although that's usually detected during installation, events like the moisture intrusion into junction boxes over time can create a ground loop or increase the magnitude of a ground loop.  So analysis could detect a ground loop affected current signal that doesn't match the HART variable.  So an analysis might catch a ground loop error.

But I suspect that the current ratio refers to signal strength, noise or signal to noise ratio of the HART signal.

A web tutorial/primer on dB (deciBel) states:
Typically the deciBel, dB is used for defining amplifier gains, component losses (e.g. attenuators, feeders, mixers, etc), as well as a host of other measurements such as noise figure, signal to noise ratio, and many others.
https://www.electronics-notes.com/articles/basic_concepts/decibel/basics-tutorial-formula-equation.php
and the primer continues on to mention current ratios.

So I suspect that the current ratio is a means of determining the signal strength of the HART signal, an FSK voltage superimposed on the mA current signal.

But how that works out to percent and what the practical real world implication is, is beyond me.

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Here's what the manual says about HART current ratio (page 62)

https://www.quicktimeonline.com/assets/images/pdf/Allen%20Bradley/1794-IF8IH%20User%20Manual%20FLEX%20IO%20Isolated%20Input%20Output%20HART%20Analog%20Modules.pdf

61e8406d47168_Currentratio.JPG.5c5b7680a

There is a current fault/fail-safe limit on most field devices that drives the analog signal upscale beyond the overrange 20.5mA level (like 21.4mA0 or downscale below the 3.7mA underrange level (like 3.3mA).   But the fail-safe limit happens when the transmitter determines that its internal functioning can not report a valid process variable.  It's telling the controller, "my process variable is not trustworthy".   Which brings up the point, what does the HART variable do when the transmitter is in fail-safe fault condition?  If the HART variable goes off-scale, too, of then there's no deviation between the fail-safe analog value and fail-safe HART value.

It might be for dealing with a situation where the 4-20mA analog output is a subset of the full range of the transmitter.   For instance, pressure transmitters can be capable of measuring 0-1 bar but can be ranged 0-0.25 bar.  I've never checked for this but it could be that the instrument senses and can report a 0.40 bar signal that would saturate the 4-20mA signal at 20.5mA or thereabouts while the HART signal could conceivably report a reliable 0.40 bar value.  The current ratio tells you that the HART signal is valid but the analog is not ? ? ? 

 

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