Foundation Fieldbus - Is there a range? 1

[roydm]

We are all used to setting the 4-20 mA output range of a transmitter.

I purchase lots of fieldbus transmitters but unfortunately don't get to play with them in the field

Example:
I have a pressure transmitter with a cell capable of measuring 0 - 100 psi
I only need to measure up to 50 psi

I maintain if I go to the DCS and select 0 - 50 it only sets the range of faceplates in the DCS and the transmitter is still capable of measuring 0 - 100. I also believe if the transmitter sees 75 psi it will communicate to the DCS that the pressure is 75 and no matter what you punch in the DCS the accuracy will not change because it's digital.

I base this on observing older 4-20 transmitters where if you have a local digital display it can read values well outside the 4-20 mA range.

As I understand it the transmitter sends a digital number in real engineering units like 45.000 psi or 3.000 bar whatever the DCS tells it to, is this correct?

I have seen a lot of articles on FF but none answered this simple question, at least not in language I could understand.

What do others think?

 

[djs]

It really depends on the transmitter, not the field bus system. You will need to consult the user manual for the transmitter in question. For example some Siemens/Milltronics units send a scaled integer value over the fieldbus. However most transmitter do send the PV as a floating point value in engineering units.

 

[danw2]

Roy, I suspect that your assessment is correct, I read a Jonas Berge (a fieldbus guru now at Rosemount) forum post (Linkedin group, maybe?) within the past 3 months on 'calibrating fieldbus instruments'. He made mention of this factoid because the word 'calibration' is so frequently used for the task of 'configuring the range', as opposed to 'determining the uncertainty with respect to a standard'.

His point was that there is no 'range' on a digital instrument, it reports what the value it has, and lets the receiver deal with scaling the value on HMI bar graphs, historical trends, proportional band, etc.

Both of Honeywell's wireless field transmitter systems (XYR5000/6000) do exactly that - report the process value, with no 'range' associated outside of the inherent measuring limitation of the instrument.

If I come across that Berg post, I'll post a link.

 

[IRstuff]

"His point was that there is no 'range' on a digital instrument"

That's not strictly true of all instruments. For example, on most digital multimeters (DMMs), each range corresponds to a different component selection or gain selection, since at some point in the instrument, it is still analog. Therefore, a range change may infer a change in a gain amplifier so that the desired signal range occupies the entire ADC range, i.e, for your example, 100 psi might digitize as FFFF, but a range change might produce a 2x gain change that causes 50 psi to digitize as FFFF and 100 psi would the digitize to "Overflow," which results in no usable measurement.

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[roydm]

IRstuff,
Sorry you miss-understood, I was referring to process control instruments, specifically those with Foundation Fieldbus communications.
Roy

 

[IRstuff]

I was merely pointing out that unless you have information about the sensor that you haven't shared, there is no knowing whether changing the range doesn't prevent reading of over-range values.

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[djs]

The range and the format of the data on the buss is entirely dependent on the transmitter. Therefor the confusion on the part of the OP. The information he is looking for (format of the data on the buss) is not found in the fieldbus documentation, but rather in the instrument documentation.

Danw2 I have seen one instrument that puts out an integer value of 0 to 20,000 on the buss. The receiver must then scale that value to engineering units in a manner similar to an analog 4-20ma instrument. Rare but they do exist. The moral is there are no absolutes.

 

[roydm]

IRsuff,
We are probably saying the same thing, I will give you an example
A Rosemount 3051 pressure transmitter range code 1 has a cell that measures 0 - 2 bar (0 - 30 psi)
In the analog version you can pick a portion of the range say 0 - 500 mbar to give you the output signal between 4 and 20 mA
So the analog output now has 4 x the resolution.
The transmitter will still read from 0 - 2 bar but for the operator it will top out at just over 500 mbar. If you were to tap into the signal all you would see is 500 mbar

In the FF version I suspect the transmitter doesn't have the equivalent of changing ranges.
I suspect there's no advantage resolution wise in selecting a different range
Does the transmitter send out a raw value or is it in engineering units, i.e. does the number change if you change units from mbar to psi or is it some unit-less number converted to EU inside the DCS (djs sort of hinted that they are in EU)
I was trying to figure out if the signal it sends is modified in any way when you change the Upper and Lower limits at the DCS
or does that just effect the range of such as a bar graph.
If you had the same variable displayed as a digital value beside the bar graph would it top out at 500 or go all the way to 2,000

I wish I could find a good explanation of this simple topic.

Thanks
Roy

 

[IRstuff]

I would think that FF does not impose limitations on your ability to command your instruments; that would be an absurdly unwieldy standard, since any change to hardware would require a new version of the spec. Therefore, your transmitter will do what it was designed to do, which may include amping up a signal to get more resolution over a smaller range.

"If you had the same variable displayed as a digital value beside the bar graph would it top out at 500 or go all the way to 2,000"

Seems to me that's dependent on what you've programmed your system to do, and what your hardware and software are capable of doing. If it were me, a range change would force a gain change, which would be reflected in an digital and/or bar graph. Otherwise, what would be the point of having a range change?

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[roydm]

IRstuff,
But in the real world with real readily available instruments I don't think you have any control over the gain.
I don't believe a modern analog transmitter changes the gain of the front end either, just the portion of the cell span it looks at
I may be wrong, that's the whole point of asking.

Roy

 

[IRstuff]

I'm not saying that you have control over the gain; I'm saying that a range change without a gain change is useless, so why would there be such a feature? When I change the range on a DMM, I expect to see more resolution if I go down, but with a smaller range, or a larger range if I go up, but with less resolution. I'm not saying that's your specific example, but it just sounds silly to do that.

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[danw2]

I couldn't find that Jonas Berge forum post that I'd read, so I went to the EDDL web site and poked around. Berge authors a lot of the EDDL info on the EDDL.org website. (EDDL is the technology used by device manufacturers to define how the system shall display the device information and functions to the technician; it's a spec and format for the file associated with a specific intelligent field instrument/device).

I found a white paper, Intelligent Device Management Tutorial: Calibration,
[URL unfurl="true"]http://www.eddl.org/SiteCollectionDocuments/TechPapers/wp_EDDL%20calibrationtrim.pdf[/url]
which reviews classic analog output 4-20mA range setting, then continues

"The analog 4-20 mA output of a transmitter is limited to the LRV to URV range. Thus the analog output does not benefit from the full LSL to USL capability of the sensor. However, FOUNDATION fieldbus, PROFIBUS, and WirelessHART transmitters as well as the digital output of 4-20 mA/HART transmitters are not limited to the LRV to URV range, but can benefit from full LSL to USL capability of the sensor."

Further on,

"Range setting is only applicable to transmitter with 4-20 mA analog output. That is, for 4-20 mA/HART transmitters, not for pure digital solutions like FOUNDATION fieldbus (FF) or WirelessHART transmitters. The reason being that FF and WirelessHART transmitters has no 4-20 mA analog output, therefore there is no need to set 4 mA and 20 mA range points. For 4-20 mA systems the range is set in both the transmitter and controller. For FF and PROFIBUS the range is set in the controller, and need not be set in the transmitter which can lead to some confusion for beginners.

The only exception for FF, WirelessHART, and PROFIBUS transmitters may be for differential pressure (DP) flow and level measurement where the end-points of the DP scale (e.g. 0-250 inH2O in XD_SCALE) and corresponding flow or level scale (e.g. 0-400 bbl/day in OUT_SCALE). This also enables DP transmitters to locally indicate in flow or level units.

FF and PROFIBUS devices have provision for setting a range in the transmitter even though it may not used for the application.
<snip>
The output of both the FF transducer block and the AI function block is engineering unit. For most applications there is no need to set range in either block in order to get the PV. However, many systems use the range in the FF transmitter AI block to scale the faceplates bargraphs. A narrower range may optionally be set to increase the resolution of the faceplate bargraph. If a range is set in the AI block, the percentage of range can be seen from the FIELD_VAL parameter." (pages 3-5)

That says to me that an FF field transmitter can report the PV within its full LSL-USL range, regardless of the range specified by the DCS side range.

The acronyms are
LRV = 4.0mA
URV = 20.0mA
LSL - Lower Sensor limit of the sensor/transmitter
URL - Upper Sensor limit of the sensor/transmitter

Honeywell uses the acronyms LRL and URL as Lower Range Limit and Upper Range Limit in the same sense as LSL and URL.

 

[roydm]

Thanks guys, I think we are in agreement that the FF transmitter doesn't have a Range as we think in terms of an analog instruments.
So you can't improve the accuracy or resolution by changing some parameter.

The reason this came up is a client asked us to calibrate a FF transmitter with a lower span for better accuracy.