Dear Friends,

Yokogawa defines pressure transmitter accuracy at reference condition as % of "span". Where span is calibrated range of the transmitter. For example,+/- 0.04% of span.

Emerson also defines transmitter accuracy as % of span, but defines span as "zero based span". They clarifies that only for Foundation fieldbus and wireless devices, span should be replaced by "calibrated range".

Question is what is the difference between "span" and "zero based span"? Why Emerson considers zero based span for analog 4-20 mA accuracy, but considers "span" = calibrated range for digital outputs such as fieldbus? An example of accuracy calculation to show the difference between the calculations will be useful.

 

Lets consider why they do that. Lets say you have a 0-1000 PSI unit with a 1% zero based span accuracy. That means that the reading can be + or - 10 PSI. If you use it at 900 to 1000 PSI, a range of 100 PSI, the accuracy is still + or - 10 PSI which is now 10% of your reading. In the absence of any other information as to the nature of the error, linearity, scaling, etc., you can't guarantee that it won't be +10 PSI at one end of your range and - 10 PSI at the other. So simply scaling to fit may not work. Digital conversions are typically guaranteed to be monotonic for some number of bits and depending on methods, the calibrated range can be effectively your zero based range because the scaling etc. is done in their unit and they can tweak transfer functions etc, even calibrate each count. With a lookup table, you can turn a pretty ugly curve into a straight line, provided it's repeatable. Generating such a function with analog electronics would be impractical. Also a given number is exact, that many counts is that many counts. Digital transmission tends to be all or nothing, you got the reading or you didn't. There are a great number of things that affect analog transmission so they control a much smaller part of the picture.

Regards
cww

 

Given the subject matter, the answer really requires a comprehensive discussion... and I don't feel like typing it all up.

Let's try to cover calibrated span vs. zero-based span first. What this is basically saying is the front end electronics are direct analog to digital via AD converter for the former and totally analog for the latter. Because all your calibration adjustments for the former are done in digital processing stages, the accuracy in units (% remains the same) will be directly proportional to the calibrated span. Because all your calibration adjustments for the latter are done in an analog measurement processing stage, raising the LRV has no effect on the accuracy.

FWIW, you can glean some additional accuracy info from this Honeywell white paper...

http://www.controleng.com/fileadmin...e/WP_HoneywellTransmitterAccuracy_Dec2012.pdf

 

> Dear Friends,

I have some update on this matter. This is not so complicated matter. Our basic understanding that span = calibrated range is correct. I contacted Emerson France for clarification. Here is their reply

Quote
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"The accuracy is based on the SPAN = Calibrated Range (= 4/20mA).

- The zero-based spans means: Zero = 4mA and Based span = 20mA.

- For digital transmitter (FF, Wireless): The transmitter hasn't a current output 4/20mA, the transmitter has only a digital output. As we refer the span to the 4/20mA, we can't use this word (span) for digital. Therefore, we refer the span as the calibrated range.

- Conclusion:
Emerson defines the accuracy as the same for Analog or Digital outputs:
Span = 4/20mA = Calibrated range"
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Unquote