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DeltaV Historian data definition

The data historian on our system appears to have data compression facilities that have been haphazardly implemented. I am used to seeing data collection of each tag being defined with items such as update time, deadbanding of the signal, smoothing of the signal etc. What I want for a small number of tags that I am investigating to have as high a definition as possible for the duration of my investigation, i.e. the minimum possible update time with the data point being read at each update with no deadbanding and no smoothing or interpolating of data.  So far I have not been able to find out how to do this. 

The symptom that I have is when I look at the tag data on a HMI screen, the data has high definition, but when it is historised, straight lines extending over long periods between data points are evident.

Any suggestions?

Regards

Jeff Richards

jeff richards

• 14 Aug 2018 9:18 AM
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6 Replies

• James Beall

  

  • 14 Aug 2018 12:17 PM

  Jeff,
  The settings for each point's history collection, including recording time and compression, is set in each control module. You can look at this in Control Studio or simply right click the module and select "History Collection". You can set the sampling rate and compression. Note that the compression setting is in engineering units of the variable. You only have to download the Continuous Historian not the individual modules. Hope this helps.
  James

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• jeff richards

  

  • 14 Aug 2018 12:42 PM

  In reply to James Beall:

  James,
  thanks for the reply. I assume by looking in the control module, that you mean to look at this is in the data historian itself and not in the DeltaV PID controller block?
  
  Regards
  
  Jeff Richards

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• James Beall

  

  • 14 Aug 2018 2:23 PM

  In reply to jeff richards:

  Jeff,
  Actually you look at the Control Module itself for the parameters of THAT module that are to be historized. When you have these like you want, then you go to DeltaV node (e.g. Application Station #1, APP01) that has the Continuous Historian being used, and download the Continuous Historian.
  James

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• Andre Dicaire

  

  • 14 Aug 2018 3:22 PM

  In reply to James Beall:

  Jeff,
  As James indicates you can view history collection defined for each module, but you can also view it by Plant Area and for the Historian as a whole. When you add history collection, it is done as part of the module, that creates the overall History collection for the Plant area holding those modules. When you assign a Plant area to a Historian, all defined history tags in the area get assigned to that Historian. With the Plant area assigned, when you add new history tags in a module or modify them, the History sever(s) will be updated with that information when you download the server's Continuous Historian.
  
  But to view all defined history tags on the server, right click on the Continuous Historian container in Explorer to view all parameters, there defined update rate, compression Enable and compression deadband.
  
  DeltaV does not use a communication deadband, so all values are communicated on any change. Some systems uses a communication deadband to minimize data reporting, especially for noisy PV's.
  
  With Compression enabled and a deadband of 0, the compression will eliminate storage of points on a straight line. the max time between values forces a value to be stored at that frequency even if the line is flat. You lose no data accuracy as any change will store a value.
  
  The fastest data collection is the slower of the Module execution time and the history tag sample time. Data can only change when the module executes. The fastest sample time is 1 second, so modules running at 500 ms or faster will see 1 second sampling in the Historian.
  
  In PHV, the trend retrieves data for each defined tag from the Historian if it exists, but the real time data for all the tags is sampled from a local OPC server on each station. This allows historized and non historized tags to be plotted together. You adjust the sample rate of the Trend in PHV. When the Historian tag is not properly configured, i.e. too slow a sample rate, too wide a compression and too long a Max Time, the run time trend in PHV can look quite different as you indicate. You can also see the opposite, when the trend update rate is set too slow, but history collection is correct, the historized data looks good but the trend real time data looks choppy or saw toothed.
  
  Anyway, if you disable compression, values are stored for every sample, and this can increase the size of the history datasets. This will affect the frequency of dataset rollover and maintenance of Extended Datasets. Doing so for a few tags is not a concern.

  Andre Dicaire

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• Casey Houchens

  

  • 14 Aug 2018 4:27 PM

  In reply to Andre Dicaire:

  Is there a way to dynamically change the history collection deviation or sampling rate based on process events?

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• Andre Dicaire

  

  • 14 Aug 2018 5:02 PM

  In reply to Casey Houchens:

  No. this is an off line change requiring download to the Historian.

  Andre Dicaire

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