4 Replies Latest reply on Sep 11, 2018 10:50 AM by Rick Davin

    CompDev and other attributes Standard

    AnantU
      compressingcompdevcompmaxcompmincompdevpercentexcdevexcmaxexcminexcdevpercent

       

      Hi,

      I recently took over our organizations PI systems operations. I noticed that there is practically no compression of the data we collect 24/7

      This is not a requirement and I want to understand more about it. I read online help and watch few videos.

      These are the devices we general use to get our data from.

      Thermocouples for temperature

      Flow meters

      Pressure Measurements

      Relays & Isolation valves status ( 0 or 1)

      PID Loops set point and CV data in eng units or %

       

      General guides would be ok, i don't need exact numbers. Some pointers or thumb-rules is what I am looking for so I can standardize the process here.

      What would be typical compression settings for above devices to collect meaningful data and not overload the PI data server unnecessarily.

      Thank you

        • Re: CompDev and other attributes Standard
          tramachandran

          Please refer:

          3226OSI8 - What are recommended Exception and Compression settings

          Compression Guidelines for PI Server

           

          OSIsoft recommends setting exception deviation to a value slightly less than the precision of the instrument. Another guideline is to set Exception Deviation (ExcDev) to half the Compression Deviation (CompDev).

          A PI Data Archive installation with default compression values is appropriate for most cases. Setting compression requires you to apply your process knowledge about the nature of the signal. One compression-deviation specification does not work for all measurements. Consider sensor type, instrument accuracy, and so on.

          2 of 2 people found this helpful
          • Re: CompDev and other attributes Standard
            Lal_Babu_Shaik

            Hi Anant

             

            Thyagarajan Ramachandran rightly pointed out all important PI compression settings. If you not aware compression settings to set then i would recommend to set compression "ON" and compmax=28800(8hr) and excmax=600(10 min) and CompDev=0 and ExCDev=0 ( this will eliminate same value). This will help if you are receiving high frequency data. 3rd party tools (PITuningtools) will help by analyzing  your historic data else you can set Compression and Exception based on tag data analysis.

             

            Thanks,

            Lal

            • Re: CompDev and other attributes Standard
              Kenneth_Barber

              CompressionInsight is another 3rd-party program that analyzes historical data to recommend exception and compression settings. I recommend it.

               

              To add to what was said above, it is probably best to leave excmin and compmin at their default value of 0. I can't think of any good reason for them to not be 0. By my understanding, excmin is the minimum amount of time that must pass before another value is stored in the PI Point, regardless of how much the value deviates from the previous value. A similar thing can be said about excmax, compmin, and compmax.

               

              For digital PI Points (that is, PI Points with point type = Digital), which record values such as "On" or "Off", you cannot adjust the exception deviation (excdev or excdevpercent) or the compression deviation (compdev or compdevpercent), since they don't make sense for digital PI Points. I actually like to turn compression off (compressing = off = 0) for digital PI Points and just tweak the excmax.

               

              Turning off compression means that none of compdev, compdevpercent, compmin, or compmax will have any effect on what data is stored or not stored. There is no equivalent setting for turning off exception (see this post). You just have to set excdev = excdevpercent = excmin = excmax = 0.

               

              Be careful when using PI Builder to edit exception and compression deviation. See this post. excmin, excmax, compmin, and compmax are specified in seconds. excdev and compdev are in the same unit as the values being stored in the PI Point ("engineering units").

               

              Also be careful when editing the span of PI Points. See this post. excdevpercent are compdevpercent are in units of % of the span. Think of "zero" as the minimum expected value and "span" as the difference between the minimum and maximum expected values.

               

              In addition to exception and compression, you might also want to consider modifying the scan rate on your PI Interfaces and PI Connectors.

              1 of 1 people found this helpful
                • Re: CompDev and other attributes Standard
                  Rick Davin

                  Hi Kenneth,

                   

                  By my understanding, excmin is the minimum amount of time that must pass before another value is stored in the PI Point

                   

                  The above is not how the exception deviation related point attributes work.  Exception deviation is performed on an interface to limit what is sent from that interface to the PI Snapshot subsystem.  Thus, it provides a benefit of reducing network traffic.  The instant a value arrives at the PI Snapshot subsystem is when the compression algorithm kicks in.