Tech tip: Datalogger Scans Campbell Update 4th Quarter 2015

Does summertime make you think of skipping? Skipping stones on a cool lake, skipping rope with friends, or skipping through cool grass in your bare feet? Ah, if only datalogging was so carefree.

Your datalogger’s program runs on a precise interval called a Scan. If your scan interval is set to five seconds then your datalogger will wake up and run its program every five seconds, exactly five seconds, synchronized to the datalogger’s clock. What happens if your program takes longer to run than five seconds? The datalogger will complete the instructions in its program then wait until the next scan to run again. It skips the scan it missed the start of. In that instance it will be ten seconds between scans.

Your datalogger’s Status table counts these skipped scans. Looking at the Station Status Summary you can see Skipped Scans and Skipped System Scans.

Skipped Scans

You’ll notice that these skipped scans occur under the Program Errors heading. Skipped scans result in skipped measurements, which affects how much data is included in your output processing, such as Average(), Maximum() and Minimum() calculations. Particular attention should also be paid to Pulse() measurements configured for frequency output.

When possible, adjustments to your datalogger program can eliminate or reduce skips. If the datalogger is running in sequential mode, look for unnecessary instructions or programmer-adjustable delays. When those cannot be reduced, the program scan interval should be increased. For programs running in pipeline mode, an increase to the number of scan buffers (using the Scan() instruction’s BufferOption) should help.

Skipped System Scans

Your Campbell Scientific datalogger automatically updates its calibration factors in the system scan. When the system scan skips, the datalogger will try to repeat that step of the calibration process next time around. This simply extends calibration time. The LastSystemScan in the Status Table reports the time of the last automatic calibration.

Don’t skip taking a look at your datalogger’s Status table for skipped scans.

Tip Tip Hooray

P.S. See also the "How to Prevent Skipped Scans and a Sunburn" blog article.