Vantage Vue wind speed

[Skywatcher_ne]

I am running Cumulus 1.8.9 and have been noticing that all of my wind gust data is usually off by 1 or 2 mph from what my Vantage Vue console is displaying vs what is getting uploaded to my web site. For example my Vue console will display last 10 min wind gust of 33mph but my web data is displaying 32mph. This happens on all web data, e.g. Weather Underground and my Cumulus web site. I have verified this by looking at my realtime.txt file. Not a big deal but was wondering why this would be.

Dave

[steve]

It's possible that the station is updating its data faster than Cumulus and the Davis DLL is reading it. There is a built in delay between reads which is mainly for Virtual VP users (as that returns the data immediately, unlike a real station). I've set the default value to 500 ms, but it's probably safe to set it to zero, assuming you're not using Virtual VP. There is a built-in delay anyway between the DLL and the station, so setting it to zero shouldn't cause the thread to sit in a tight loop and use excessive CPU. You should be able to tell if this is what's happening by carefully watching the console updating and the Cumulus 'last read' time in the bottom right hand corner (or just watch and compare the 'latest' wind speeds). See if occasionally the console updates twice between Cumulus updates.

Or you could just go ahead and do the edit anyway:

With Cumulus stopped, edit cumulus.ini and enter a line in the [Station] section

VP2SleepInterval

The other possibility is that it's simply a rounding error, but you would only expect a 1 mph difference in that case.

[Skywatcher_ne]

I tried your suggestion and the last data read went from every two seconds to every second. I am still seeing a 1 mph difference most of the time between the Vue console and Cumulus. CPU usuage went up only a couple of percent. Am I right in assuming that every event that is displayed on the Vue console is written to the data logger? If so doesn't Cumulus read every event from the data logger? Like I said in my first post it's not a big deal. Just trying to understand a little more about how these things work.

Dave

[steve]

I tried your suggestion and the last data read went from every two seconds to every second. I am still seeing a 1 mph difference most of the time between the Vue console and Cumulus.

The 'last 10 minute wind gust' on the console must be something new introduced for the Vue, as the VP2 doesn't have it. Cumulus calculates it for itself, so I think the difference is down to rounding errors.

Am I right in assuming that every event that is displayed on the Vue console is written to the data logger?

No, that would result in the data logger filling up very quickly. It creates an entry in the data logger at the interval you have configured in Weatherlink - minimum 1 minute, and I think the default if you haven't set it is 10 minutes.

If so doesn't Cumulus read every event from the data logger?

No, while running it reads data 'live' via function calls provided for that purpose, and it tries to read every update. If it's updating once a second it must be getting every reading. The Vue must operate differently from the VP2; with a sleep interval of zero, the VP2 returns data every 2 to 3 seconds; this is the actual update interval of the station. For some reason the Vue must return its current data immediately, without waiting for the next update, which is how the VP2 appears to work. Cumulus only uses the data from the logger when it starts up, to fill in any missing data from when it last ran.