erratic wind values

[opticsman0127]

Since installing Cumulus about 2 weeks ago it has given me a great deal of fun. I think everything is now sorted in terms of educating me, primarily by Steve but, there is still one problem which baffles me completely. I have searched all previous posts and can't find where anyone has had this peoblem.

Here it is:

I have Cumulus set up as follows:

1. Calculate 10 minute average ticked on station settings. Tried it unticked also.
2. Plot latest gust ticked on display settings
3. Wind calibration, speed and gust set at 1.2 as my anemometer is a bit short of 30 foot above ground.
4. Vantage Pro 2 console is set to 5 minute update as is Cumulus

The problem is that if Cumulus is switched off for any length of time and then switched back on, the wind values jump up and then, over a few minutes, drop back down. The 'base' level is our actual wind average, the higher, peaked levels are significantly higher. I have attached a screen shot where I have carried out this test over the last 3 hours or so. Also, the frequency of the plotted points changes from one switch off to the next. This is definitely a Cumulus anomoly because I have downloaded the data in Davis Weatherlink software and the values seem to be correct and a relatively 'flat' line.

Can anyone solve the problem please.

Barry

[steve]

The parameters that are available to Cumulus are different depending on whether the data is coming from the logger. When using the data from the logger, the nearest thing corresponding to 'latest gust' is 'high wind speed', which is the peak wind speed over the logger interval (5 minutes in your case), so that's what I use. In normal running, the gust is simply the wind speed over the previous three seconds or so. So there are inevitably differences between what is logged and hence plotted when using logger data compared to 'live' data.

The plot frequency is different for similar reasons. When running, points are plotted at 1-minute intervals. When plotting data at start up, the frequency is that of the logged data (whether this came from a previous 'live' run or from the station logger).

[opticsman0127]

Thanks for the concise reply Steve. I think I follow it, receiving logged gust data from the console gives different results from receieving live data from the console?

I can see the logic there but, would this also give large discrepency readings for average wind speeds. When downloading the same set of logged data into both Weatherlink and Cumulus, the wind speed values are considerably different.

Barry

[steve]

Basically, the VP2 doesn't actually have anything called a "gust". Cumulus is trying to fit all of the various wind readings that it gets from different types of station, into the same display. Weatherlink has it easy because it only drives Davis stations, and it only displays one wind value - the "speed". Cumulus doesn't have the same data available to calculate the various types of "speed" that it displays, when the data comes from the logger, which is why the calculations come out differently.

If you prefer the way Weatherlink handles the wind speed, I could put an option in to do it the same way in Cumulus. You would then get only one gust/speed/average value (and hence just one line on the graphs) that would be read directly from the station and would be consistent whether it came from the logger or from live data.

[opticsman0127]

I understand your dilemma trying to make Cumulus equally receptive to every weather station data coming to it. Your suggestion to add the extra function is very kind but, I would hate to say yes and then decide against using it after a while because the separate values for 'speed' and 'gust' are both individual and interesting. Having a singular set of data plots might reduce the value of the information.

Having said that, if its very little trouble, it would be interesting to see what things look like running the option.

Forgot to say thanks for adding the feature of widening the legend lines on the graphs, piece of cake now reading the plots.

Barry

[steve]

It's not a lot of work. Once I've got 1.8.8 properly out of beta, I can look at putting it into the next round of changes.

[gemini06720]

Steve, if I understand correctly you reply above (and please correct me if I am wrong), the Davis Vantage Pro station only provide wind readings - it does not know what a gust is.

My understanding of a gust is a burst of wind that only last for a few seconds. Can you explain why gust is as important if not more important than just the wind readings.

If you prefer the way Weatherlink handles the wind speed, I could put an option in to do it the same way in Cumulus. You would then get only one gust/speed/average value (and hence just one line on the graphs) that would be read directly from the station and would be consistent whether it came from the logger or from live data.

Once again, please enlighten me. The Davis Vantage Pro does not produce gust information but, Cumulus using the data from the Davis Vantage Pro, produces a gust value! You must have something in Cumulus that does the transformation from wind speed to gust speed...

[steve]

Yes, a gust is a short burst of wind, typically measured over a period of 3 seconds, and this just happens to be what the VP2 supplies, except that it refers to it as the 'speed'. So Cumulus calls this the 'latest' reading, and the peak value of this over the last 10 minutes Cumulus calls the 'gust', in an attempt to give a feeling of the speeds that the wind is 'gusting to'. Quoting a 'gusting to' speed as well as the average speed just adds more information to how strong the wind is.

The other speed that Cumulus displays is the average speed. This is either calculated by the console from the last 10 minutes worth of the 3-second speed, or calculated by Cumulus in exactly the same way. It's this figure that is missing from the VP2 logger data, which is why Cumulus fudges things a bit when using logger data and all the figures come out differently.

[gemini06720]

Steve, thank you for the explanation.

...a gust is a short burst of wind, typically measured over a period of 3 seconds, and this just happens to be what the VP2 supplies, except that it refers to it as the 'speed'...

But, does the display of the vantage Pro2 console not show the wind speed?

Is there a place where someone could get more information on all the data that is exported from a Vantage Pro2 console (through either the serial, USB or IP datalogger)?

[opticsman0127]

Steve, If its no trouble, please go ahead, it may throw up some interesting results.

Barry

[steve]

But, does the display of the vantage Pro2 console not show the wind speed?

The 'speed' that it shows is the current 3-second speed. Also, if you push the right buttons, you can get it to show you the current 10-minute average speed.

Is there a place where someone could get more information on all the data that is exported from a Vantage Pro2 console (through either the serial, USB or IP datalogger)?

Yes, it's in the "Serial Communications Reference" document, available here: http://www.davisnet.com/support/weather ... llsdk.asp

[gemini06720]

Steve, once again, thank you for the information.

And, as indicated by Barry, if it is not too much of trouble, having an option to chose between what is presently available from Cumulus and the way it is done with the WeatherLink software ... please...

[opticsman0127]

Steve,

Attached is a screen shot of the wind speed/gust graph from my station. It was produced this morning after switching on Cumulus after a night's switch off. Is this what you would have expected in terms of the general appearance of the plots, especially the 'flat' sections for wind speed.

I have taken on board your comments of yesterday and am finding it difficult to relate to this somewhat, 'block' appearance, predominantly in the midnight to 2:00 am period. If it is all connected with sampling frequencies then its something we can definitely compare if you can offer the 'Weatherlink option'. I think a comparison of your 2 differing methods will be extremely interesting.

Barry

[steve]

Attached is a screen shot of the wind speed/gust graph from my station. It was produced this morning after switching on Cumulus after a night's switch off. Is this what you would have expected in terms of the general appearance of the plots, especially the 'flat' sections for wind speed.

It's hard to say what to expect, because it depends on the 'profile' of the wind speeds at your location. I haven't been turning my PC off overnight for the last few days (I normally do) so can't compare with mine. Oh, and I use a different setting for 'plot latest gust' anyway, which has a big effect on the graph. I'll be sure to switch my PC off tonight, and set 'plot latest gust' (I already use a 5-minute logger interval) then we can compare graphs tomorrow. I've got the station calculating my 10-minute average. But my wind profile will be different anyway, because of my location.

Basically, the differences are (with the settings you said you were using):

During live running, plotted at 1-minute intervals:
"Gust" = Speed over previous 3 seconds
"Speed" = Average of 3-second speed over previous 10 minutes

With data from logger, plotted at 5-minute intervals:
"Gust" = Peak 3-second speed over previous 5 minutes
"Speed" = Average of 3-second speed over previous 5 minutes.

You can see what the difference would be with the 'weatherlink' option, if you imagine the following: There will be a single value plotted; during 'live' running, it will be what is now plotted for "Gust" and during "logger" periods, it will be what is now plotted for "Speed".

[steve]

Just to clarify (hopefully!) what I said about the live data being plotted at 1-minute intervals; this does of course only apply to data that was read 'live' during the current session. The graph that is drawn at start up may have data that was logged 'live', but is now plotted at 5-minute intervals (because it's taken from the log file). So, a summary of the plot intervals is:

Live data read during the current session: 1 minute
Live data from a previous session : Your Cumulus log interval
Logger data : Your VP2 log interval