Davis VP2 vs Weatherflow comparison

I did a short initial comparison report between my brand new WeatherFlow station and Davis VP2.
Keep in mind:

  • Sky and Davis anemometer are mounted on the same pole, side by side
  • Air is located in the stevenson screen about 1 meter (3 ft) from Davis sensors (standalone)
  • Data from Davis VP2 is continuously stored by Weather Display
  • Data from WeatherFlow was downloaded from WF servers later on
  • Time axis is aligned to minutes
  • There was a (thunder)shower event that included low gusts (20 km/h)

See attached plots. I think they both match pretty well. However, after the rain (it was 8pm and dark), the Sky started reporting wind which did not exist. Even now (morning) it is still reporting wind. This is an issue which I don’t understand and would like to hear some opinion from community or from the @WFmarketing

03_rain_acc 01_gust_speed 02_avg_speed 04_temperature 05_humidity


not bad in terms of temperature humidity might need a calibration. i have a solution for erratic wind and dropouts . tried and tested under flash flood bursts . i will try and publish it next week with some photos.it does not involve dismantling the sky or opening it up or risk of damaging,its really simple and idea used in similar products . cost your time and about a $1 …

I think the humidity readout from WF shall be alright. My Davis hardly goes 100% but my other station does. I’m not concerned about this at all. Also, the Davis’ sensor is in its own shield and WF is in the screen.

I’m more curious about the fake wind :thinking: Personally, I don’t want that my station simply takes (or compars) data from “neighboring” stations since our territory differs from site to site vastly, especially the wind.

My report didn’t include the solar and lightning data. The solar is comparable (with some offset) and the lightning is OK. The Air logged about 60 strokes I think and my other sensor logged 70. The third sensor logged 350 but that one is more sensitive.

absolutely nearby stations can be misleading and send you down the wrong path , all sorts of different scenarios,installations.the lightning sensor in the air if you can get it in a quiet nosie free environment which is hard todo today so many electronics and leds give off spurious harmonics the led bulbs can be the worst in poorly designed circuits. the air can exceed the 40km specification ive seen it many times albeit im in a 270 deg sea view and only have a poor reception in the north direction has the main housing in the area is northerly direction… all in all the product has come a long way and slowly most teething discrepancies are getting resolved…

i dont know if you use NexStorm i had an email from the developer a day ago he has something quite interesting but as much i can say its too early to see if it materializes…but in short another piece of software and another device for the weather enthusiast community on the arisen …have a nice day…brian

Thanks for the great side-by-side data, Peter! Having a Davis VP2 co-located with your WeatherFlow station produces some great data. Wish you had been part of the field test!

Your rain graph actually highlights something interesting: the SKY is a lot more sensitive than a tipping bucket. Note how, even in this relatively strong rain event, the start of rain happens significantly earlier by SKY. In lighter rain this is even more extreme, since the Davis tipper needs 0.1 mm (or 0.01 inch) before you know it’s raining, but the SKY will report much lower rain amounts - it will actually let you know (via the “rain start” event) within 5 seconds of rain starting. This higher sensitivity is also what makes the SKY’s curve so much smoother than the Davis’s. It looks like the total accumulation is a bit low with the out-of-the box calibration, but that will improve when we roll out the rain component of our CL system (real soon now!).

Your wind graphs provide a great case study too: At these low speeds it’s easy to see how the Davis reports very discrete gust values: 0.0, 1.6, 3.2, 4.8… km/h (0, 1, 2, 3… mph). That’s because the gusts are measured by counting the number of times the cups spin over a fixed time period. The resolution of the Davis is 1.6 km/h (1.0 mph) - the Davis can’t measure gusts anywhere between multiples of that value. The SKY, on the other hand (thanks to the fact that sound waves move much faster than spinning cups) has 10 times higher resolution: 0.16 km/h (0.1 mph).

As for the SKY reporting wind when the Davis did not, there are two possibilities. First, as @weather34 alludes to, when there’s rain (or anything that’s not air) on the reflective plate, that can cause wind readings to be less accurate. SKY has filter algorithms that attempt to correct wind data when it thinks there may be rain (or any other object) in the path of the ultrasonic sensor signal. These filters are not perfect, but they do a pretty good job. However, after looking over your SKY’s debug data during this period, we can see that these filters were NOT in effect. That is, the filters had no effect on the wind values from your SKY during this period.

The other possibility has to do with physics. A new Davis anemometer has a cut-in speed of 3 km/h, thanks to the inertia and static friction inherent in the spinning cups. That cut-in speed increases with age as the bearings wear and corrode. On the other hand, the ultrasonic anemometer in the SKY does not have any inertia or bearings that need to be overcome in order to detect wind (or bearings to wear and corrode). They do have a noise floor that defines the cut-in speed which varies by device, but this is in the neighborhood of 1-2 km/h.

The wind speeds shown in your analysis period are very light. In fact, many of the 3-sec readings report 0, but it’s very rare for outdoor wind to be truly zero for a full minute at a time. There may have been some very light wind - enough that the SKY was picking up, but not enough to get the Davis cups going. Next time the Davis reports zero and it seems like there is virtually no wind, experiment by lightning a match and watching the smoke.

PS: Note, your Davis will still report speeds lower than 3 km/h, but only as the already-spinning cups slow down.

PPS: I’m really looking forward to @weather34’s sub-$1 solution to the rain on the plate issue (even though it doesn’t look like that was the issue in this case)!!


The other thing is corrosion and cable fraying. My VP2 is in that state currently I think. I can see the cups spinning but it goes to 0.0 mph for many many hours, then seemingly breaks free (reading waaaay high) then settles down to reasonable mph readings. I really need to get a diagnostic cable from them to verify the sensor board is still ok after just shy of 10 years.

Hoping WF gets their auto-cal stuff enabled so I can believe the wind/rain/solar readings real soon now.

I’m getting good agreement vs. Davis and little breadboard sensor IoT things I cooked up with temperature/humidity.