[quote author=nikoshepherd
But it’s not as cute as a Cambell Stokes http://www.russell-scientific.co.uk/meteorology/campbell_stokes_sunshine_recorder.html
But it will still produce a curve, just with shallower gradients?
BTW the VP2 is serial…but I’m using a serial to USB converter because the PC only has a single COM port and I need two ports.
That was my opinion a few posts back, I don’t think he is taking the atmosphere into account.
[quote author=nikoshepherd
It will also depend on the construction of the sensor and sensor housing. The sensor may be 3D or if it is horizontal it may be mounted under a lens which focuses any sunlight at most angles onto the sensor. I’ve not seen the VP2 sensors, but the docs suggest they have diffusers which I assume are used to help get light at low angles to land on the sensor?
Yes, no lens, it has a horizontal diffuser, and they expose a little of the vertical edge to enhance the performance at very low angles where the light will be bouncing off the top surface. There’s a good write up on this type of design here http://www.for.gov.bc.ca/hfd/pubs/Docs/Wp/Wp53.pdf
[quote author=nikoshepherd
[quote author=daveq link=topic=13090.msg101306#msg101306 date=1134846905]
You’re correct.
You do. The straight line is effectively the inverse of cloud cover (although other things like pollution could also affect the reading). The maximum number changes throughout the day and if you sensor is receiving the maximum (or very near maximum) then the sky is clear.
I’m glad we all meant the same thing once we worked out what we were saying :lol:
Best of luck with your mounting Chris, nearly got blown off my ladder yesterday with a 30 mph+ gust 
I’ve not used a UV sensor for insolation, but when doing UV emission spectroscopy our apparatus utilised quartz because glass is a bigger UV absorber.( Have never seen a VP UV detector, so have no idea about it’s structure. )
Whew! Thought I was really being dense. I’ve been on a 5 hour road trip and haven’t thought about anything else since my last post. Glad you guys were able to work it out. 
Thanks for the discussion.
–Dave
Derivatives can be tricky to get your head round…comparing a maximum expected value (following a curve) with a maximum measured value (following a curve) can give a straight line.
However, it looks like the VP solar sensors are designed to try to give a reasonably flat response for a large range of angles of incidence, so they’re really measuring atmospheric effects for most of the day.
I have the 1-wire from Hobby Boards and it appears to be very sensitive but the bell curve is pronounced due to the design of the sensor, I believe. (Not due to the Hobby Board’s board, BTW.) Can’t speak to the VP sensors. But… I clearly (pun intended) understand about the ‘reasonably flat response for a large range of angles of incidence’. I think my saying 100% is what caused all the discussion, because in practice it would never be 100% due to the atmoshpere or any other attenuation.
I don’t use it for anything important but for ‘fun’. It is really interesting to run the mouse over the graph and see the clouds corresponding to the graph peaks and dips. Just another enjoyable part of WD and weather.
–Dave
If the sky was perfectly clear from sunrise to sunset the curve (of watts/meter sq) from the Davis sensor will be a sine curve.
I’m one step nearer to ‘VP2 go’. I’ve spent the afternoon putting up a wall bracket and an 8ft metal pole. The ISS is now mounted on it.
Next job is to get an aerial rigger to replace my WMR928 anemometer with the VP2 one.
Then it’s the big job…swap over from the WMR928 to the VP2 in WD. I might need some clues along the way there!
if you didn’t do it already I suggest testing the anemometer before it gets installed on the roof 
When I swapped from a LaCrosse to VP2 all I had to do was show it the correct comm port and change the station type. It was very easy from a WD standpoint.
–Dave
The anemometer did work when I put it together, but I’ll try it again before it goes up in the sky.
The kind of things I’m thinking about with the swap over are (and these are to remind me as well!)…
- With the faster wind speed updates I could tweak my clientraw update frequency
- I’ve got some extra sensors with the WMR928 which don’t have equivalents on the VP2
- The WMR928 needs a big baromete offset which will need removing.
- I’ll need to make sure the VP2 is using the same barometer values (local v SLP) in WD as the OS station
- The COM port will need to change
- I’ll have to work out how to enable the VP2 data logger
- I can presumably grab the VP2 forecast text now and make it available on my web page
- My web page says the data is from a WMR928
- Probably some more things!
One word - “backup”