What shape do you think the daily solar plot would be if the sensor tracked the sun? Maybe I’m misunderstanding what you’re saying. It seems like you’re saying that on a clear day with a sensor tracking the sun you’d get a straight line rather than a curve? Experience suggests that’s not the case. On a sunny day stand in the sun at 0800, 1200 and 1600. You will feel much hotter in the sun at 1200 than at 0800 or 1600 because the solar radiation for the day is at it maximum (approx) at that time.
[quote author=daveq link=topic=13090.msg101228#msg101228 date=1134830910]
Just a thought on my part and could be absolutely wrong.
Right, you would get a different curve due to the changing attenuation through the atmosphere and then some other effects at really low angles.
I’ve made a bit more progress with the VP2 now. I’ve added the USB to serial adapter to the WD PC, loaded up WL and connected the console to the PC. I’ve also put the ISS outside (just sat on a wall for now further away than it will be when it’s mounted) and I’m getting a decent signal from it.
This is about as far as I can go without climbing ladders and drilling walls to mount it properly. I’m going to tray to get that done on Monday or Tuesday.
One final question…I’ve put my height ASL into the console. Does this mean that the console displays SLP rather than local pressure? WL and the console show the same value, but did I read somewhere than WD sees the raw local pressure rather than SLP? The pressure shown by my WMR928 is about 2mb lower than the VP2 and that’s the kind of difference I’d expect between SLP and my local pressure (I’m at around 20m ASL).
Yes, the console will compensate based on the altitude you plugged in. WD gets the raw value and does its own compensation.
[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.