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Davis VP2 Relay Device

For discussion of DIY weather equipment - sensors, accessories, improvements to existing kit etc
Herbaldew
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Joined: Sun Mar 12, 2017 8:33 pm
Weather Station: VP2+
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Location: US Mid-Atlantic

Re: Davis VP2 Relay Device

Postby Herbaldew » Wed Aug 23, 2017 3:34 pm

mcrossley wrote:only one question are they far enough in front of the mast so they aren't shadowed at midsummer?

At my latitude the Sun rises/sets at midsummer approximately 45 degrees either side of North.


Turns out the answer is "no". Looks like I have about the same measurements here and I have a little bump in the chart at sunrise and sunset.

How far out from mast do you have your sensors?

Looking at the photo below, wouldn't I be better of with them on the north side of the mast?

Thanks - Herb

Image
Last edited by Herbaldew on Wed Aug 23, 2017 5:24 pm, edited 1 time in total.

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mcrossley
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Re: Davis VP2 Relay Device

Postby mcrossley » Wed Aug 23, 2017 4:24 pm

Herbaldew wrote:Looking at the photo below, wouldn't I be better off mounting the sensors on the north side of the mast?


Nope! Then the sensor would be shaded every day!

As you have your sensors side-by-side they will have to further out than if they were inline with the mast. This very poor sketch shows where they need to go...
IMG_20170823_171332-01.jpeg
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Herbaldew
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Joined: Sun Mar 12, 2017 8:33 pm
Weather Station: VP2+
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Re: Davis VP2 Relay Device

Postby Herbaldew » Wed Aug 23, 2017 8:20 pm

I think this will do it - find out for sure next June ;)

Image

AllyCat
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Re: Davis VP2 Relay Device

Postby AllyCat » Thu Aug 24, 2017 8:27 am

Hi,

Herbaldew wrote: - find out for sure next June ;)


Or tell us your site Latitude and we can probably work it out now. ;)

Cheers, Alan.

Herbaldew
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Joined: Sun Mar 12, 2017 8:33 pm
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Re: Davis VP2 Relay Device

Postby Herbaldew » Thu Aug 24, 2017 1:31 pm

38 North -75 West

I'm confident it is good with an inch or two to spare. I printed out that "rose" above and taped it to a large piece of cardboard. Then drew lines like in Mark's sketch laying out my "safe zone". Then took the actual mount and laid on the cardboard sliding it out from center (actually from the where the south side of the mast would be) until the sensors where in the "SZ", then added a couple inches just to be safe.

AllyCat
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Re: Davis VP2 Relay Device

Postby AllyCat » Fri Aug 25, 2017 12:44 am

Hi,

For Latitude 38 degrees, the maximum day length appears to be 14.75 hours or about 220 degrees (i.e. from 70 to 290 degrees azimuth). So it looks as if you should be alright, but may need to also consider the diameter of the mast and sensor, and possibly reflections from the mast.

BTW a maximum of 18 hours daylight / 270 degrees appears to correspond with latitude 57 degrees, which is about mid-Scotland for us and Canada for you.

Cheers, Alan.

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mcrossley
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Re: Davis VP2 Relay Device

Postby mcrossley » Fri Aug 25, 2017 9:17 am

AllyCat wrote:BTW a maximum of 18 hours daylight / 270 degrees appears to correspond with latitude 57 degrees, which is about mid-Scotland for us and Canada for you.
<pedantMode>Taking refraction into account 54.5N gives 270 degrees, but only 17h16m of daylight?</pedantMode>

Herbaldew
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Joined: Sun Mar 12, 2017 8:33 pm
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Location: US Mid-Atlantic

Re: Davis VP2 Relay Device

Postby Herbaldew » Fri Aug 25, 2017 3:15 pm

Herbaldew wrote:Oops! I had a max wind of 253 MPH today.


Phantom gust number two last night - at least it was slower at a mere 190 MPH this time lol

AllyCat
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Re: Davis VP2 Relay Device

Postby AllyCat » Fri Aug 25, 2017 3:43 pm

Hi,

mcrossley wrote: <pedantMode>Taking refraction into account

Is it actually refraction, or mainly due to the "complexities" of the 3D geometry? I believe the (atmospheric) refraction is around 0.6 degree, which I wouldn't have thought could account for more than a few minutes of day length. But I realised when writing my post above, that the calculation of the azimuth for any given time might not be as simple as implied.

Although I have written (or to be honest adapted) a program to calculate sunrise and sunset times for any location and date, I've not yet extended it to calculate the sun's azimuth for any given time. One reason for that is the "model" calculation (written by a University lecturer) that I originally adapted, proved to be demonstrably wrong! So I then planned to "borrow" the nicely-documented calculation in your "sun plot" application, but haven't got around to doing that yet.

Cheers, Alan.

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mcrossley
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Re: Davis VP2 Relay Device

Postby mcrossley » Fri Aug 25, 2017 5:06 pm

Hmm, fag packet calculation, at mid-summer for 54.5N, refraction extends the day approximately 22 mins, the azimuth angle by ~ 5.5 degrees (11 minutes, and 2.25 degrees at both sunrise and sunset). Is that enough I wonder?


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