Topic: DIY 'Rain Gauge'

[Mr Smith]

Any tips how to make a 'Rain Gauge', please :)

[Bjerreby]

I'd use a plastic funnel that empties into a translucent tube. The tube should be much smaller diameter than the funnel, as that works like an amplifier.

The trick is to mark the correct scale on the tube, and that is dependent upon the diameter of the funnel and the diameter of the tube you use. The scale (marked in mm of rain) is.....

"the diameter of the tube squared, divided by the diameter of the funnel mouth squared, multiplied by the depth of rain in the tube (mm), then take the square root of the lot".

Put mathematically, rainfall = [d^2 y /D^2]^0.5, where d = tube diameter, D = funnel diameter, and y = depth of water in the tube.  :)

[Baccy Man]

Place a container outside, you now have a rain gauge. ;)

If you want to calibrate it then the necessary calculations are given on this site.
http://bobscrafts.com/bobstuff/raingage.htm

[Mr Smith]

Bjerreby
             Excellent reply just what I was looking for(can't work the maths out),  I only left school with two 'O' Levels one in metel work and the other one in Biology, do you know anyone who wants a cat welding :),

[lushy86]

Blimey BJ - that's brainy!!  ;D ;D ;D

Lushy x

[Bjerreby]

Oh!..........thank heavens it's morning again.

I had a dream last night about the maths and the rain gauge, and I knew something wasn't right about taking the square root of d^2*y/D^2.

The fact is, if you read the rain gauge the moment it stops raining, the scale should be d^2*y/D. Here is the reason:



Most allotment owners however will only read their rain gauge a good few hours after it has stopped raining, and some evaporation will have taken place, so it could be worth adjusting the scale to allow for some evaporation. That's why I put in the square root.

That's my excuse and I am sticking to it! Anyway, apart from my careless inclusion of the square root, it raises the question of "when did it stop raining".  ;D

The suggestion to use any old container without a funnel will work, but I think it is very coarse, and you will have difficulty getting a reasonably accurate reading from it, not only because of surface tension, but also because of increased evaporation. Furthermore, the container would need to stand completely level I think.

Another good thing about my oversight was, that I now have confirmation about a decent dream theory. It seems many dreams are caused by anticipation of things the day before, but the anticipation is not fulfilled. The brain gets programmed in preparation for something to happen, and when it doesn't, it has to be de-programmed (by dreaming) to make space for events the next day. In my case, in my rush to show off on this message board, I made a maths mistake, and my brain wasn't satisfied..........so I dreamed about it and put it right again! :)

Mr Smith. If you let us know the diameters of your funnel and your collection tube, I shall gladly draw a scale for you and post on here.

[Bjerreby]

And I forgot to mention that the funnel and tube principle can be applied to your water butts if they are filled from a roof.  That's how I do it :)

[Mr Smith]

Bjerreby
             Excellent reply just what I was looking for(can't work the maths out),  I only left school with two 'O' Levels one in metal work and the other one in Biology, do you know anyone who wants a cat welding :),

[M.S.gardener]

Bjerreby
             Excellent reply just what I was looking for(can't work the maths out),  I only left school with two 'O' Levels one in metel work and the other one in Biology, do you know anyone who wants a cat welding :),

Thats cool ,cos i left with them aswell plus art and geography so we could start a little buisness of......fancy welded cats that look good anywhere in the world ::),what ya think  ;D

[Mr Smith]

BJ
      Thanks for the offer I will be in touch :)

[Mr Smith]

BJ,
         I have sorted a clear plastic fishing rod tube for my rain gauge which will be mounted on a metal frame, it is 45mil in diameter and 220mil long, is the rain gauge more accurate with a funnel ? :)

[Bjerreby]

BJ,
         I have sorted a clear plastic fishing rod tube for my rain gauge which will be mounted on a metal frame, it is 45mil in diameter and 220mil long, is the rain gauge more accurate with a funnel ? :)

I'd say yes, it is. The place where the raindrops land is a bit random, so the bigger the collection area, the better measurement you will get.

[Bjerreby]

Looks like the recession is forcing me into early retirement Mr Smith, AND I AM BORED! So I have been busy. All I need is confirmation of your funnel and tube dimensions........

[Mr Smith]

BJ,
         Thanks for your time and trouble but I have managed to get hold of a rain gauge and outside thermometer off e-bay, for every one  mil of rain it relates to one meter of rain over a square meter, :)