Hi folks,
I have a standard Davis at 0.2mm and a OS at 0.1mm, they are about 3ft apart.
Today we have a bit of rain, Davis says 1mm and OS says 1.4mm
Now is this because the OS has a larger bucket, therefore a larger area to catch more rain? Or is this cause I may have a is call with the tip size on the OS?
Many have found that the VP2 rain gauge under-reads compared to say, a manual rain gauge, maybe by as much as 10%. There are 2 small “screws” - one under each tipping bucket that can be turned to raise or lower the resting point to adjust how much rain would be needed to tip a bucket.
5-6 tips is much too small an amount of rain to make a conclusion.
Ah OK, will wait for more rain, just out of interest, maybe a daft question but, how do you measure x mm of liquid
Well, you could google it 
or:
Collect rainfall in a straight sided container and measure it with a ruler, but it’s close to impossible to measure 1mm of water that way.
Collect rainfall over a known area and measure the volume of water collected (in ml) and do a bit of math to convert that volume back to the depth in mm.
For example use a funnel, and a measuring cylinder.
But what about the math? Well, as you’ve pointed out recently, I’m very old so to make it easier I’ll use cm, not mm. 1 ml = 1 cubic centimeter (cc).
Let’s suppose the funnel is actually a Davis rain gauge collector and it’s feeding the water into a measuring cylinder instead of the tipper.
Davis tells us the collection area is 214 cm2, and suppose we found 107 ml (cc) of water in our cylinder. We divide the volume (107) by the area (214) so (107/214) = 0.5 cm or 5 mm of rain fell.
This is how a gauge like the CoCoRaHS works, a funnel feeding into a smaller diameter cylinder. Except they did the math already and conveniently marked the measuring cylinder directly in mm of rainfall instead of volume of water collected.
Thank you Niko, or i could just get a CoCoRaHS but it looks like its just the US, shame, they look quite decent 
There is one on ebay but with shipping it turns out quite dear, will have a look at the Weather Shop, see what they have
Cheers Niko, sadly a bit more than I would like to spend on a manual item, but I have found these
https://www.climemet.com/collections/rain-gauges/products/cm1016-professional-manual-rain-gauge
There ya go. Of course neither it or the CoCoRaHS are the same diameter as the Davis, so that’s a possible error. Comparing results from different types of gauges, of different sizes is pretty much guaranteed to drive you nuts - you have been warned :lol:
So, going back to my original my original question then, would be the bigger area that caused the higher reading? That’s the reason I think it is, but I can never get my head around it lol :?
As I said above it’s too small an amount of rain to make any conclusion. You need to record the data over a period of time, with storms of at least an inch to really see the trend. Even then, within a small, obstructed yard there can be many reasons (turbulence, shadowing, etc.) why two gauges would not receive the same amount of rain.
Ah OK, cheers
You might find the rain gauge topic on our Davis knowledgebase useful as background reading:
www.manula.com/manuals/pws/davis-kb/1/en/topic/rain-gauge-faults
Thanks John, will have a look
Interesting reading, thankfully i dont think its the Davis at fault, well, shouldnt be for the price.
Gonna get a manual so that i can compare then between the 3,
Both my buckets are Davis, one is now using an OS 968 transmitter and modified to 250mm funnel
I had a wee bit more rain last night (before midnight) Davis 3mm, OS 4.5m thats a big difference
I must have the setting wrong for the OS rain tip in WD (0.160), couldnt see me getting that wrong, could you Niko???
EDIT:
Ran the calc again and it looks like it should be 0.087, when i was messing with setting up 2 WDs it looks
like i did sommat wrong with the calc 
Let’s get the facts straight and I’ll check the numbers.
You have a 250mm diameter feeding an OS tipping mechanism - right?
What is the original tip size for the OS gauge?
What is the original diameter of the OS gauge?
:oops: :oops:
Original OS bucket 1mm tip @ 100mm diameter, so now ignore that i beleive?
But, where i got confused is, i am now using a fully converted Daivs bucket, os bucket no longer involved just the electronics
i got it into my head that cause it was os electronics that it still went by the OS sizes but it doesnt does it #-o
So i think its ignore anything OS now… Davis 165mm at 0.2mm tip with a new funnel at 250mm
OK, thanks for the clarification, after posting that I was thinking it wasn’t correct :roll:
25 cm diameter = 490.87 cm2
Davis original = 214 cm2
491/241 = 2.294 times bigger area.
0.2 mm tip / 2.294 = 0.087 mm tip
You’re right 
Phew, thanks a lot for your brain “again” lol 