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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 11:32 am 
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Location: Sanday, Orkney
Weather Station: Davis VP2
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SUBYDAZZ wrote:
So far as I see it, this should give me slightly more accurate data with respect to having less likelihood of say 0.1 or 0.2mm of rain not being counted because previously it would only measure at least 0.3mm, and thereby giving me nil rain.

Yes, that's correct. I did in fact add a funnel to my Oregon rain gauge (my first station) because that had a 1mm bucket, and a lot of the time small amounts of rain just weren't being registered at all.

Quote:
With respect to heavy rain, if you have a large catch area does this cause any problems like the reservoir overflowing because the bucket can't tip fast enough or any spillage through the bucket area as a result of the moving bucket itself?

I don't think it's likely to overflow, but one problem is that while it's tipping it's not measuring rain, so that rain is lost. This can be significant if it's tipping frequently.

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 11:42 am 
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Joined: Sun Apr 18, 2010 9:47 am
Posts: 251
Location: Karoonda, SA
Weather Station: WH-1081
Operating System: Windows 7, Home Premium
During this last week I have had extremely heavy rainfall rates recorded by my guage (Fine Offset with a modified larger catchment funnel). It has registered a rainfall rate of 131.9 mm/hr just a few days ago. The total rainfall I recorded matched that of an official weather station less than 1 km away to within 1-2%. I figure that is not a bad result. The calibration I currently use is .37, so bucket tips are recording as .111mm. I arrived at the .37 multiplier not by a calculation based on area of the modified and original collector alone, but by comparing the Fine Offset rain readings to a manual rain guage placed along side my stations modified rain collector. Over time I have settled on the .37 multiplier.

Bob

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 12:05 pm 
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Joined: Fri Jan 01, 2010 1:04 pm
Posts: 169
Weather Station: Digitech XC-0348
Operating System: Windows 7
tjaliwalpa wrote:
During this last week I have had extremely heavy rainfall rates recorded by my guage (Fine Offset with a modified larger catchment funnel). It has registered a rainfall rate of 131.9 mm/hr just a few days ago. The total rainfall I recorded matched that of an official weather station less than 1 km away to within 1-2%. I figure that is not a bad result. The calibration I currently use is .37, so bucket tips are recording as .111mm. I arrived at the .37 multiplier not by a calculation based on area of the modified and original collector alone, but by comparing the Fine Offset rain readings to a manual rain guage placed along side my stations modified rain collector. Over time I have settled on the .37 multiplier.

Bob


How much bigger than the original area is your catchment funnel?


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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 12:57 pm 
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Location: Dudley, West Midlands, UK
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Operating System: XP SP3
Quote:
... I arrived at the .37 multiplier ...
So approx. 3x

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 8:16 pm 
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Joined: Sun Apr 18, 2010 9:47 am
Posts: 251
Location: Karoonda, SA
Weather Station: WH-1081
Operating System: Windows 7, Home Premium
SUBYDAZZ wrote:
How much bigger than the original area is your catchment funnel?


Approximately 5 times the area. I expected the multiplier to be .33, but in fact I needed it to be .37. This may have something to do with the water lost during the tipping process (as Steve suggested).

Bob

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 8:19 pm 
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Joined: Sat Feb 21, 2009 12:41 pm
Posts: 1859
Location: Devon UK
Weather Station: FO WH1081PC (Maplin)
Operating System: OS X, Linux Mint, Win7 & XP
For an area ratio of 5 I would expect the factor to be 0.2 ie. 1/5

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 8:35 pm 
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Joined: Sun Apr 18, 2010 9:47 am
Posts: 251
Location: Karoonda, SA
Weather Station: WH-1081
Operating System: Windows 7, Home Premium
Gina wrote:
For an area ratio of 5 I would expect the factor to be 0.2 ie. 1/5


Oops, your correct. I stuffed up. Must be the early morning as I have just woken up. In fact the area is closer to 3 times, not 5 (55cm² to 154cm²) Does that make sense. Actually, it is not 3 times, it's 0.3571428571428571 to be exact. I've just been out and remeasured it. Sorry for misleading you all.

By the way, the official data from the BOM stattion for December rain so far is 131.4mm and my station (less than a km away) is 130.6mm

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 9:36 pm 
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Posts: 1859
Location: Devon UK
Weather Station: FO WH1081PC (Maplin)
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tjaliwalpa wrote:
Sorry for misleading you all.
No problem :)
Quote:
By the way, the official data from the BOM stattion for December rain so far is 131.4mm and my station (less than a km away) is 130.6mm
Well pretty close :)

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Dec 09, 2010 11:39 pm 
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Joined: Sat Dec 26, 2009 8:52 am
Posts: 588
Location: Crafers, South Australia
Weather Station: Fine Offset WH1091
Operating System: Windows XP
Hi Subydaz, I've increased mine by 3 times as well with calibration of .35. Originally it was really accurate compared to my 3 manual rain gauges in anything but really heavy falls. So I increased the sides of the original collector bucket and since then it has improved a lot so I can only assume I was getting some overflow.

You can see how in this thread: viewtopic.php?f=13&t=3074&

You can also see my modifications under the "Sensors" link on my website in my sig.

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Sat Jan 01, 2011 2:28 pm 
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Joined: Mon Dec 20, 2010 12:12 pm
Posts: 446
Location: Palmerston, NT, Australia
Weather Station: WS-1081 with rain gauge mod
Operating System: Windows 7 & 1.9.3 b1059
SUBYDAZZ wrote:
OK, so by adding a larger catchment area and using the calibration factor in Cumulus to compensate for this (eg: If I triple the catchment area of my gauge and then go under Multipliers for rainfall in Cumulus Calibration and put in 0.33) then it will be more sensitive to smaller amounts of rain and variations in the rain rate?

So far as I see it, this should give me slightly more accurate data with respect to having less likelihood of say 0.1 or 0.2mm of rain not being counted because previously it would only measure at least 0.3mm, and thereby giving me nil rain.

I take your point though on the accuracy of the device as far as measurement goes, if the original gauge is not perfectly accurate (and of course it wouldn't be as it's hardly a precision instrument) then any error will be propagated if you go changing the catchment area etc.

With respect to heavy rain, if you have a large catch area does this cause any problems like the reservoir overflowing because the bucket can't tip fast enough or any spillage through the bucket area as a result of the moving bucket itself?


Hi Subydazz, I bench tested my WH1081 after I had increased the catchment area 3 times (55cm2 to 165cm2) and found that each tip (0.1mm) was (trying to remember.....) 12 drops, and that I had no issue with wasted drops (a drop falling into the bucket that was already tipping) up to a rate of 50mm/hr. However, upto 130mm/hr you would get a drop wasted, which would be a 1/13 or 7.7% under reading. Any rate way above this you would probably get a 15.4% under reading. Out of 16 days I've been using Cumulus in December, I had 12 rain days of which 3 times I had it go over 50mm/hr (and up to 118mm/hr). I am currently happy with this trade off. I also like the fact that not only is it more sensitive and accurate (bench tested below 50mm/hr), it also records rain earlier (and sets of my voice confirmation alarm! He he :D ) Obviously everyone will be different. If you never get above 50mm/hr then do it. If you always get above 50mm/hr, or crikey! 130mm/hr, then you may want to think twice.
I've added a photo. My mod is in two parts. The first mod was to prevent splash out by increasing the wall height (100mm). The 2nd part (done later) was to increase the catchment area. To make the rectangle (50mm x 110mm) proportionate I just made the larger area (50mm x the square root of 3) x (110mm x the square root of 3) using laminating sheets. To make them stronger I laminated a laminating sheet for each side and hot glued them together. It's very strong and total cost was $2 for 9 sheets (the first part was only made with one sheet). I found that making the area excactly 3 times bigger, then using a Calibration multiplyer of 0.33 to give a 0.1mm reading is very neat. I have read others make mods and get each tip of the bucket giving a funny reading like 0.111mm? Weird. If your area is slightly under, you can modify the tipping bucket to make it more accurate (initially mine, unmodified, was 9% under).

Michael (Palmerston, NT, Australia)


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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Sat Jan 01, 2011 2:40 pm 
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Joined: Mon Dec 20, 2010 12:12 pm
Posts: 446
Location: Palmerston, NT, Australia
Weather Station: WS-1081 with rain gauge mod
Operating System: Windows 7 & 1.9.3 b1059
[quote="hills"]Hi Subydaz, I've increased mine by 3 times as well with calibration of .35. Originally it was really accurate compared to my 3 manual rain gauges in anything but really heavy falls. So I increased the sides of the original collector bucket and since then it has improved a lot so I can only assume I was getting some overflow.quote]

G'day Hills, love your mods, particularly to the temp/hum gauge. In regards to the rain gauge mod, did you increase the sides of the tipping buckets? What I found is that adding weight to the buckets meant that it required more drops of water to overcome the extra weight on the other side of the bucket (thus making the reading read under actual). Also, with the extra weight, the buckets would tip slower, and I've found the inaccuracy comes from when the rain rate is high, drops will be wasted into a bucket that is already tipping. The fact that your mods have made it more accurate is amazing :clap: . Well done.

Michael.

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Mon Jan 10, 2011 11:46 pm 
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Joined: Fri Jan 01, 2010 1:04 pm
Posts: 169
Weather Station: Digitech XC-0348
Operating System: Windows 7
So would you be able or not to modify the bucket to catch less before it tipped rather than increasing the catchment area to give the same results?


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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Jan 13, 2011 12:29 am 
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Joined: Mon Dec 20, 2010 12:12 pm
Posts: 446
Location: Palmerston, NT, Australia
Weather Station: WS-1081 with rain gauge mod
Operating System: Windows 7 & 1.9.3 b1059
SUBYDAZZ wrote:
So would you be able or not to modify the bucket to catch less before it tipped rather than increasing the catchment area to give the same results?


Yes, reduce the tipping angle. You could throretically reduce the tipping angle so much (raising the height of where the bucket will rest can be up to 10mm each side), which would only require a few drops to tip it (instead of like 12 drops, you would get it to tip at 12/3=4 drops ((thus reducing the tip from 0.3mm to 0.1mm)). But you need to remember that it will be more susceptible to a false reading if your rain gauge will rock in winds.

I hadn't actually thought of it that way, this would solve any problem with high rain rates giving flase readings of 7% or more, however, since you are only dealing with very small measurements that there is a high possability of not being able to fine tune it and it may well consistantly give false readings. Eg. 1 drop over and your out 20%, 1 under and your out 25%.

Another thing you will have to check is that with a reduced angle by this much you would have to make certain that the drops that a falling in a FULLY falling into the bucket and not getting split by the middle of the buckets.

I would still increase the wall height of the catchment area though.

I can forsee to get this right, and I think you could, that you will need to get the measurement correct, not only to the mm, but maybe even as close to the tenth of a mm. Good Luck, perserverence is the key.

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 Post subject: Re: increase accuracy of fine offset rain tipper
PostPosted: Thu Jan 13, 2011 9:57 am 
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Joined: Sat Feb 21, 2009 12:41 pm
Posts: 1859
Location: Devon UK
Weather Station: FO WH1081PC (Maplin)
Operating System: OS X, Linux Mint, Win7 & XP
If you reduce the amount required for the buckets to tip you will increase the error caused by tipping and worsen performance in high rainfall conditions even more. In light rain, accuracy is limited to one drop. The only way of increasing accuracy in light rain is to increase the catchment area. Reducing the bucket tipping volume will reduce the time to detect the onset of rain but overall will reduce the measurement accuracy.

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