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Miscellaneous Debris => Weather sensors, design, analysis, discussion, home brew => Tech Corner => Rain Gauges => Topic started by: SLOweather on November 23, 2016, 05:36:16 PM
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Need a lot of 4" gauges to check the rainfall around your land?
Cut 4" plastic drain pipe in 12" lengths, and glue on a flat end cap. Carefully smooth and bevel the inside of the top lip if you desire. Figure out a way to mount it that suits you.
Read using a Stratus inner cylinder.
In a pinch, you could use a lab graduated cylinder, and divide the reading in mls by 2 to get inches in hundredths.
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Interesting, so have you compared for accuracy, or is this just an in the ball park gauge? Would make a good snow catch gauge if it was accurate with the Stratus inner cylinder.
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I haven't built one yet. However, the IDs of a Stratus funnel and the 4"pipe I measured differed by about a sixteenth of an inch, making the area of the pipe about 3.1% smaller. I think you'd get that back by beveling the edge.
Or, just apply the 3.1% (multiply the reading by 1.031) as a correction factor.
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Well, I built 2 of these to test in tonight's rain. More tomorrow. Plus maybe other semi interesting rain discussions. Been playing with some historical dats.
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OK. I made 2 and tested them side by side by side with the Stratus in the rain a couple of nights ago.
(http://www.sloweather.com/blog/2017/170326pvcgauges.jpg)
Stratus 0.45"
PVC "A" 0.46" 2.2% higher
PVC "B" 0.47" 4.4% higher
All measured using the Stratus inner cylinder.
However, after the fact, I realized that I should have dried out the measuring cylinder between readings. 0.01" in a 4" gauge amounts to 2.05 ml, which is probably what gets left behind after quickly dumping the cylinder.
Plus, having straight sides, the home made gauges have little opportunity for splashout compared to the Stratus funnel. This may well be an issue with funnels, as has been discussed here before. In this rain, the Stratus and adjacent 4" Imhoff cone matched exactly, while in a heavier storm, the Imhoff cone recorded several hundredths more rain.
I'll run them one more time and see what happens.
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Construction hints if you decide to make one or more of these, and a question.
It was hard for me to mark an accurate line or otherwise get a square cut. I finally cut a marking ring off a 4" coupling to make a good perpendicular mark on the pipe, and cut it with a hacksaw.
Spread PVC cement around the bottom corner of the cap before it gluing onto the pipe. That will form a good seal at the bottom of the gauge.
I filed a Stratus like bevel around the top with a half-round file, and then finished it with fine sandpaper.
Now then, got any good ideas on a mounting method for these?
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Great tip Chris. Think I'll try a couple of these on my property.
Should mention though that without the stratus cover/funnel you will lose accuracy from evaporation if not checked in a timely manner.
To remedy that you could use your average automotive type funnel and just cutoff the top portion so it sits in the pipe snugly.
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Yeah, I was going to mention that, as well as your fix. I'm gonna have to see what funnels Harbor Freight has. I was tardy a few days ago an both the PVC tubes were dry, while the Stratus and cone still had the 0.01" in them.
BTW, in testing, in light rains the PVC gauges track the Stratus almost perfectly. However, in yesterday's rain,
0.66" Stratus
0.67" Imhoff cone
0.70" PVC "A"
0.70" PVC "B"
We had a couple of downpours in excess of 1.5"/hr rate. I'm attributing the differences to Stratus splashout. And the Imhoff cone has a flat top edge, not beveled.
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Chris,
If you have a metal or wood working lathe with a 4" chuck, you could probably mount the PVC pipe in it and use the lathe tools to cut off the end as well as to create the 45° edge. That would ensure a perfect and level beveled edge to the gauge.
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Now then, got any good ideas on a mounting method for these?
Sorry for my lack of timeliness, but I do have a mounting suggestion. You could get a couple of 4" conduit hangers, mounted vertically. I personally haven't used them this large, but I have used smaller ones for a variety of "unintended" applications, including antenna supports and snow sensor mounting. You could leave them just slack enough to slide the conduit out. The end cap might be problematic, but maybe wingnuts might offer a quicker disconnect method. You might need some kind of "L" bracket to support the bottom so it doesn't slide out.
(http://www.garvinindustries.com/images/itemimages/ch-400.jpg)
Greg H.
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FYI you can get 4" CLEAR PVC - but it is about $25/ft.
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FYI you can get 4" CLEAR PVC - but it is about $25/ft.
Considering that an entire Stratus gauge can be had for about $30, that pretty much defeats the "inexpensive" part of this. :)
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Harbor Freight has a set of 2, 3, 4 and 5 inch funnels for $1.19.
(http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_18517.jpg)
The 4" is a nice fit inside the PVC. The 5" would need to be trimmed.
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Here's the 4' HF funnel in a tube:
(http://www.sloweather.com/blog/2017/1704204inch.jpg)
and the untrimmed 5":
(http://www.sloweather.com/blog/2017/1704205inch.jpg)
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Heat gun
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4 inch conduit strut clamp and strut material