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[wxsig] Rain Gauge Calibration

Ed L. Locklear, KF4CHG kf4chg at earthlink.net
Wed Feb 23 04:43:10 UTC 2005

William Beals wrote:

> Gary and Clay:
> Russ and I remember this being a VERY hot topic earlier, even before the
> T238 got involved with the rain sensor.  The question got answered, but
> we're having trouble finding the archives that "proved it".
> Russ remembers that it is not the inside diameter (5 7/8"), but rather the
> diameter to the centerline of the lip (6.00") that matters, but that only
> gets you to 15.67oz, not quite 16oz.  Several experts have been questioned,
> hopefully an answer soon (if Gary doesn't beat me to it).
> will
>>Trying to understand the theory behind the operation of the rain gauge.
>>I have the original Dallas Semiconductor rain gauge. The instructions that
>>come with it state to "Adjust the calibration screws so that 16 Oz at the
> 1
>>inch per hour rate produces 100 counts." This appears to correspond to
> 1.00"
>>indicated on the T238 display.
>>Where I am confused is this: Taking the measurement of the radius of the
>>rain gauge and caculating the area exposed and converting to ounces gives
> me
>>about 15 ounces per inch of rain:
>>5.875 inches diameter
>>2.9375 inches radius
>>= 27.109 square inches
>>At 1" of rain, this would be 27.109 cubic inches
>>Converting cubic inches to ounces;
>>1.80468751 cubic inches per ounce
>>= 15.021 ounces.
>>Why then do you calibrate with 16 ounces?
>>Want to make sure I get this right before I go mount it on the pole!

Clay, Gary, and Will,

I really don't understand why this is so complicated for calibrating a 
rain guage.  The diameter, radius, nor circumference have nothing what 
so ever to do with calibrating the rain guage.  It is mearly a funnel to 
guide the water (rain) to the tipping bucket.  As per the instructions 
quoted below:

"To measure rainfall accurately, you will need to calibrate the tipping 
bucket by making sure it is level and dripping water through the 
collector top. Adjust the calibration screws so that 16 oz at the 1 inch 
per hour rate produces 100 counts. This can be measured using the rain 
guage display in the weather station software. Use a paper cup with a 
pin hole in the bottom to supply the water at the proper rate."

So get 16 oz. of water and a paper cup or whatever prick a pin hole in 
it and start pouring water (this will take a long, long time) and 
calibrate the tipping bucket screws while monitoring with software. 
Once you have it calibrated, then hook it up to the T-238.

Now if you do not want to waste your time refilling a paper cup over and 
over.  Go to the local pet store (even Wal-Mart) buy some air hose a air 
value (plastic regulator), and your favorite 2 ltr. drink on sale. 
Drink the soda or pour it out, drill hole in cap for hose, seal it, fill 
bottle with 16 oz measuring cup.  Put cap w/hose back onto the bottle 
and adjust the regulator for the proper flow to produce 100 counts.

Hope this is of any help to anyone.  Else you're going to go bald or 
gray haired way before your time.  I know, I am already there!


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