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[wxsig] Rain Gauge Calibration

William Beals will at beals5.com
Wed Feb 23 05:01:09 UTC 2005


Ed:

The question isn't how to calibrate the rain gauge to 16oz--which I agree
isn't too hard.  The question is "Is 16oz of water the correct amount of
water to use for the calibration process"  In theory it should be the
equivilent of 1 inch of water at the diameter of the rain gauge catcher.
Where we're having fun splitting hairs is if the really correct amount is
16oz, 15oz, or 15.67oz.  Part of the question is based on defining the
"diameter" (inner edge vs center of lip) and I think there is another tweek
too.

will

-----Original Message-----
From: Ed L. Locklear, KF4CHG [mailto:kf4chg at earthlink.net] 
Sent: Tuesday, February 22, 2005 9:43 PM
To: will at beals5.com; TAPR Weather Station SIG Mailing List
Subject: Re: [wxsig] Rain Gauge Calibration

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!
>>
>>Thanks!
>>
>>Clay
>>

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!

Ed, KF4CHG






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