[wxqc] Rain gauge cal

Ted Lum gladstonefamily45.net at tedlum.com
Tue Oct 11 22:32:28 CDT 2011

But in all seriousness, some things don't carry over. In the calibration 
lab its important to account for all of those little items, whether it 
be the accuracy, tolerance, resolution, uncertainty, etc. Becasue in the 
calibration lab you need a standard that is many times more accurate, 
sensitive, resolute, etc. in order to measure that 5% accuracy of the 
sensor under test. That's how you know its +/- 5% and not +/- 6% or +/-2%.

So, when you are talking calibrations and standards measurements, yea, 
all those little fractions are important. In the field is a completely 
different situation, you don't expect the same specs. as you had in the 
lab. But too often people change context not taking that into account.

The most glaring this here, though, is that we all - myself most 
centrally included - have been running around calling the Davis gauge an 
8" literally for years. Leave it to someone to actually measure the damn 
thing... what a concept. Not sure where 8" got stuck in my head, but it 
did, and I've been real wrong for years. That in and of itself renders 
the whole argument moot, because that's a pretty big error before you 
even get started.

But, regardless of the specifics the point that I have been trying to 
get across is this. Think of a VHS tape... remember those? Every time 
you made a copy of a copy you would loose 1/3 or the resolution. Same 
idea with references and standards. Every time you use a standard to 
judge another instrument expect to loose something... You can't use a 5% 
instrument to judge the accuracy of another instrument to withing 5%. 
And for each generation you lose more and more. But this is for 
calibrations and standards,  it does not carry over into other contexts.

On 10/11/2011 6:20 PM, Paul Grace wrote:
> Thanks-  It's humorous to see people attempting to compensate for 0.1% 
> errors due to the variable density of rain, against the basic 5% 
> accuracy of the sensor.
> *From:*wxqc-bounces at lists.gladstonefamily.net 
> [mailto:wxqc-bounces at lists.gladstonefamily.net] *On Behalf Of 
> *jdr1 at metdata.com
> *Sent:* Tuesday, October 11, 2011 1:35 PM
> *To:* wxqc at lists.gladstonefamily.net
> *Subject:* [wxqc] Rain gauge cal
> I noticed there are several people that are trying to calibrate their 
> Davis gauges to a 4"gauge.  Victor referred to the Davis gauge as an 
> 8" gauge, it's not an 8", it is a 6 ½ inch gauge I have two of the 
> Davis tippers on the same post mounted at 5' above the ground  on two 
> different Davis Stations. They rarely have had the same rainfall 
> amount over the three years they have been mounted together.  I  had a 
> 8" NWS standard non-recording gauge only a few feet away several years 
> ago and had the same problem of differences in the amount the tippers 
> recorded and the 8" recorded.
> I have been a Davis dealer for 18 years and have talked to their 
> engineers on several occasions about these differences.  To check the 
> calibration on the Davis tipper, 5.44 ml equals one tip.  The accuracy 
> of the Davis Tipper is +/- 4%, +/- 1 rainfall count between .01"and 
> 2.00" per Hour. +/- 5%, +/-1 rainfall count between 2.00" and 4.00" 
> per hour.  If you want to be absolutely sure that the Davis gauge is 
> accurate, you can send it back to Davis and have it NIST certified, or 
> if you are purchasing a new station then request a NIST certified rain 
> gauge.  Davis Instruments web site has a page on rain gauge 
> calibration, www.davisnet.com <http://www.davisnet.com/>.
> Rainfall is extremely variable and can vary in amount over a few feet 
> or even inches, this depends on the wind or like of.  The NWS people 
> even admit that the tipper is sometimes not accurate in high rainfall 
> rates or high wind.  If a tipper is mounted on a post that can be 
> shaken by the wind there will false tips and inaccurate rainfall 
> readings.  Mounting and siteing of a rain gauge is very important, 
> NOAA has several publications on  the proper siting or weather 
> instruments.

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