[wxqc] Rain gauge cal

Paul Grace paulgrace at lookoutranch.com
Tue Oct 11 19:13:30 CDT 2011



From: TJ [mailto:tjslagle at cox.net] 
Sent: Tuesday, October 11, 2011 3:50 PM
To: paulgrace at lookoutranch.com; 'Discussion of weather data quality issues'
Subject: RE: [wxqc] Rain gauge cal


Oh come on don’t be such a kill joy!!!  LOL


We haven’t even discussed accounting for expansion and contraction based on
temperature of the Davis or Cocorahs range gauge. When it is hotter both
gauges would be slightly larger in diameter than when it is colder
potentially affecting the amount of rain they collect. And let’s not forget
with the Cocorahs gauge that larger diameter would affect the volumetric
measurement which would also be affected by the fact that the Cocorahs gauge
would be slightly taller when it is hotter once again affecting the
volumetric measurement.


This would continue on to the tipper in the David gauge as it would also
have expansion and contraction based on temperature that could potentially
affect that 5.44 ml needed to make it tip not to mention the actual
cleanliness of the tipper itself.


This is just getting started. LOL




From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Paul Grace
Sent: Tuesday, October 11, 2011 5:21 PM
To: jdr1 at metdata.com; 'Discussion of weather data quality issues'
Subject: Re: [wxqc] Rain gauge cal


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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