[wxqc] CWOP Information for CW4490 (C4490) in Leander, TX US

Sun Jun 1 14:53:51 CDT 2008

I guess the 52 week analysis graphs would be appropriate for this
discussion. Here's the graph for my station:

http://weather.gladstonefamily.net/site/C6155?tile=10;days=364#Data

Victor

On Sun, Jun 1, 2008 at 2:47 PM, Victor Engel <brillig at gmail.com> wrote:

> Please note that when I said I noticed a suspected variation in pressure, I
> was referring to the difference between my reading and the analysis, not a
> variation in pressure. I know there is a daily fluctuation in pressure.
> Presumably it should be the same for everyone in the vicinity.
>
> Also, just because a sensor is indoors doesn't mean that it's immune to
> temperature variations. Such a notion presumes indoor temperature is
> constant. Over the past 6 months or so my indoor temperature has varied from
> about 46 to 86. I allow indoor temperature to vary for economical reasons.
>
> Anyway, I haven't confirmed the effect I described. It might not even be
> there. What I seem to notice, though, is that my readings are more likely to
> be lower than the analysis during warm weather than during cold weather. The
> difference is slight, since my QC tends to hover between 98% and 100% for
> barometer.
>
> Perhaps I shouldn't have brought it up at all until I verified it.
>
> Victor
>
>
> On Sun, Jun 1, 2008 at 2:23 PM, Evan Bookbinder <Evan.Bookbinder at noaa.gov>
> wrote:
>
>>  > Since the pressure sensor is located in the console, NOT the station,
>> if you see a variation with OAT then almost certainly its coming from a
>> software calculation and not a temperature affect on the sensor its self.
>>
>> The location of the sensor indoors has nothing to do with the fact the
>> barometric pressure *is* impacted by outside air temperature. Air pressure
>> is the weight of a column of air above a sensor. 99.999% of the air weight
>> is not in your house, it's outside. Barometric pressure is VERY MUCH altered
>> by outside temperature. Because we're not dealing with an enclosed volume of
>> space, when you heat the air, molecules expand and thus there are less of
>> them in a given volume of space. As such, the pressure falls. When you cool
>> the air, air molecules contract, occupying more per unit volume and thus
>> weight more (pressure increase).
>>
>> In the absence of any significant airmass changes, a diurnal barometric
>> pressure curve will show a MINIMUM in the mid afternoon at peak heating, and
>> a maximum around daybreak during peak COOLING. This is for outdoors, with
>> the greatest impact occuring where there are the most air molecules -- in
>> the lower atmosphere. Changes to air temperature INSIDE your home are
>> generally considered negligeble given the vertical depth of air in your home
>> when compared to the entire atmosphere above you (yes, even with cathedral
>> ceilings).
>>
>> All things considered equal however, you should not be seeing the
>> barometer rise in the afternoon, unless you are a coastal location
>> experiencing an afternoon seabreeze (which is accompanied by a drop in air
>> temperature and rise in pressure).
>>
>> Evan
>>
>>
>>
>>
>>
>>
>>  ------------------------------
>> *From:* wxqc-bounces at lists.gladstonefamily.net [mailto:
>> wxqc-bounces at lists.gladstonefamily.net] *On Behalf Of *Merton Campbell
>> Crockett
>> *Sent:* Sunday, June 01, 2008 2:09 PM
>> *To:* Discussion of weather data quality issues
>> *Subject:* Re: [wxqc] CWOP Information for CW4490 (C4490) in Leander, TX
>> US
>>
>> Davis Instruments' Application Note 28 explains how the barometric
>> pressure that is reported is derived.  The barometric pressure reported has
>> been reduced to report sea level barometric pressure.  On page 11 at the end
>> of the discussion of barometric pressure, a formula is provided for
>> converting the reported barometric pressure to the altimeter reading used
>> for CWOP reports.
>> I don't understand, exactly, how temperature effects the pressure sensor
>> in the Davis Instruments Vantage Pro2 console but my station tends to report
>> higher barometric pressures in the afternoon.  The console is in a room with
>> cathedral ceilings on the west side of the house.  The room heats up in the
>> afternoon and doesn't cool down until mid-evening.  During this period, the
>> QC graphs show the widest divergence between my readings and the predicted
>> values.
>>
>> Merton Campbell Crockett
>>
>>
>>  On 01 Jun 2008, at 11:31:02, Ted Lum wrote:
>>
>>  I don't know. That thread is more than two years old and I don't know if
>> it was ever resolved. I have not been able to find anything in the Davis
>> release notes that say they made a change, but that does not mean they
>> didn't. Since the pressure sensor is located in the console, NOT the
>> station, if you see a variation with OAT then almost certainly its coming
>> from a software calculation and not a temperature affect on the sensor its
>> self. This is only circumstantial, but your observations seem to imply they
>> have air density in the calculation. We should find out for sure, I don't
>> have first hand knowledge. As a Davis owner/operator I want to know.
>>
>> Victor Engel wrote:
>>
>> OK. But I thought the latest version of Weatherlink was sending altimeter
>> -- and VWS as well. Do I have a wrong understanding there?
>>
>> Victor
>>
>> On Sun, Jun 1, 2008 at 12:37 PM, Ted Lum <gladstonefamily.net@
>> tedworld.com> wrote:
>>
>>> You might want to re-check this thread.
>>>
>>>
>>> http://server.gladstonefamily.net/pipermail/wxqc/2005-November/003212.html
>>>
>>> Sam Drinkard wrote:
>>>
>>> Victor Engel wrote:
>>>
>>>
>>> I'm going to answer your question specifically and then also raise a
>>> more generic question.
>>>
>>> First, it looks like your pressure reading is too low. I believe if
>>> your mean error exceeds 2 millibars, you will get the message. When I
>>> first set up my stations (C6155) in 2006, I noticed it tracked another
>>> stations AS425 very closely in all data points, including pressure.
>>> Although we tracked closely, the analysis stated our readings were too
>>> low, so I made an adjustment upwards. My stations has had good QC for
>>> pressure ever since.
>>>
>>> I wonder, though, about something. All three of us use Davis Vantage
>>> Pro hardware, and all three of us were too low by similar amounts. Do
>>> these stations generally ship with a slight adjustment error? Could
>>> there be an adjustment needed at the factory? Certainly, we can't
>>> determine this from just three data points. I'm curious of others'
>>> experiences around the country, though, with this hardware.
>>>
>>> An alternative, of course, is that our hardware is correct and the
>>> analysis is wrong. Were that to be the case, though, the airport would
>>> be too high. I think that's unlikely, since aircraft rely on accurate
>>> altimeter readings to navigate.
>>>
>>> The other thing I've noticed but haven't spent time to check is that
>>> it appears the error in my readings may be correlated to temperature
>>> -- the higher the temperature the lower my reading with respect to the
>>> analysis. I may just be imagining that, and the magnitude of the
>>> effect certainly isn't large, if present. If this really is true,
>>> though, I wonder what would cause it.
>>>
>>> Victor C6155
>>>
>>>
>>>
>>>     Victor,
>>>
>>>     I've noticed the same thing with my station, which is a Davis
>>> WM-II.  If I look at the long term analysis of pressure, I can see a
>>> trend in the summer months that *seems* to degrade the sensor, but like
>>> you, I've not investigated this aspect.  As most of us know, nearly all
>>> electronic components have some variant or change with changes in the
>>> component's temperature.  Perhaps it would be a starting point to
>>> contact Davis and ask if there is any compensation built into the
>>> pressure sensor, or if ambient air temp changes would in fact, cause the
>>> sensor to not read in a non-linear fashion.
>>>
>>> Sam
>>>
>>>
>>> _______________________________________________
>>> wxqc mailing list
>>> Post messages to wxqc at lists.gladstonefamily.net
>>> To unsubcribe or change delivery options, please go to:http://server.gladstonefamily.net/mailman/listinfo/wxqc
>>> To search the archives: http://www.google.com/coop/cse?cx=008314629403309390388%3Aknlfnptih9u
>>>
>>> The contents of this message are the responsibility of the author.
>>>
>>>
>>>
>>>
>>> --
>>> This message has been scanned for viruses and
>>> dangerous content by *MailScanner* <http://www.mailscanner.info/>, and
>>> is
>>> believed to be clean.
>>> _______________________________________________
>>> wxqc mailing list
>>> Post messages to wxqc at lists.gladstonefamily.net
>>> To unsubcribe or change delivery options, please go to:
>>> http://server.gladstonefamily.net/mailman/listinfo/wxqc
>>> To search the archives:
>>> http://www.google.com/coop/cse?cx=008314629403309390388%3Aknlfnptih9u
>>>
>>> The contents of this message are the responsibility of the author.
>>>
>>
>>
>> --
>> This message has been scanned for viruses and
>> dangerous content by *MailScanner* <http://www.mailscanner.info/>, and is
>>
>> believed to be clean.
>>
>> ------------------------------
>>
>> _______________________________________________
>> wxqc mailing list
>> Post messages to wxqc at lists.gladstonefamily.net
>> To unsubcribe or change delivery options, please go to:http://server.gladstonefamily.net/mailman/listinfo/wxqc
>> To search the archives: http://www.google.com/coop/cse?cx=008314629403309390388%3Aknlfnptih9u
>>
>> The contents of this message are the responsibility of the author.
>>
>>
>>
>> --
>> This message has been scanned for viruses and
>> dangerous content by *MailScanner* <http://www.mailscanner.info/>, and is
>>
>> believed to be clean. _______________________________________________
>> wxqc mailing list
>> Post messages to wxqc at lists.gladstonefamily.net
>> To unsubcribe or change delivery options, please go to:
>> http://server.gladstonefamily.net/mailman/listinfo/wxqc
>> To search the archives:
>> http://www.google.com/coop/cse?cx=008314629403309390388%3Aknlfnptih9u
>>
>> The contents of this message are the responsibility of the author.
>>
>>
>>  Merton Campbell Crockett
>> m.c.crockett at roadrunner.com
>>
>>
>>
>>
>> _______________________________________________
>> wxqc mailing list
>> Post messages to wxqc at lists.gladstonefamily.net
>> To unsubcribe or change delivery options, please go to:
>> http://server.gladstonefamily.net/mailman/listinfo/wxqc
>> To search the archives:
>> http://www.google.com/coop/cse?cx=008314629403309390388%3Aknlfnptih9u
>>
>> The contents of this message are the responsibility of the author.
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://server.gladstonefamily.net/pipermail/wxqc/attachments/20080601/e0935781/attachment-0001.html 