[wxqc] CWOP Information for CW4490 (C4490) in Leander, TX US
Ted Lum
gladstonefamily.net at tedworld.com
Sun Jun 1 16:01:19 CDT 2008
That's right. What I mean to say is that since the pressure data which
should be sent to APRS is altimeter (which is only altitude and not
temperature corrected from the sensors raw reading) we should see no
variation due to OAT from the actual local reference altimeter. In other
words, we assume altimeter will vary based on OAT to begin with, what we
would not expect would be variation from a local altimeter references
from either a correction (software) or stray variation (sensor or
electronics drift). The complaint here is that a station is not tracking
to other local stations. Assuming that all stations are reporting
altimeter, and are seeing the same variation in altimeter due to
temperature, then any single station showing variation from the sample
group with respect to temperature would be suspect. When I say variation
I AM NOT referring to "a" stations pressure, I am referring to the delta
withing the control group. I apologize for my oversimplification.
So, we assume altimeter is affected by OAT and we assume that we know
what the local altimeter reference is. We know that the Davis pressure
sensor is in the console, not the station, which is inside and (likely)
in a climate controlled environment that would limit thermal variation
of the sensor or electronics. So, statically significant altimeter
variation from the local altimeter standard that correlates to OAT would
likely be caused by some software correction. Now, if the room
temperature is highly variable, and the delta variation correlates to
room temperature then there is a likelihood you're looking at thermal
drift in the sensor or electronics. Also, a positive pressure drift with
respect to a positive temperature swing is typical of a correction since
the actual effect of temperature on atmospheric pressure is the inverse,
although this might also be indicative of sensor or electronics drift.
The problem here is that we are comparing to a control group and not a
local calibrated, traceable, standard, and it could just as easily be a
problem with the control group as with the single station.
Evan Bookbinder 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
>>> <http://gladstonefamily.net>@tedworld.com <http://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
>>>>
>>>>
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>
> Merton Campbell Crockett
> m.c.crockett at roadrunner.com <mailto:m.c.crockett at roadrunner.com>
>
>
>
>
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