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[wxsig] Barometer calibration

John Bennett jabennett at insightbb.com
Tue Jan 8 00:51:25 UTC 2008


Several points...

* When the program asks for a barometric pressure, it would be best to
use a NWS or airport reading that is closest to you. This will insure
you have a fairly accurate value. The altitude does not matter. All
pressure readings are corrected for ASL. Do be sure you get the reading
just after the top of the hour (or whenever it was recorded) for the
most accurate setting. I would stay away from consumer station readings
if at all possible. The pressure transducers used in commercial stations
are expensive for a reason.

* Accuracy will depend mostly on how much patience you have when doing
the calibration and how accurately you match up the levels. It can be
very frustrating. I used a column of water that was as high as I felt
workable. The greater the distance between the two setpoints, the
greater the accuracy. However, this does not mean you can use a column
of greater length. It must be exactly as the program/docs call for. The
internal calculations depend on that.

* When I first developed the sensor, I graphed the pressure on a five
minute basis for the day. What this revealed was a graph with small
cycles showing up with a period of anywhere of 20 minutes to an hour. I
talked with a meteorologist at the Indianapolis NWS office about this.
This is a known phenomena and is caused by eddies and currents within
the atmosphere. Also note that a building that does not breath and/or
the HVAC can affect the readings. Different locations separated by a
mile or two can therefore have different readings. However, the overall
trends should be the same. What I finally did in my software (WXN
weather suite) was to plot only the daily average. It was too confusing
otherwise.

* Accuracy will be best nearest the calibration point (i.e., the
barometric pressure you entered in the last step). The design specs and
calculations are shown either in the manual or to a link on my website -
can't remember which. I believe Will found that this was so close to
what he had done with the Bray sensor, he did not re-work the algorithm
to match this one. Point being, that as you move away from the
calibration point, there will likely be some error but not enough to
amount to much.

* Mesowest captures and analyzes most APRS weather stations. You can get
a good feel for how you are doing by looking at this site. Here is my
station. Note that my humidity sensor sometimes gets saturated and puts
out some weird readings. Otherwise, since the end of November everything
looks pretty good:

http://www.met.utah.edu/cgi-bin/droman/regress_database.cgi?stn=AR446&time=GMT

Here's a link to the same site for WB9TLH. Dwight is running the same
software (i.e., wxn):

http://www.met.utah.edu/cgi-bin/droman/regress_database.cgi?stn=AP832&time=GMT

You will need to do some hacking, but should be able to find your
station if you've been on the air very long.

Hope this helps.

73, John
n4xi

On Mon, 2008-01-07 at 10:11 -0800, John Yost wrote:
> Finished the  TAPR X1W-1 kit.
> 
> Followed the procedure using baromcal.exe.  That
> process went well, until it wanted a barometric
> reading.
> 
> I found another wx station close to me at about the
> same altitude and entered their reading(1021.1mB
> converted to 30.15 inHg).  So far ok.
> 
> When connected to the T238+, the reading was 30.21
> which showed up as 1044mb on aprs wx site.  I adjusted
> the offset to make my reading agree with or at least
> in range with other stations around me.
> 
> Is there a better way to get a local pressure reading
> to compare to, or is ball park close enough for this?
> 
> The calibration was done with the unit in place, on a
> shelf above my desk in the basement of the house. 
> Didn't think that should be a factor. 
> 
> Thanks for any suggests or recomendations
> 
> john
> K3YJP-4
> AS577
> 
> 
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-- 
John Bennett
Evansville, IN
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