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[wxsig] Hobby Boards Barometer

Dave Barton dbarton at nettally.com
Wed Apr 30 11:07:14 UTC 2008


that sounds great.  thank you, sir!

Dave
AI4GF

William Beals wrote:
> Folks:
>
> I did find the coding error.  Long story, but boils down to not handling the
> remainder right on some fixed point math for calculating metric BP.  It only
> affects the fractional part, and the way the error manifests itself, only
> decimal values of .0, .1, and .2 are possible!  I'd say that pretty much
> describes the problem!
>
> I'll work on the fix over the next week.  The tricky part for me will be
> testing it given my wx station is down right now, but there are ways.  I've
> had some other features coded up but never released and so there will be
> more changes than just the BP fix.
>
> will
>
> -----Original Message-----
> From: William Beals [mailto:will at beals5.com] 
> Sent: Monday, April 28, 2008 9:57 PM
> To: 'TAPR Weather Station SIG Mailing List'
> Subject: RE: [wxsig] Hobby Boards Barometer
>
> Dave and Ken:
>
> Eeuw, this problem now sounds very familiar!
>
> I went back in my own archives and this was reported by Ken Brown back in
> March 2006!
> I thought I had fixed this problem, but I still have it on an old "to do"
> list and I don't see the fix listed anywhere in my release notes.
>
> I'll go look at my code and see what I can figure out.  It has been a while
> since I've looked at that fixed-point math!
>
> will
>
>
> -----Original Message-----
> From: wxsig-bounces at lists.tapr.org [mailto:wxsig-bounces at lists.tapr.org] On
> Behalf Of Dave Barton
> Sent: Monday, April 28, 2008 7:50 PM
> To: TAPR Weather Station SIG Mailing List
> Subject: Re: [wxsig] Hobby Boards Barometer
>
>
>    Mr. Brown stated the problem very clearly and succinctly. Thank you,
>    sir.
>    To use empirical data,  as noted in my previous email, you can view
>    the data of multiple T238 stations that are reporting barometric
>    pressure.  Here are a few:
>    [1]http://www.findu.com/cgi-bin/wx.cgi?call=k3yjp-4
>    [2]http://www.findu.com/cgi-bin/wx.cgi?call=w0dvm
>    [3]http://www.findu.com/cgi-bin/wx.cgi?call=w0maf-1
>    [4]http://www.findu.com/cgi-bin/wx.cgi?call=wa2gug
>    [5]http://www.findu.com/cgi-bin/wx.cgi?call=ai4gf-4
>    The vast majority of the pressure readings end with a zero tenths
>    value.  However, a small number end in xxxx.1 or xxxx.2.  These values
>    seems to occur when the value is trending up or down, as in
>
>    1023.0
>    1023.0 1023.0 1022.2 1022.1 1022.2 1022.2 1022.0 1022.0 1022.0
>
>    The APRS standard allows the tenths digit, the english inches of
>    mercury values show a decent resolution, and there is even one T238
>    station that  reports over TCP/IP with good resolution.  I am just
>    stumped to figure out how we can occasionally have xxxx.1 or xxxx.2
>    values, but not xxxx.3 through xxxx.9.  If we had the .3 through .9
>    values, presumably we'd have a smoother graph.  Since we have
>    acceptable resolution for the inches of mercury, it seems like we
>    should be able to convert to millibars.
>    Inches       Millibars
>
>    29.91 1012.9
>    29.92 1013.2
>    29.93 1013.5
>    29.94 1013.9
>    29.95 1014.2
>
>    My last assembly programming was 25 years ago and although I have
>    looked at your code, I doubt that I can be of any help, but I am duly
>    impressed with the effort that has gone into the code for the T238+!
>    Regards, Dave
>    AI4GF
>    [6]Ken.A.Brown at noaa.gov wrote:
>
> Will,
>
> The reason for the scaling needed on the barometer was explained
> well on the Jennings web site.  It was indeed a pain to get the
> Bray barometer calibrated for my elevation and took several weeks
> to do so with fine adjustments to the span and offset values.  I
> did a comparison for several weeks with four local aviation reporting
> stations and it now falls in well when I use the LCD BP value.
>
> What the sequence of previous message concerned is the conversion
> of the LCD BP value in inches of mercury to the APRS output value
> in millibars.  The LCD BP value seems to increase and decrease
> smoothly where as the millibar value seems to "stair step" in
> mostly whole numbers instead of millibars and tenths.  Using
> conversion tables from the Smithsonian Meteorological Tables
> shows that the conversions do not seem consistent.  I was never
> able to get the LCD to display millibars correctly so it is
> always indicating altimeter setting inches of mercury values.
>
> Ken Brown, n5kuk
>
>
> ----- Original Message -----
> From: William Beals [7]<will at beals5.com>
> Date: Monday, April 28, 2008 0:57 am
> Subject: RE: [wxsig] Hobby Boards Barometer
>
>
>
> Dave (and all):
>
> It took longer than expected, but here is what I was able to dig up
> from my
> archives on barometric pressure.
>
> The root issue being bantered around at the time revolved around
> resolutionof the A/D converter.  The problem was that we didn't
> have or wanted to
> afford an A/D that could cover the entire range of BP values from
> sea level
> to 10,000+ feet and do so with enough resolution to be able to
> accuratelydiscern trends.
>
> The problem is if you can only count on about 9 bits of data (512
> steps), if
> you decided to digitize all possible values at any altitude, any given
> person at a fixed altitude would have only a very small number of
> thosesteps available for local measurements.
>
> This wasn't a revelation of mine (though I was having similar
> concerns), it
> was from Jim Jennings.  From his design and the folk that copied
> him, the
> answer was to use resistors and op-amps to take the output of the
> sensorthat could cover all pressures and normalize the output of
> the sensor so
> that it will put out a specific voltage if the local pressure
> compensatedfor sea level was around 27 inches.  In this way, the
> analog electronics
> compensate for the differences in altitude and then the A/D
> converters range
> only deals with local variations from that baseline.
>
> In my code, the equation is Baval= 27.46+(Bavad*0.429).  I am
> pretty sure
> (but didn't document well), Bavad has a range of 0.0 to 5.0.
>
> This is actually a very way of doing things!  Changes due to local
> pressurechanges are MUCH smaller than changes due to altitude, so
> "nulling it out"
> in the analog domain means we get to use a much cheaper (and 1-wire
> powerable) A/D converter and as it is based on local conditions and
> neverchanges, kind of a waste to use such an A/D for something that
> can be
> calibrated out.  Also, what it also meant was that for programmers
> like me,
> we got to write one piece of code and it will work everywhere
> without the
> need for compensating for different locations.
>
> will
>
>
> -----Original Message-----
> From: Dave Barton [[8]mailto:dbarton at nettally.com]
> Sent: Thursday, April 10, 2008 5:47 PM
> To: [9]will at beals5.com; TAPR Weather Station SIG Mailing List
> Subject: Re: [wxsig] Hobby Boards Barometer
>
> I tried searching the TAPR archives and found one reference to the
> problem  from Mr. Brown, N5KUK, but nothing further.  No rush, Mr.
> Beal,
> not a high-priority, I am just curious.  The APRS specs allow for
> the
> digit, I assume, since it is displayed on the log files.
>
> Dave
> AI4GF
>
>
> [10]will at beals5.com wrote:
>
>
> Folks:
>
> There is a lot of history on this!  Like everything else it was a
>
>
> conpromise. I'm on a trip now, but can see if I can dig them up this
> weekend.
>
>
> will
> Sent with thumbs from a PDA
>
> -----Original Message-----
> From: John Yost [11]<k3yjp at yahoo.com>
>
> Date: Thu, 10 Apr 2008 10:10:12
> To:TAPR Weather Station SIG Mailing List [12]<wxsig at lists.tapr.org>
> Subject: Re: [wxsig] Hobby Boards Barometer
>
>
> I am using a T238+ with the TAPR X1W-1 barometer kit.
>
> I see the same behavior, the local display in inches
> of mercury seems to be ok but loses the resolution
> when sent to aprs.
>
> my .02
>
> john
> K3YJP
>
> --- Ken Brown [13]<Ken.A.Brown at noaa.gov> wrote:
>
>
>
>
> The problem with the pressure reading only reporting
> whole millibar
> readings is not in the barometer but in the T238
> code that converts
> the reading received from the barometer to the
> millibar reading
> transmitted.  At my station, n5kuk, I have two T238s
> each with a
> Bray barometer. One is on APRS and one is a test
> unit on the bench.
> Both exhibit the same problem. The problem is seen
> on the APRS output
> but not on the displayed value in inches of mercury.
>  I use the APRS
> T238 as a standalone with it's own tnc and radio.
> On another APRS
> station with WinAPRS the nice weather graph clearly
> shows the stair
> step of the pressure reading. My units are T238 and
> not T238+.  On
> the bench unit the reading were noted on the rad
> RS-232 output that
> would go to a tnc in APRS service.  Both T238 units
> running the latest
> firmware.
>
> Ken, n5kuk
>
> Dave Barton wrote:
>
>
>
> More information on this issue.  This web site,
>
>
>
>
> [14]http://www.wxqa.com/callsminmax/index_callsminmax.html,
>
>
>
> has logs of
>
>
>
> T238+ weather stations.  I've only checked a
>
>
>
> handful of T238+ stations,
>
>
>
> but I have found a couple of stations which seem
>
>
>
> to to have the same
>
>
>
> issue as mine, N5KUK and K3YJP-4.
>
> There is another station, KA1GEU*, *that has a
>
>
>
> normal range of first
>
>
>
> decimal places.  However, his station is reporting
>
>
>
> via the Internet, not
>
>
>
> radio.
>
> I'll try to find more examples after work today.
>
>
>
> Now I wonder if it's
>
>
>
> the modem board?
>
> Dave
> AI4GF
>
>
> Gerry Creager wrote:
>
>
>
> I'll have to get some sleep, then check but I
>
>
>
> think the APRS spec, and
>
>
>
> therefore FindU, resolve to 1 mB.
>
> More's the pity.
>
> gerry
>
> Dave Barton wrote:
>
>
>
> I am puzzled by something.  I recently bought
>
>
>
> and hooked up a Hobby
>
>
>
> Boards barometer (B1-R1-K) and it seems to be
>
>
>
> working OK.  One thing
>
>
>
> I am seeing is that the barometric pressures
>
>
>
> reported by FINDU almost
>
>
>
> always have a .0 for a decimal millibar value.
>
>
>
> In other words,
>
>
>
> 1016.0, 1017.0, 1018.0.  I have occasionally
>
>
>
> seen xxxx.1 or xxxx.2,
>
>
>
> but never xxxx.3 through xxxx.9.  My FINDU plots
>
>
>
> (AI4GF-4) have a
>
>
>
> very jumpy barometer chart.  The barometer is
>
>
>
> supposed to have a
>
>
>
> resolution of 0.007 in.Hg.
>
> I looked at the raw packets and the anomaly is
>
>
>
> reflected in the raw
>
>
>
> packet.   I have the latest firmware, 1.15.8R.
>
> I saw a suggested mod of a resistor in parallel
>
>
>
> with R1 but have not
>
>
>
> done that because I don't quite understand the
>
>
>
> mod.  Could this
>
>
>
> problem somehow be related to not having done
>
>
>
> the mod?
>
>
>
> 73, Dave
> AI4GF
> Tallahassee, FL
>
>
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> References
>
>    1. http://www.findu.com/cgi-bin/wx.cgi?call=k3yjp-4
>    2. http://www.findu.com/cgi-bin/wx.cgi?call=w0dvm
>    3. http://www.findu.com/cgi-bin/wx.cgi?call=w0maf-1
>    4. http://www.findu.com/cgi-bin/wx.cgi?call=wa2gug
>    5. http://www.findu.com/cgi-bin/wx.cgi?call=ai4gf-4
>    6. mailto:Ken.A.Brown at noaa.gov
>    7. mailto:will at beals5.com
>    8. mailto:dbarton at nettally.com
>    9. mailto:will at beals5.com
>   10. mailto:will at beals5.com
>   11. mailto:k3yjp at yahoo.com
>   12. mailto:wxsig at lists.tapr.org
>   13. mailto:Ken.A.Brown at noaa.gov
>   14. http://www.wxqa.com/callsminmax/index_callsminmax.html
>   15. mailto:wxsig at lists.tapr.org
>   16. https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   17. mailto:wxsig at lists.tapr.org
>   18. https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   19. mailto:wxsig at lists.tapr.org
>   20. https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   21. http://mail.yahoo.com/
>   22. mailto:wxsig at lists.tapr.org
>   23. https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   24. mailto:wxsig at lists.tapr.org
>   25. https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   26. mailto:wxsig at lists.tapr.org
>   27. https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   28. mailto:wxsig at lists.tapr.org
>   29. https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
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