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[wxsig] Hobby Boards Barometer

John Yost k3yjp at yahoo.com
Wed Apr 30 14:26:26 UTC 2008


Will,

I'd be happy to help test the new code.

john


--- Dave Barton <dbarton at nettally.com> wrote:

> 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
> 
=== message truncated ===



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