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[wxsig] Hobby Boards Barometer

Ken.A.Brown at noaa.gov Ken.A.Brown at noaa.gov
Mon Apr 28 16:34:14 UTC 2008


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 <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 [mailto:dbarton at nettally.com] 
> Sent: Thursday, April 10, 2008 5:47 PM
> To: 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
> 
> 
> 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 <k3yjp at yahoo.com>
> >
> > Date: Thu, 10 Apr 2008 10:10:12 
> > To:TAPR Weather Station SIG Mailing List <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 <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, 
> >>>
> >>>       
> > 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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