Order Tray | Contact Us | Home | SIG Lists

[wxsig] Hobby Boards Barometer

William Beals will at beals5.com
Mon Apr 28 00:57:16 UTC 2008


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 resolution
of 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 accurately
discern 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 those
steps 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 sensor
that could cover all pressures and normalize the output of the sensor so
that it will put out a specific voltage if the local pressure compensated
for 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 pressure
changes 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 never
changes, 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
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> wxsig mailing list
>>>>> wxsig at lists.tapr.org
>>>>>
>>>>>           
> https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   
>>> _______________________________________________
>>> wxsig mailing list
>>> wxsig at lists.tapr.org
>>>
>>>       
> https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   
>> _______________________________________________
>> wxsig mailing list
>> wxsig at lists.tapr.org
>>
>>     
> https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   
>
>
>
> __________________________________________________
> Do You Yahoo!?
> Tired of spam?  Yahoo! Mail has the best spam protection around 
> http://mail.yahoo.com 
>
> _______________________________________________
> wxsig mailing list
> wxsig at lists.tapr.org
> https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>
>   
> ------------------------------------------------------------------------
>
> _______________________________________________
> wxsig mailing list
> wxsig at lists.tapr.org
> https://lists.tapr.org/cgi-bin/mailman/listinfo/wxsig
>   






More information about the wxsig mailing list