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[wxsig] Hobby Boards Barometer

Ken Brown Ken.A.Brown at noaa.gov
Thu Apr 10 15:36:47 UTC 2008

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

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
> 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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