[aprssig] barometric pressure units
steve at dimse.com
Sun Nov 4 23:13:18 EST 2012
What is going on is a math deficiency on your part.
The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101.325 kPa. The following units are equivalent, but only to the number of decimal places displayed: 760 mmHg (torr), 29.92 inHg, 14.696 psi, 1013.25 millibars or hectopascals
In other words, normal atmospheric pressure is 101.325 kPa, or 101325 Pa. If you wanted to express this in tens of Pa you would divide by 10, giving 10132.5 tens of Pa. APRS drops the decimal, so normal is 10133. This is what is transmitted, not Pa as you state. Since 1 hPa equals 100 Pa, normal pressure is 1013.25. hPa (same as millibars or mb) is the usual display unit, but it is transmitted in 10s of Pa, Pa/10, or tenths of hPa, hPa*10.
All stations near you are transmitting in the correct units, though one does seem to have a calibration issue.
On Nov 4, 2012, at 10:36 PM, Andrew P. wrote:
> That's what I said. They are reporting in pascals, not in 10s of pascals (i.e., drop the units digit). For example, if W3HOA-2 was reporting in 10ths of hectopascals, the value would have been 01016 (assuming rounding), not 10156. If you look at page 64 of the APRS 1.0.1 specification, it says (I quote):
> b = barometric pressure (in tenths of millibars/tenths of hPascal).
> So, what's going on? Has the protocol spec changed since 1.0.1 and I haven't found the change notice yet? Or has there been a de-facto change and the spec hasn't been updated to account for it?
> Andrew Pavlin, KA2DDO
> > From: steve at dimse.com
> > Date: Sun, 4 Nov 2012 22:18:37 -0500
> > To: aprssig at tapr.org
> > Subject: Re: [aprssig] barometric pressure units
> > On Nov 4, 2012, at 9:47 PM, Andrew P. wrote:
> > > But, even so, _no_ station in my area is reporting in 10s of pascals, as specified by APRS 1.0.1.
> > Sure?
> > http://www.findu.com/cgi-bin/wxnear.cgi?call=KA2DDO
> > closest station is
> > http://www.findu.com/cgi-bin/raw.cgi?call=EW0301
> > presently reporting
> > EW0301>APRS,TCPXX*,qAX,CWOP-3:!4002.66N/07527.34W_319/002g010t042r000P000p000L000h62b10150.VWS-DavisVP2+
> > The baro pressure is 5 digits following lower case b, or 10150, which sure looks like Pa / 10 (or hPa x 10) to me.
> > next closest is W3HOA-2, reporting 10156
> > In fact, of the 20 closest, the only one with data that appears bad is N3NTP-2.
> > Steve K4HG
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