Barometer different in graph

[thosj]

I use VWS 14 p42, but have had this issue with several builds. I have a Davis VP2 and this is on Vista.

Once in a while, on a computer startup, the barometer displayed in a graph or alpha numeric display is off from the actual reading and off from the Broadcast Display, upper left (for me). The Broadcast display always matches the station, but the graph or numeric display is off. It's always around 32 - 34 MB. I can to into calibration and put that amount in barometer offset and they match again, and the Broadcast display still matches as it always does. Then, MAYBE, on the next reboot of the computer, the graph will be off by the amount I have in the offset so I have to go in and take it out!! That offset amount also drifts a little, if/when it needs an offset at all. If I put in 32.65, maybe a day later it'll be off by .13 and I'll have to go into calibration and change the amount slightly. This has been going on forever, years, thru upgrades of VWS. I'm just curious what this could be, and if there's something I can set to remedy it. Normally, when it doesn't need an offset, everything matches the VP2, if it's 1023.1 on the station, it's 1023.1 in VWS. Then, suddenly, and only on a reboot, it's off by 32 or so.

What IS this?

[weather9]

You may have already checked this, but make sure the graph is SL Barometer also and not Barometer.
given the difference it sounds like the graph is displaying absolute pressure and not sea level corrected pressure.

[thosj]

Hmmm....putting up a graph with SL Barometer instead of Barometer fixed it. Why would that be? It wasn't ALWAYS off, just once in a while on a reboot! Maybe sometimes it showed absolute pressure and sometimes SL pressure. I'm only at 890 ft above sea level, would it be off around 32MB at that altitude?

[weather9]

Not sure why it would change on a reboot. I never use absolute pressure for anything. It has applications, but for most purposes, everybody uses corrected sea-level pressure. Not sure how much a difference occurs from 890 feet. It is probably rather significant though.

[WeatherBeacon]

Hmmm....putting up a graph with SL Barometer instead of Barometer fixed it. Why would that be? It wasn't ALWAYS off, just once in a while on a reboot! Maybe sometimes it showed absolute pressure and sometimes SL pressure. I'm only at 890 ft above sea level, would it be off around 32MB at that altitude?

Yes. At 890 feet the offset from standard atmospheric pressure would be about 32.507mb.

Kevin...

[NiceBill]

The table below sets out the corrections (in inches)required. Should you live below sea level then subtract 0.06" per 50 ft (15m). To convert the corrections to mm mercury multiply by 25.4 & to millibars by 33.86,
 
Altitude       ADD to barometer   Altitude     ADD to barometer
in ft.(m)      reading                  in ft.(m)     reading

50(15)        0.06                      550(169)     0.59
100(30)      0.12                      600(184)     0.64                                    
150(46)      0.17                      650(200)     0.69    
200(61)      0.22                      700(215)     0.74    
250(77)      0.27                      750(230)     0.79    
300(92)      0.33                      800(246)     0.84            
350(107)    0.38                      850(261)     0.89
400(123)    0.43                      900(276)     0.94
450(138)    0.48                      950(292)     0.99
500(153)    0.54                      1000(307)   1.04

Bill.>>>>>>>>>>>

[thosj]

Yes. At 890 feet the offset from standard atmospheric pressure would be about 32.507mb.

Kevin...

Well, that explains my ~32 off, but still no clue on why sometimes on reboot it was off the ~32, sometimes not. But not a problem now that I have it set to SL!!

Thanks for the replies.