Pressure Altitude in Loggers

I take as a fact that all flight loggers use a pressure transducer to
record altitude versus GPS altitude(true?). We all get a calibration
document with the recorder which spells out how accurate that
transducer is.

So why do some loggers (i.e. Cambridge 302 - a flight computer) ask
you to set the barometric pressure much as we do with an altimeter?
Others (i.e. Cambridge 302A, EW Microrecorder - both flight loggers)
do not require this. NOTE: I have flown with both a 302 and a 302A at
the same time ... and the overlapped log files in SeeYou were
identical. Therefore I the pre-flight barometer setting is not
incorporated into the FAI flight log.

I can only assume that the pre-flight barometric settings is to
enhance the accuracy of a flight computer, not a simple flight
logger. Is this increase accuracy important? Some flight software
also has the capability of setting "Altitude" (i.e. Glide Navigator
II). Why is that?

Thanks, John "67" DeRosa

On Jun 11, 3:47 pm, ContestID67 wrote:
So why do some loggers (i.e. Cambridge 302 - a flight computer) ask
you to set the barometric pressure much as we do with an altimeter?

Because it has an altimeter (actual altitude) display on the front
face. The pressure setting does not get logged in the IGC file, it's
only there so the pilot may use this instrument to augment the
mechanical altimeter, which may not be as easy to read as a digital
display.

Some flight software also has the capability of setting "Altitude" (i.e.
Glide Navigator II). Why is that?

When providing final glide info, GNII can be configured to either
provide altitude required, or differential altitude. For differential
altitude to be meaningful, the software must know your actual
altitude, corrected for local barometric pressure. And remember,
pressure changes over the course of a day, so be sure to get an
accurate altimeter setting for the home field just prior to making
that final glide. I've seen typical changes of 2-300', but sometimes
more.

-Tom

On Jun 11, 5:47 pm, ContestID67 wrote:
I take as a fact that all flight loggers use a pressure transducer to
record altitude versus GPS altitude(true?). We all get a calibration
document with the recorder which spells out how accurate that
transducer is.

So why do some loggers (i.e. Cambridge 302 - a flight computer) ask
you to set the barometric pressure much as we do with an altimeter?
Others (i.e. Cambridge 302A, EW Microrecorder - both flight loggers)
do not require this. NOTE: I have flown with both a 302 and a 302A at
the same time ... and the overlapped log files in SeeYou were
identical. Therefore I the pre-flight barometer setting is not
incorporated into the FAI flight log.

I can only assume that the pre-flight barometric settings is to
enhance the accuracy of a flight computer, not a simple flight
logger. Is this increase accuracy important? Some flight software
also has the capability of setting "Altitude" (i.e. Glide Navigator
II). Why is that?

Thanks, John "67" DeRosa

The altimeter setting is also useful for post-flight analysis of
the log file (in StrePla, SeeYou, or whatever). The ATS
altimeter-setting entry in an IGC file (for example, in an H
record), may be used by a flight recorder to pass this info
forward. Then, StrePla or SeeYou can use this to adjust
pressure altitude to MSL...

Not sure which recorders or analysis programs actually
implement this correctly !

Hope that helps,
Best Regards, Dave "YO"