What I meant to say concerning OAT is that it's used in computing TAS
(horizontal or vertical).

I also agree with Don in that I'm really only concerned with "more"
rather than "how much".

Still, just for jollies, I'm going to compare my mechanical vs
electronic varios just to see if the difference between the two
increases with altitude.

On 12/7/2014 1:25 PM, wrote:
On Sunday, December 7, 2014 12:00:08 PM UTC-8, Dan Marotta wrote:
...And it's not even officially winter yet...

Well, it's raining here in northern California, I can't even work on our winch 8^(

OK, I understand about correcting the transducer based altitude,
corrected for ISA, so I guess you're using your static and OAT inputs to
calculate altitude before calculating and displaying the rate of change.

Actually, OAT isn't used to convert pressure to ISA altitude, as what matters is the mean temperature of the column of air between your altitude and sea level, not OAT at that altitude. So, one would need to drop periodic sounding probes to correct for "true" altitude, but just applying a basic mapping from pressure to ISA altitude (or one_over_sigma as Dave indicated) is "good enough" for our purposes (if one wants to know the actual altitude, as such, applying a QNH or QFF setting gets you closer, yet). Likewise, for a transducer variometer hooked to a TE probe, static pressure is typically not used, instead one ends up with effectively "total energy altitude" (where you would end up if you could convert all (?) of your airspeed to altitude), which is again "good enough".


So am I to understand, then, that my mechanical vario is displaying
*indicated* rate of lift/sink while my electronic vario is displaying
*true* rate of lift/sink? I guess I'll take a close look on my next
wave flight.

I think it may be a bit more complicated than that, but I've never really thought about what a mechanical flow-based variometer actually measures...

Marc


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Dan Marotta