On Sunday, December 7, 2014 11:46:38 AM UTC-5, Dan Marotta wrote:
Seems to me that if the vario measures a volume rate of flow, then at
higher altitudes, for a given change in altitude, the change of pressure
would be less (recall that pressure change is non linear with altitude),
hence a lower volume flow, hence a lower rate of change reading (lift or
sink).

So, if I'm climbing at 10 kts at sea level and my vario reads 10 due to
some finite number of moles or molecules of gas exiting the flask, past
a sensor, to the atmosphere, I'd think that, at 18,000' MSL where the
static pressure is roughly half what it is at sea level, then the volume
of air leaving the flask and passing the sensor would be roughly half of
what it was at sea level, then an absolute climb rate of 10 kts would be
displayed at something more like 5 kts. It probably doesn't make a damn
bit of difference anyway.

Flash update! I passed this by my wife (who's a current student glider
pilot) and she opened the topic of indicated vs. true speed. I think
that's the answer. We'll be reading an indicated 10kts regardless of
altitude but our true rate of climb will depend upon altitude. I think
that supports what I said above but I'd still like a mathematical
explanation.

Would somebody with an actual education in the subject please explain it
for the rest of us?

On 12/7/2014 7:39 AM, Dave Nadler wrote:
On Saturday, December 6, 2014 12:55:05 PM UTC-5, Jim Lewis wrote:
I understand now that the TE probe (and the pitot probe,
for that matter) cannot help but sense static pressure -
in addition to dynamic pressure.

Right! This is easy to understand by considering the pressure
sensed by TE or pitot probes with glider on the ground.


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Dan Marotta

What you have described is generally correct for FLOW type variometers such as mechanical instruments and early electric instruments. Almost all modern electric varios use pressure transducers and do not have the change in flow volume to consider.
UH