Sharing static and pitot line

On Thu, 10 Mar 2005 19:33:57 -0800, Eric Greenwell wrote:

Roger Druce wrote:
While on the matter of line sharing, the following is relevant to sharing
the Total Energy line between a number of flow variometers, mechanical
(Winter, PZL, etc.) and / or electrical (Cambridge CAV II etc.)

Each of the variometers fed by the one TE source have flow through the
instrument to their own flask.

All flasks running off one TE source must be the same physically and
thermodynamically. If you mix flasks with different characteristics then
you will get cross flow between the instruments. You can try this test on
the bench provided you do it carefully. Get two identical pneumatic varios
(with hopefully little inherent instrument error) and two diferent flasks,
say one vacuum flask with heat sink material inserted and the other a same
type vacuum flask without heat sink material inside. Apply a signal via a
Tee junction to the two variometers. Note the variometer readings of the
two variometers at different flow rates, ie calibrate one vario relative to
the other. They won't read the same! Then swap the flasks between the
variometers and repeat the calibration. The relative calibration will swap
over between the varios showing that the flasks are influencing things
strongly due to their different characteristics. Install identical flasks
and the varios will resume responding together.

So use identical flasks to avoid cross flow when using a common TE source.

If you have to worry about the flasks cross-flowing, you have another
problem: your TE source isn't good enough to supply the flow the two
flasks need, and both your varios will operate more slowly than they
would alone. The TE system must be able to supply the TE pressure to the
vario tee, regardless of the flow the varios require. If it does this,
then the flasks can't affect the pressure at the tee, and it won't
matter what size each is. This might require a probe with a larger
hole(s) in it, shorter or larger diameter tubing from the TE probe to
the tee for the varios, or varios that use smaller flasks (i.e., require
less flow).

If the varios all require the same sized flask, why not connect the
varios in series to a single flask ? The small additional volume seen
by the early instruments in the chain, due to the volume of the later
varios will be a source of error, but shouldn't be a large error
unless a small flask is used.
Cheers, John G.