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FOR LML

On 9/20/01 Bill Kennedy described a fuel level measuring system using a
capacitance probe mounted in a standpipe which is located in the wing root
and then asked if he was missing something.

Yes, Bill, either you are missing something or I am.  My presumption is that
you have based your design on that old truism that a liquid will always seek
its own level. If this truism were true in this case, then the level in the
standpipe would be the same as the level in the tank and your system would
work fine. Trouble is, there is a caveat that goes along with that truism
that is often ignored and that is that the gas pressure on top of the liquid
on both sides must be EXACTLY the same if the levels are to be the same. If
the pressure above the fuel in the tank is different than the pressure above
the fuel in the standpipe then you have created a manometer and the pressure
difference will result in a corresponding difference in level. In this case,
the pressure above the fuel in the tank is the pressure created by the ram
(dynamic) air pressure on the fuel tank vent. Theoretical ram air pressure is
about 1.5 inches of mercury above static at 200 mph and sea level. This is
equivalent to a column of gasoline about 26 inches above static pressure.
While a fuel tank vent will not convert all of this ram air pressure to
static, it will recover a significant part of it. If we assume that half of
it was recovered and your standpipe were vented to an area of pressure which
was equal to the free stream static pressure then the level in the standpipe
would be some 13 inches above the level in the tank and, unless the vent was
very high, fuel would be coming out of the standpipe vent! Keep in mind that
the level you are trying to measure only varies about 9 inches (IIRC) from
full to empty so the introduction of even very small pressure differences
will result in a large error in the fuel quantity measurement.

Since you didn't explain where or how or if you vented the standpipe, I can't
comment on your particular design any further but I think most venting
options have been considered in the past and none, to my knowledge, have ever
been successful. These include not venting the standpipe at all, venting it
in the cabin (!), putting a ram air collector on the standpipe vent, venting
the top of the standpipe to the tank vent space by routing tubing down to the
wing root and then out to the wing tip, etc. If you wish to pursue this
further, please describe how you intend to vent the standpipe and I will
comment in more detail.

Jim McKibbin

LML website:   http://www.olsusa.com/mkaye/maillist.html
LML Builders' Bookstore:   http://www.buildersbooks.com/lancair

Please send your photos and drawings to marvkaye@olsusa.com.