I see this subject has reared it's head once again. Some may remember my
proposal to use a permanent air dip tube down to the bottom of the tank and
use an ASI to measure the head (a 130 kt instrument matches the 15 inch head
of fuel).
This obtained PFA approval after the provision for a tap to enable the air
column to be recharged in the event of leakage (or more likely as experience
shows, the loss of some of the column due to turbulence when reading low
levels)
My intention had been to change the sensor over to the ASI port of the RMI
Encoder.
If you have built one of these you will know that the sensor is a solid state
device, similar or identical to that being talked about by Tony K. There
several points about this.
Firstly RMI categorically refuse to take any liability for exposing such a
device for fuel (vapour in my case) and have themselves been searching for a
long time for one that is KNOWN to be so proof. I bought some and find the
spec. passes the buck by merely saying "Limited to (media) which will not
attack polyester, silicon or fluorosilicone". Some someone has got to do
some extensive life tests. It would be prudent otherwise to house any
experimental units in a sealed enclosure just in case. One also has a
potential bleeding problem to avoid air being trapped in the sensor which has
a similar effect to loss of column in the vapour case.
But if you stay with an air (fuel vapour) column, you can rather easily
isolate the sensor from the vapour by arranging a U section in the "pitot"
return to contain a plug of oil or glycerine.This can be either horizontal,
or the system recalibrated to allow for the head change when it moves along
slightly. It also provides a degee of damping which is useful.
Secondly you can see from the RMI circuit diagram that it is surrounded by
some pretty elaborate temperature compensation circuitry, in spite of the
presence of two sensors (another for the static) which work differentially.
This is the answer to the question raised by Nick Hammond - you either have a
second sensor to measure the static pressure above the fuel, or at least lead
the second port of a single differential sensor back to the top of the tank.
For anyone who wishes to experiment in this field see RS components part no.
216-6253
Graham C G-EMIN
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