mprather(at)spro.net
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 Posted: Thu Aug 13, 2009 6:55 am Post subject: AeroElectric-List Digest: 12 Msgs - 08/12/09 |
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Any chance of a bad ground or some other noise working its way into the
system? It doesn't sound like sloshing error, and maybe not a sensor
problem - though I don't know enough to rule that out. It also doesn't
sound like a calibration problem. I would stop calibrating as I don't
think it will fix the problem (until you fix the real problem). 
Matt-
| Quote: | Bill and Matt,
I, too, visually check the fuel quantity as always, but it is nice to have
some reasonably accurate indication of fuel quantity in the cockpit while
flying. My AF-3400EM records fuel flow, quantity used/remaining, etc.,
and
it is also a reference tool for decision making.
The Princeton sensors for the RV are very short and they fit vertically
at
the wing root. So, the sloshing error should be minimized as compared to
the Lancair version. There is also a dampening built into the sensor
circuit to help eliminate the sloshing error. So, the sloshing error is
largely
mitigated. The problem I've had is getting the Princeton sensors to send
consistent signals to the AF-3400 engine monitor. I calibrated the
sensors
seven times by draining the tanks and adding measured 2 gallon increments
to each tank. After each addition, I let the fuel settle down before
taking the reading being sent to the engine monitor and entering the
reading
into the engine monitor non-volatile memory. Above about 12 gallons, all
readings are the same because the fuel level is above the top of the
sensor.
The problem I'm having is that the sensor is sending signals to the EM
that
are up and down the scale. The EM is simply displaying the quantity it
recognizes in relation to the calibration entries. I am convinced the
problem lies with the Princeton sensors. Yes, I've talked to the
manufacturer
and he says, "It should work. Try calibrating again."
The fuel sensors are mounted in the wing root and, thus, are difficult to
get to for removal and replacement. So, I'm not excited about pulling out
the current ones and replacing them - too much Pro-Seal.
I'm going to burn the tanks down some more and dip check the tanks and see
how the readings compare.
Stan Sutterfield
<I have the Princeton capacitance sensors in my Lancair. I have not yet
calbrated them, but I would be very shocked if the fuel slosh didn=92t
cause a
lot of errors. The sensors are about 6 feet long and must be no more
than
20 degrees off of horizontal. Fuel sloshing would move a great distance
up
and down the tubes. I have them in there because FAA says I need a fuel
guage. I will check the level visually before each flight and refill
when I
should be at =BC to =BD tank. Reguardless of what the fuel gage says!>
<Seems like sloshing can be dealt with either/both mechanically or/and
electrically:
- A restriction or a baffle can be used to slow the flow of fuel into and
out of the tube(s). I'd be slightly surprised if the manufacturer didn't
include this.
- A low pass filter can be used to average and damp the display in
response to the raw input from the sensor. Again, I'd be slightly
surprised if the manufacturer didn't include this.>
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