Electronics International FL-1 - Manual (Page 26)

Capacitive and Resistive F Capacitive and Resistive F Capacitive and Resistive F Capacitive and Resistive F Capacitive and Resistive Fuel Level Sensor Issues uel Level Sensor Issues uel Level Sensor Issues uel Level Sensor Issues uel Level Sensor Issues
The dihedral of the wing can affect full fuel level readings. Once the fuel level in the tank goes above
the sensor, the fuel level reading on the FL-2 will not read any higher. (For example: if a tank holds 20
gallons of fuel but at 18 gallons the fuel is at the top of the sensor, the
maximum the FL-2 will read is 18 gallons.) The FL-2 reading will only
be accurate for tank levels of 18 gallons and lower. This is a common
problem for most resistive fuel sensor installations. By programming the
FL-2 with a slightly lower Sensor Count for the full fuel level calibration
point, the FL-2 can be made to read the Full Fuel Level.
To minimize this problem with a resistive sensor some aircraft manufacturers will install two fuel
sensors in a single tank; one at the in-board and one at the out-board
portion of the tank. The sensors are wired in series (one sensor must be
insulated from ground). They must not have a combined series resistance
of over 270 ohms to work with the FL-2RA instrument (a paralleling
resistor can be used to fix this problem). Since there are two resistive fuel
sensors in a single tank, there is twice as much to go wrong. It is very
important to use very reliable and repeatable resistive fuel sensors (if there
is such a thing) in this design.
Fuel slosh to the in-board or out-board portion of the tank can also cause inaccurate readings. For
example: depending on the fuel sensor placement in the tank, as the fuel sloshes to the out-board
portion of the tank during a turn, the fuel level reading on an in-board mounted sensor will lower,
indicating an inaccurate low-fuel level reading on the FL-2. It might not seem that this would be a
problem in a coordinated turn but it happens (try it for yourself). This is a common problem in most
resistive fuel sensor installations or for long wet wing tanks with little dihedral. A compensating factor
is that when one wing’s fuel level reads low the other reads high. Therefore, the FL-2 will read cor-
rectly when displaying “Total.” For the most accurate readings, read your fuel level only in straight
and level flight.
If the fuel sensor cannot measure the lowest portion of the tank, the FL-2 will display inaccurate low
fuel level readings. For example: if a tank level of 4 gallons or less does not produce a change in the
output of the fuel level sensor, the FL-2 will display a fuel level of “0” gallons for tank levels of 4
gallons and below. Be sure the fuel sensors are mounted and/or adjusted to measure fuel at the lowest
point in the tank as possible.
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