Electronics International FL-2 - Manual (Page 25)

T TT TTechnical Notes echnical Notes echnical Notes echnical Notes echnical Notes
Page Three
As a general rule, resistive sensors can be unreliable. We tested a number of NEW resistive fuel
sensors and only found two that passed our tests. That was the Stewart Warner F-385-CP05 and the
Rochester 7740-304. We do not know the long-term reliability of these sensor.
Some problems with resistive fuel sensors are listed below:
A. Linearity - This is NOT a problem when used with the FL-2. The FL-2 compensates for sensor and
tank non-linearity.
C. Repeatability - If a sensor cannot repeat its resistive output for a given fuel level, your fuel level
readings on the FL-2 will be inaccurate. Some factors that affect repeatability are:
1. Side loading of the float arm - Most resistive fuel sensors fail this test. This problem is usually
caused by poor design of the float arm linkage and/or wiper.
2. Temperature Changes - Most resistive fuel sensors pass this test.
3. Moisture Changes - Most resistive fuel sensors pass this test.
4. Intermittent opens in the wiper - Many resistive fuel sensors fail this test. This problem can be
caused by low wiper force, poor wiper design, height variation of the resistive element or poor
linkage design. Also, as the wiper bumps onto the terminator (end stops) the wiper can go
open (high resistance).
5. Long term reliability - As the wiper, wire wound resistor and linkage wear, intermittent
“opens” and side loading effects can become a problem. Also, varnish and other products in
the fuel can deposit on the wiper and wire wound resistor. In addition, if the fuel level in the
tank leaves the wiper and wire wound resistor exposed to air for long periods of time, they can
corrode.
The P The P The P The P The P-300C Capacitive F -300C Capacitive F -300C Capacitive F -300C Capacitive F -300C Capacitive Fuel Level Probe uel Level Probe uel Level Probe uel Level Probe uel Level Probe
The P-300C is a capacitive fuel level probe. It does not have any moving parts and, therefore, does
not have any of the failure mechanisms of a resistive fuel level sensor. The electronics that convert
the probe capacitance to a pulse is external from the probe (P-300C Interface Module) and, there-
fore, can be changed without removing the probe. The minimum probe length for
the P-300C probe is 2 feet.
Not all capacitive probes are the same. There are sev-
eral subtle manufacturing design issues that must be
addressed to produce a reliable and repeatable ca-
pacitive probe. These design issues will not be
covered here.