Personally, I like the Pillar Point optical fuel sensors.  So much so that I even had our guys install them on the prototype Javelin Jet.  These are simply economical versions of the technology that is in many bizjets today.  However, I cannot speak to their in-service longevity as I am not flying yet, and the Javelin only flew six times before we retired it.

 

I have them in multiple places in my Lancair 360.  They are wired into an annunciator system.

 

1)  Header tank low fuel sensor.  I have a capacitance fuel probe in my header tank that displys fuel quantity and also turns the fuel transfer pumps on and off.  However, at a somewhat lower fuel quantity (30 minutes at cruise power, estimated at 5 gallons) the optical fuel sensor becomes uncovered and a red LO FUEL warning comes on.  This is the "no kidding, land right now" warning.  This may happen due to running out of gas, or an unnoticed failure in my fuel transfer system, or a dead capacitance fuel probe in the header tank. 

 

2)  Wing tanks empty sensor.  These are actually installed in the transfer line from the Facet fuel pump outlet to the header tank, immediately ajacent to the fuel pump outlet.  (...then comes a T-fitting with the sensor, and then comes the flexible fuel line up to the header tank...)  A ran a test using the Facet pump to pump water uphill.  When the inlet unported, the Facet pump definitely blew air thru the line so I know the sensor will become uncovered when the wing is empty.  This goes to an amber LEFT WING EMPTY or RIGHT WING EMPTY light, alerting me to turn off the transfer pump.  Otherwise the fuel system is fully automatic with manual override.  Just like my Cessna 150 -- turn it on and forget about fuel transfer, except my Cessna didn't have a "you're about to run out of gas, you moron!" warning. (...and I remember a couple of times when I almost did...)

 

Pillar Point used to sell "modified" T-fittings that their optical sensor screwed into.  They did this to deal with reflections internal to the fitting which spoofed the sensor -- at least that's what they told me then.  I do believe that they sell T-fittings now with the sensor integral to it.  Same concept, but they believe it is a superior implementation.

 

The reason I did this -- and I followed this logic elsewhere -- is for the safety of flight warning sensors (ok, safety of engine operation) to be separate from the normal instrumentation.  If my engine monitor craps out, I still have the annunciator to tell me that I still have fuel and the oil pressure is non-zero.

 

- Rob Wolf