Fuel Probes Background
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For years, while visiting air shows, I always visited vendors looking and comparing various remote capacitance fuel probes (because I wanted some in my own LongEZ).   They were very simple, small and reliable devices and VERY EXPENSIVE ($150-195 each) and I to cheap to spend mucho dinero for such a simple product which doesn't even include the electronics.    What is worst is that they are not really designed for canards.  They have to be inserted through the tank side wall, or you have to somehow fair the modules or probe heads into the airframe.  For a LongEZ there is no solution which will work and eliminate possibley of fuel leakage into the cabin.   A few months ago I had an idea to make a probe setup specifically designed for canard installation though the top of the strake.  What I ended up with is a fuel probe system which would work in any canard, eleimiates any possibly of leakage, does not require fuel removal during installation and is completely invisible.

 LongEZ/VariEZE's are built in way which does not lend itself to the easy installation of capacitance fuel probes (CFP).  Cozy’s have a nice fairing forward of firewall (turtleback side walls) which can be used to cover and hide the probe installation.   I have seen a few attempts to install probes in LongEZ’s, but have never liked the final look of the available products.   Either they are puck type which is harder to seal, or they are WAY too tall to hide.  The shortest probe I have seen was at least 1.5″ tall (or a puck at 1″ tall.   The only advantage to these type systems is that they are removable after installation.    My design can be removed, but not easily, but what can go wrong with a simple tube and with a rod in it?

Design requirements:  A very short probe connection height of less than 3/8″ tall to fit into the space of the 10mm foam core of the strake. 

After many designs a working prototype was finished


Less than 3/8″ tall it can be easily and quickly installed in a new plane or an existing bird.  Is completely invisible after installation and fits nicely in the foam core space.  Installation is invisiable when done, no chance of leakage, does not require retesting of the tanks or draining of fuel to install.

Next, some electronics are needed to make these probes work.

I knew Princeton Technology built capacitance electronic fuel probe modules.   Princeton builds puck styles ($95 for two set points and $145 for five set points) and a remote electronics type with a remote probe ($195 with five set points) and has a great reputation.   At the SNF air show last year,  I talked to  Todd Stehouwer (the owner of Princeton) and finally convinced him to develop a new system specifically for a canard installation.   I could easily see a wide open market for a Canard type probe system as our installation requirements are totally different from other type aircraft.    As many canards owners rewire their canards to install improved electronic engine monitoring systems such as Grand Rapid, Dynon, ECI there is a need for fuel probes but none designed for strake installation.

After many months of emails, calls, a few miss steps, Todd finally completed the engineering for the module needed allow this probe to properly function.    He used the probe test cell I sent him (MGS with AVGAS) for setup, calibration and testing.    The nice thing about his electronics module is that it can be used with either AVGAS or MOGAS (you have to recalibrate it for the specific dielectric of the fuel).   If you change fuels, you have to recalibrate the probe (5 setpoint module) or recalibrate your EFIS.  Not a big deal as one only has to take off the cover and press a button at specific set points as you fill your tank.

Well, call me an over cautious engineer, but I always have to check everything out before I put my foot out there to get stepped,   I check the height dimension,  the dielectric constant of  different epoxies and the accuracy of the voltage output for the two common fuels I use, MOGAS and AVGAS..    I want to be sure I would be as happy with this design as if someone else presented me with a similar product for design evaluation.  So I built a couple of test cells to check on the systems operation.


Here is a couple of issues of concern to me.

1.  Will fuel resistance of either WEST, MGS or EZPoxy  (common epoxies) work for this application?  I know MGS and EZPoxy is multi-fuel safe.  A quick call to WEST Tech Support confirmed WEST resin is fuel safe for AVGAS or MOGAS (NO Ethanol) fuels.   Epoxy fuel compatibility check  …good.

2.  Next does the epoxy affect the overall capacitance of the system?   Epoxy is a dielectric.  Is the dielectric (hence functionality) a factor and/or epoxy dependent?  I built three test cell to evaluate.  One using WEST, MGS and finally EZPoxy epoxy. 


After testing myself I gave my test cells to by Jack Wilhelmson (electrical engineer and developer  of the EZNoselift)  we found the epoxy made no difference to a calibrated probe in AVGAS or MOGAS.  Epoxy/Capacitance  check …good.

3.  Does the OVERALL system work reliably?  A test rig was set up and after gathering about 40 data points, I found overall the system works very well.  Voltage output for different heights were exactly repeatable about 85-90% of the time.  Sometime, it would just be off just a small amount 0.1-0.2 V or so, but no surprises.  I don't even know if that small a voltage would make a difference on a display.   This could be partly due to the difficulty of measuring the capacitance on a 7.75″ probe (the height of the strake) or the accuracy of the VOM.    Todd and Jack also tested the electronics.    Accuracy check.  …good.

Here you see the probe sitting in MOGAS at “FULL”  tank  level in my home test rig.  The voltage shows is 4.96v.  

In my opinion everything in life has Ying, Yang…..Good and Bad sides.  Here is this probe’s good/bad side.

The BAD:
1.  This is a PERMANENT install.  Everything is glassed into place.  You can get it out, but not easily.  Be smart and careful with the installation..  

1.  It is the only probe/electronic system specifically designed and built for a canard aircraft (Varieze’s to Cozy 4)
2.  The electronics are easily removable/serviceable (there is a quick disconnect on the fuel probe wire).
3.  It is VERY easy to install in a few hours.  Installing the first two probes took me around 2 hrs and I didn't know what the heck I was doing!

The Installation:
Now the fun part.  The entire process took about 2 hrs start to finish.  

Top skin removed

Hole cut and prepared

Probe installed and wired

Hole is filled with flox and ready for finishing or an Al disk cover

Install time is about one hour per side.  No tank retesting necessary.

 One afternoon while Todd and I were talking and trying to find a suitable name to order this specific electronics modification, and we decided to just call them the "Longez" modules.

I am now selling fuel probes for Rutan canards and Velocity’s.   If you are interested in buying some, please read the installation instructions before you order.   The pricing is at the end of the installation instructions.