Breakthrough Capacitive Fuel Probe

Product Preview:  Belite Capacitance Fuel Probe

Last Revised March 8, 2016


Cap Probe

Capacitive Fuel Probe controller module.  Functional prototype shown.  Case not shown.


Probes for fuel tanks have often used the principal of “capacitance” to determine the level of fuel within the tank.  These probes are usually constructed of a thin conductive center probe surrounded by a liquid permeable sheath and a grounded jacket.  Commonly available in lengths from 12” to 24” (or longer), these probes suffer from the following problems:

  • Although many claim to be bendable, in our experience we’ve found them to frequently short and fail when bent.  Bendability is a requirement to fit through access holes and at a diagonal positioning from the hole to the bottom of the tank.
  • Presence of minute amounts of water within the fuel may cause these probes to fail. Water is a very good conductor of electricity (specifically when non-pure or contaminated.)
  • Calibration of capacitance probes is affected by changes in the dielectric quality of the fuel in the tank. Specifically, a mix of AvGas with autogas, or a mixture of alcohol within that autogas will cause capacitive probe readings to fail.
  • We believe that almost all capacitive probes on the market are designed with older technology circuits that are less immune to high electrical noise environments, such as what is found in aircraft. While this is a more subjective claim, we have solid foundation in making new claims of noise immunity with our new design.
  • Most capacitive probes require intrusive, large holes in the top of the tank in order to mount the probe puck.


Belite’s new Capacitive Fuel Probe Solves Problems

Our new probe resolves these issues and offers substantial benefits.  In particular, the probe has the following features:

  • The probe is a flexible wire assembly. It may be routed from one corner of the tank to the opposite corner, and held in place at the far corner using a simple mechanical hook.  It does not care what path it takes, as long as that path is surrounded by the fluid to be measured.  It also does not care (within reason) the length of the wire within the tank, and will accommodate any length between 6 inches and 40 inches (one meter).    The wires are jacketed with Fluorinated Ethylene Propylene (FEP).  This material is amazing.  Quoting one of the manufacturer’s websites:

For instance, in the automotive industry, chemical transference is often needed to deliver vital fluids throughout a vehicle’s complex engine. With this in mind, the flexibility and high working temperature characteristics of fluoropolymer tubing can provide custom applications in long lengths on a variety of vehicles.

Since fluoropolymer tubing is a fantastic insulator, it can offer electrical insulation and chemical resistance that makes it a great splicing aid for high continuous service temperatures in corrosive environments.


  • Our probe will not fail in the presence of water. (Water will affect the reading, but the unit will continue to function in an accuracy degraded mode.)
  • Our probe electronic module includes a calibration button which is used to set the Empty and Full position of the fuel tank. This button may be optionally routed to a pushbutton switch in the cockpit, so that the fuel probe may be recalibrated to Full after the tank is filled.  Therefore, full position calibration is automatically recompensated for type of fuel in the tank – 100LL, AutoGas, with or without alcohol.
  • This probe was designed using an integrated circuit with a novel tuned inductive filter on the probe input. This has demonstrated a great deal of noise immunity.
  • The wire assembly may exit the tank via a vent hole or any other hole. In a good tank design, this would be in that portion of the tank reserved for fuel expansion as the fuel warms up on a hot day.

Cap Probe Wire Module

Prototype wire assembly.  Not final shipping version, but a pretty good representation.

How does this new fuel probe work?  What are its features?

  • The design uses a very stable temperature compensated capacitive oscillator in conjunction with a tuned inductive filter. As fuel surrounds the wire probe, the capacitance changes.  The circuit is capable of resolving very small changes in fuel height and converts this to an output voltage, which is linear to the fuel level.  The more fuel, the higher the voltage.
  • The circuit is compatible with any type of gasoline, alcohol, or jet fuel. Kerosene is just fine.
  • The fuel probe unit provides a linear +5 volt output, compatible with almost all modern fuel display gauges and EFIS displays.
  • We specifically recommend our new RADIANT fuel gauge as a companion to the fuel probe.  While the product preview document you are reading is about the Fuel PROBE, we’ll interject for a moment and show you a few photos of our RADIANT fuel GAUGE.  Here is a screen shot of the fuel gauge:


Screen shot of RADIANT fuel gauge.  Left tank is empty and right tank has been drawing down fuel for approximately 10 minutes.  
We recommend this RADIANT fuel gauge for these reasons:  it has a great 2 minute sloshing filter built in; it shows 15 minutes of fuel trend history, and it is a full color, it is nearly weightless, it is sunlight readable display.  $200.

Now, back to the fuel probe:

  • The controller board on the fuel probe has one switch. For calibration: after powering up, press it quickly to set the “low” fuel point.  After filling with gas in the tank with the probe, press it longer (a few seconds) to set the “high” fuel point.
  • A remote switch may be mounted in the cockpit. Pressing it will reset the “high” calibration point.  To avoid accidental high point resets, this requires a five second touch on the switch.  This allows automatic recalibration after every tank filling.
  • For hard core techies: the unit’s connector also provides a +5v regulated output.  This allows a differential analysis of the signal level, as the output DAC is technically ratiometric to the +5V supply.  If you understand that, you’re an electrical engineer.  If you don’t, just ignore it.
  • For hard core OEMS: this product is also available with digital RS-232 serial output, periodically broadcasting the fuel level at 9600 baud.  This allows direct digital attachment to a variety of EFIS systems, assuming support from the EFIS vendor.


What does the Belite Capacitive fuel probe cost?  When is it available?

  • The unit is priced as follows: Unit + 6” probe — $200; Unit + 12” probe — $210; Unit + 40” (1 meter) probe — $250.  Custom lengths are available; contact factory.  Includes electronic unit in small case; wire probe of 6”, 12” or 40” length; power connector and pigtail harness; short coaxial cable to connect together probe to module.
  • First shipments are projected in Q2.
  • Orders received between March 8 and April 30 receive a 25% discount. In order to reserve your delivery position and receive this discount, we must charge your card at time of order.  All such orders are expected to ship by May 30.
  • A similar special is available on our RADIANT fuel gauge.   Orders received between March 8 and April 30 receive a 25% discount. In order to reserve your delivery position and receive this discount, we must charge your card at time of order.  All such orders are expected to ship by May 30.

Want to see a demo video?  Navigate here:

Cap Probe Demo Video Screen Shot

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