How Can a Float Based Sender be so Accurate

Accuracy in Fuel Level Sensing 

Absent the aircraft substantiation that is protected intellectual property at the present time,  it is hard to demonstrate what this system does for the average pilot.  

This fuel system component will generate considerable interest, but fuel level senders of the past have been, an ineffective tool for pilots to utilize.

We can all point to or tell stories of classic flying films where the lead has - tapped the fuel gauge to see if it was reporting correctly.

A technological advancement in this field may not just improve an existing fuel quantity indication system  - but may become a new component with the potential to be disruptive to aviation safety. 

Why? 

In earlier dialogs we discussed the digital aspect of the fuel level sensor output - but what does that mean.

In the diagram above illustrates a distinct position output of the float arm as represented as a ray on the hemisphere.

The middle point being represented by the binary 10000000  - the next ray above is 01111111 the next ray above by the binary 01111110.   Each of these is different.

What this provides is a positive address for the float position for every ray shown on the diagram.

By combining a digital address to a non contact level system.  We allow free motion of the float - no discernible wear - no wear that would affect this digital output.

A measuring system with many discrete addresses over the rise and fall of the float makes it is easier to carefully describe the tank volume into usable information for the pilot.

More information allows complex tank shapes and configurations commonly found in aircraft to be described in more controlled and accurate manner.

The non-contact part of the sensor takes the fuel contents of the tank and it's varying electrical and physical properties out of the equation.  

The controlled float finds the fuel / air boundary in all flight conditions.

Legacy Systems 

Resistive Systems 

So lets compare to a resistive based float system with the resistance trace in the tank  - most general aviation aircraft in the field use this or some variation.

So the manufacturer of the fuel sender will talk about how this resistor card is laser trimmed and it has 50 or 60 precision resistive steps from empty to full when the unit is brand new.  

So that appears to be similar to the example above - yes the digital example has more steps but it is more expensive - yes.  

What is not revealed  is that the resistive steps in output are not distinct or different  - but rather a subtle step increase or decrease in the electrical property when new - after use or wear the subtleties are easily blurred or worn away.  So we in actual practice have a system that now may incur discontinuities in the stepwise output - and provide a less than adequate reading or even deceptive reading for fuel level .   

When we talk about modern general aviation aircraft and the resistance traces become much smaller as shown on the right.   The need for intrinsic safety - explosion proof requirement brought about some subtle changes in aviation fuel systems.  It was no longer acceptable to have wires or traces in the fuel tank proper.  The general aviation industry turned to propane gauges that had the wiper driven by a magnetic couple to an external wiper system enclosed in plastic.


Capacitive Systems

Capacitive Systems - this is the domain of larger aircraft and some small aircraft.  It is the defacto system for aviation.  The designs have no moving parts and are reliable in principle.

Again we are not dealing with distinct positions but an electrical subtlety between one level and another.  So while the fluid provides a good dielectric - the qualities of the fuel become a critical component in how the system works.  Therefore what you add to the tank is a measurable component for the fuel gauge system and is known as the k factor in a capacitance equation.

So what you add - Fuel -  Quality, Temperature, Composition, Entrained Air, Water and Temperature  have a direct bearing on the output of the gauge.

Components of a good Capacitive level systems contain the following:

• Probe Compensators - measure permittivity of the fuel  - ability to carry a charge
• Densitometers  to determine the specific gravity of the fuel
• Temperature compensation - direct though linear effect on k 
• Compensation of tube diameters to provide a linearized output

If your capacitive level system does not have compensation,  it has limited value in aircraft applications. 

Capacitance probes while highly developed and may utilize segregated DC or AC power, can meet requirements for intrinsic safety.   Capacitive systems however will never overcome the fact that we have separated metal tubes connected to external wiring in the aircraft.

Capacitive systems have difficulty with

• Fuel Stratfication - hot fuel added over cold soaked fuel 
• Contamination
• Corrosion 
• Indifferent fuel quality or in non aviation applications alcohol percentages

Capacitance systems in transport aircraft are redundent in that there are two systems for each tank to insure dispatch reliability for transport aircraft.  

Intrinsic Safety for Aircraft Fuel Tanks

INTRODUCTION    
A flammable mixture of fuel vapor and air can exist at times in a partially filled aircraft fuel tank containing jet fuel or much less so Avgas.  Research has been done to develop methods to eliminate or reduce the risk of having an explosive condition in the fuel tank. There are a few different approaches to preventing fuel tank explosions. Explosions need three conditions to occur simultaneously: a flammable fuel source, sufficient oxygen to react with fuel molecules, and an ignition source to start the chemical chain reactions. Eliminating any one of these conditions will prevent a fuel tank explosion. 

REDUCING OXYGEN CONCENTRATION

Recently, attention has been focused on developing a low-cost, low weight, high-efficiency fuel tank inerting system for use in large transport airplanes. This system uses high temperature bleed air from the engines to create nitrogen-enriched air (NEA) with as high as 98% nitrogen concentration. The NEA is plumbed into the ullage space above the liquid fuel in the fuel tank, forcing air out the vents and creating an atmosphere with a maximum oxygen concentration of 12%. This value has been shown to be the lowest oxygen concentration that will support ignition of  fuel vapors. This approach eliminates one of the key ingredients required to have a fuel tank explosion (sufficient oxygen). 

So now we have added a component to the aircraft to address what can't be addressed with a capacitive system in the fuel tank.

REDUCING IGNITION PROBABILITY
Ignition of fuel vapors can occur as a result of several different mechanisms. Voltage sparks, thermal sparks, and hot surfaces are the most probable ignition sources present in or around a fuel tank. Any of these ignition sources could occur due to lightning strikes, electrical faults in fuel tank electronics, or short circuits caused by cleaning debris, such as steel wool or other small conductive filaments that may have been inadvertently left within a fuel tank. Combined with fuel tank inerting, reduction or elimination of the likelihood of ignition sources could provide an additional safety factor to preclude virtually any fuel tank mishaps during the life of an aircraft.

Electrical spark has been the standard method of determining ignition energy required to ignite a flammable mixture. The generally accepted minimum ignition energy for a hydrocarbon/air mixture is around 200 micro Joules (μJ) for a specific mixture of fuel and air, usually at a stoichiometric mixture or slightly richer. The 200-μJ energy in most experiments is the energy stored in a capacitor and discharged across an electrode gap as a voltage spark. It should be noted that the stored capacitor energy is not the exact amount of energy deposited into the spark, as there are always losses between the capacitor and the electrodes. Nevertheless, the capacitor energy is a very good approximation of the minimum ignition energy of a mixture and the relative ignition strength of a voltage spark.

Flammable mixtures can also be ignited by means of thermal or friction sparks. Thermal sparks are different from voltage sparks; they are very small burning particles of metal that radiate bright colors due to high temperature burning. Thermal sparks are produced either by two hard surfaces sliding against each other creating a shower of sparks or a wire or filament making or

Spontaneous ignition of flammable vapors can also occur due to heat transfer from a hot surface to fuel molecules. A standard test method has been developed to measure the autoignition temperature of a liquid fuel by dropping a small amount of fuel onto a flat, heated surface and noting the temperature at which a flame is observed. It has been accepted that the autoignition temperature of jet fuel is around 450 ̊F and Avgas is around 536 ̊F , although these are not exact figures. Many factors can affect the ignition of the fuel vapors and the propagation of a flame front from the hot spot. The design of the test apparatus will determine the type of combustion that will occur. Cool flames can develop and propagate through a flammable mixture without creating an explosion as long as the rate of heat generated is not much greater than the rate of heat lost; explosions can only occur if significantly more heat is generated than lost.

Currently, the Federal Aviation Administration (FAA), guidance for electrical systems that introduce electrical energy into fuel tanks, such as fuel quantity indication systems (FQIS), provided in draft Advisory Circular (AC) 25.981-1C, states a maximum steady-state current of 10 milliamps (mA) root mean square (rms) is considered an intrinsically safe design limit for FQIS. It also states that current levels above 10 mA rms, particularly for failures and transient conditions, could also be considered acceptable, provided that proper substantiation by test and/or analysis justifies them as intrinsically safe. As an example, the AC states that for transient conditions, it is acceptable to limit the transient current to 150 mA rms, and failures that result in steady-state currents above 10 mA rms should be improbable and not result in steady- state currents greater than 30 mA rms. These values were determined after a considerable factor of safety was applied to the lowest values found from previous tests using Jet A vapors and steel wool filaments as the ignition source. The experimentation presented in this work was performed using a calibrated gas mixture with a predetermined minimum ignition energy to solidify the confidence in the electrical current guidance in draft AC 25.981-1C. 

CIES Inc FQIS SENDERS - 
Our senders do not have any electrical components in the tank, and no generated heat energy to ignite fuel in the tank.   The system measures the position of a magnetic pair on a float arm located inside the tank from a location outside the fuel tank proper.   This method of sensing is Anisotropic Magneto Resistive technology and is exclusive to CiES Inc.   

The CIES Senders eliminate the in tank ignition sources that could occur due to lightning strikes or  electrical faults in fuel tank electronics.   As the sensor does not rely on the fuel interface for measurement,   corrosion removal is not an issue.   Cleaning materials like steel wool that are used to clean capacitive sensors are not required with the CiES sender design.