While easy to use by a non-technically trained operator, the information gained by these inventions is very limited. Patents have been issued to inventions which determine the water content of materials by measuring the electrical properties of the materials and relating these properties to water content. However such system would not be able to detect the phase separation if its formation has started. Such system would be useful only if historical information of the measured fuel composition is taken into account.
Many patents have been issued that employ sensors of dielectric properties to measure the water content associated with hydrocarbon liquids, especially crude oil. The percentage of water may be determined in both water continuous and oil continuous samples. Methods of determining water content in oil streams also include microwave technologies.
Unfortunately, microwave technologies are often expensive to implement. A particular complication found with some non-aqueous liquids, such as gasoline and diesel fuels, is that their chemical composition and resulting physicochemical properties constantly change due to the variability in the raw materials of their manufacture, the variability in processing procedures and parameters, and variability in the type and amounts of any blending chemicals added.
Additionally, Federal and state laws may require the manufacture of gasoline that meets specific oxygen levels at different times of the year. Such requirements can be met through the use of various ethers or alcohols. The use of alcohols rather than ethers can have an enormous effect on the ability of the gasoline to dissolve water.
Owen and T. The present invention uses multiple point physicochemical measurements that would allow calibration of capacitance-based level meters.
Calibration of the level sensor for changing dielectric permittivity of the fuel would improve their accuracy in measuring oxygenated fuel blends. The table values of dielectric permittivity for such liquids are 2.
The resistivity of these liquids is dependent on the presence of trace-level chemicals and is not generally assumed constant. However, it can be safely assumed that the resistivity of gasoline will be higher than the resistivity of either ethanol or water. Also, the bottom layer will become electrically more conductive. The formation of multiple layers with considerably different dielectric properties makes it impossible to accurately estimate the liquid level using a conventional capacitance level sensor.
Such migration may lower the dielectric permittivity as evidenced by capacitance values of the bottom layer below that of ethanol and also may significantly lower the resistivity of the bottom layer as was observed experimentally. However this effect is in no way prohibitive to this invention and can even form the basis for the phase separation detection using resistivity values.
The positions of the measurement points need to be selected in a way that the top and the bottom layers in the case of the phase separation would be measured distinctly. Increasing the number of measurement positions might improve the accuracy of the system for example in the case when there is significant diffusion between the top and the bottom layer.
Specifically, it is expected that the measurement at the bottom of the tank will indicate a considerably higher dielectric permittivity as measured by capacitance due to a higher concentration of water and ethanol if phase separation exists.
Also, it is expected that the resistivity at the bottom position will be lower than the resistivity of the fuel at an upper portion of the tank. For automated phase separation detection, the values of dielectric permittivities as measured by capacitance of the top and the bottom layer can be compared subtracted from one another.
If the difference is higher than some predetermined threshold difference value, the phase separation event is identified. Alternatively, the dielectric permittivity of the bottom layer can be compared to a predetermined value.
If the permittivity measurement exceeds that predetermined value, the phase separation event is identified. Similarly, the dielectric permittivity of the top layer can be compared to a predetermined value. Once the permittivity of the layer falls below that value because of ethanol migration into the bottom layer , phase separation is also identified. It should be noted that monitoring the changes in the top layer alone will result in lower detection sensitivity compared to monitoring the dielectric changes in the bottom layer.
Other, more complicated algorithms may be utilized employing the combination of measurements of capacitance and resistance of the top and the bottom layers.
There are many types of sensors that can be used to measure the dielectric properties of the liquid phases in the tank as may result from a determination of capacitance and resistance. One of ordinary skill in the art can readily select which sensor would be appropriate to use in the present invention to achieve the desired result. Of course, as the temperature of the contents of the tank are also measured at the same time, one of ordinary skill in the art can also readily determine such temperature by use of conventional temperature sensors.
The simplest sensors include parallel-plate and cylindrical capacitance sensors. Fringing electric field sensors might be utilized as well. It might be possible that in addition to measuring the resistivity and capacitance of the liquid, the fringing electric field sensor might be capable of measuring the degree of cloudiness of the liquid and the properties of the colloidal phase.
This might become important for applications where significant mixing of the separated phases occurs. It is also possible that gasoline degree of cloudiness may be an indicator of an onset of phase separation, and if its measurement is combined with multipoint dielectric measurements, a greater phase-separation detection system accuracy can be achieved.
The capacitance and resistance measurements might be performed using many different measurement approaches including spectroscopic and microwave techniques. These and other advanced techniques might improve the accuracy of the detection method, but even the simplest capacitance and resistance measurements techniques should be sufficient for the detection of the phase separation.
The choice of the sensor and the measurement technique should depend on the particular application requirements and limitations. One example is the presence of other capacitive sensors in the tank such as level measurement sensor. In this case it is important to select a different operating frequency for the two phase separation detection sensors or one sensor which would be mechanically moved between the two measurement positions in order to avoid the interference from the capacitance-based level sensor.
Since the measurement system described above provides the bulk capacitance and resistance properties of the two layers in the case of phase separation, or the properties of the single gasoline layer if there is no phase separation, it can be used to improve the accuracy of the capacitance-based level measurements by use of a continuous capacitance-based level probe as shown in Figure 4.
As mentioned above, the accuracy of the level measurements is negatively affected by any change in the dielectric properties of the liquid being measured. The intermediate values are simply mapped in between using a linear model. Therefore, any changes in the liquid dielectric properties or formation of separate layers with different properties will create a condition that cannot be estimated using such a simple model and calibration procedure.
For example, if the dielectric constant of gasoline is to increase due to higher ethanol concentration, a typical capacitance level sensor will produce a measurement that is higher than the actual liquid level. By combining the dielectric values obtained with the phase separation detection system with the capacitance of the conventional liquid level probe a much greater accuracy in determining the liquid level can be achieved, hi addition, it might be possible to simultaneously detect the levels of both the top and the bottom layers in the cases when phase separation actually occurs.
Such measurement capability may employ an advanced mathematical algorithm. One approach is to adopt the "Maxwell capacitor" model which predicts the capacitance of a multi-layered sample with layers having arbitrary dielectric properties. By adapting such model, a mathematical relationship can be established to convert the three dielectric measurements two from the top and the bottom sensor of the disclosed system, plus the capacitance of a common liquid level sensor into a pair of liquid level values - one for the top and one for the bottom layer.
Another possible way of accurately detecting the degree of phase separation is to increase the number of measurement positions as shown in Figure 5. For example, a vertical array of capacitance sensors can be utilized at positions of different heights as depicted in Figure 5. The height of the array should be similar to the maximum expected depth of the bottom layer.
The number of sensors in the array should be chosen appropriately for the desired accuracy of the phase separation level measurement. It should be noted that further increase in the number of sensors can improve the system performance only if there is very little mixing of the top and the bottom layers. An algorithms can be applied to each of the sensors in the array. Such algorithms would need to differentiate and account for the "in-between" dielectric properties that will be detected by the sensors located in the area where the layers are mixed.
One of ordinary stall in the art can readily arrive at such desired algorithms. One of ordinary skill in the art is readily capable of accomplishing such temperature-adjustment so that the capacitance and resistance values may be appropriately compared to such pre-determined values.
Such temperature adjustment is necessary as the capacitance and resistance are each temperature-dependent values. It has also been determined that the sensing and comparing of both capacitance and resistance enables advantageous results to be achieved not otherwise obtainable by sensing and comparing only one of capacitance or resistance.
While the use of only a single sensing of capacitance or resistance at multiple points within the tank may be of some value in determining the presence or extent of phase separation, it has been found that various water- soluble fuel additives migrate to the ethanol-water phase, thus potentially skewing any resulting capacitance or resistance values that may be obtained in the absence of such additives. The fuel hoppers are isolated compartments within the tanks that are continuously supplied with fuel by gravity and a fuel ejector pump system.
Fuel is removed from the fuel hoppers to supply the engines and APU. To prevent the outward flow of fuel during in-flight turns, ribs within the tanks are fitted with baffles and check valves hinged to open toward the wing root, thus ensuring the hoppers remain full at all times. Fuel is contained within most of the interior of the wing, with the tank dimensions defined by the front wing spar, rear wing spar and the upper and lower wing skin.
The interior of the wing is coated with a sealant during manufacturing to prevent fuel leakage. The shape of the wing accommodates the installations necessary for efficient operation of the fuel system.
The tank area near the wing root has the largest volume and houses the fuel boost pumps and fuel feed lines. To prevent the outward movement of fuel during flight maneuvers involving turns, the five wing ribs within the tank that form the contour of the wing are fitted with baffles hinged to open only in the direction of the wing root.
The wing ribs also contain a series of holes above the baffles to allow fuel to flow outboard during single-point pressure fueling, since the single-point access is located near the wing root. A second set of holes penetrate the wing ribs below the baffles to allow residual fuel and any accumulated water to drain inboard to the wing root as fuel quantity decreases. NOTE: The size of the wetted area described for seep, slow seep or heavy seep is limited by evaporation of fuel. When fuel evaporates from a wetted surface at the leakage rate, the wetted area will not enlarge.
For more about fuel leaks, see: G Fuel Leak Inflight. The hot hydraulic fluid is cooled by fuel in the tank and tank fuel is slightly warmed, although the high ratio of fuel to hydraulic fluid results in a minimal temperature rise then returned to the system reservoir, after passing through a system return filter located in the filtration module.
The right system also includes plumbing to the motor section of the PTU and return. The hot hydraulic fluid is cooled by fuel in the tank and tank fuel is slightly warmed, although the high ratio of fuel to hydraulic fluid results in a minimal temperature rise then returned to the system reservoir, passing through the return filter en route.
A residual benefit is heated fuel. The G places on-side hydraulics to the on-side hopper, the G crisscrosses. The heated fuel is sprayed on top of the wing fuel throughout the tank. The illustration shows those piccolo tubes in the outboard section labeled "A" but there are also piccolo tube assemblies at positions "B" through "G" on both wings.
The probes are placed in positions that allow determination of fuel quantity for the full range of tank capacity, from full to lowest usable level. The probes are connected through a wiring harness that enters the tank through the forward wing spar The harness leads from the wing inboard, through the fuselage and up to the FQSC unit in the LEER. The probes measure quantity as a function of electrical capacitance. The FQSC transmits a low voltage signal nominally 5 volts to each probe through a wire in the harness and measures the capacitance through another wire in the probe.
The ends of the two wires are separated by a fixed distance so that the capacitance between the wires is dependent upon the characteristics of the medium between the wires. The capacitance is higher when the probe is immersed in fuel and lower when the probe is exposed to air. The FQSC receives the capacitance signals from all probes in the wing and converts the combined signal to fuel quantity. The capacitance of the probes changes as the dielectric constant of the insulating medium between the capacitor electrodes changes.
When the fuel tanks are full, the capacitance of the probes is greater than the probe capacitance when the fuel tanks are empty. Changes in fuel level cause a change in probe capacitance. This capacitive signal is supplied to the FQSC. What is a capacitor? It is two conductors separated by an insulator. By placing different electrical charges across the conductors you establish an electrical field. The capacitance probe is a cylinder one conductor surrounded by a dielectric the insulator which is surrounded by another cylinder the other conductor.
The capacitance between the two tubes varies depending on how much of the tube is submerged in fuel. Note there are no moving parts but aircraft attitude can influence readings since more or less of the probe will be covered by air. The low level probe is installed in the hopper, near the lowest level of the tank. When the fuel level drops to expose the probe to air, the capacitance falls and a low level signal is provided to the FQSC.
The low level probe is positioned so that the low fuel level signal will occur at approximately lbs. The high level probe is positioned in the vent plenum so that when fuel quantity has reached tank capacity, the probe will be immersed and provide a high capacitance signal. The high fuel level signal is provided to the external refueling panel at the right wing root in order to illuminate the high level warning light on the panel door.
The low level probe is positioned so that the low fuel level signal will occur at approximately six hundred fifty pounds lbs. What about the fuel sloshing from tank to tank? When you roll crisply after takeoff you may notice a growing fuel imbalance of lbs or more. You can transfer the fuel but will end up having to transfer it back. Or you can wait five or ten minutes and it takes care of itself.
With the intertank and crossflow valves closed, there is no connection between tanks so do you have a problem? A crisp turn can put more fuel in the top part of a probe and momentarily fool the FQSC into thinking there is more fuel. Just give it a few minutes to figure it out. And stop rolling the airplane like a fighter. Each tank has a densitometer that determines the density of the fuel in the tank to improve the weight calculation of the FQSC.
The densitometers are electromechanical oscillators that vibrate at a frequency proportional to the density of the medium surrounding the oscillator. The FQSC translates the frequency to density.
The left tank densitometer is located in the tank hopper to monitor existing fuel density and the right tank densitometer is located outboard in the wing tank to measure fuel density as it is loaded in the tank. The dielectric constant is dependent upon the chemical makeup of the fuel, and different fuel brands or types have different additives that effect the dielectric constant. The right wing has an additional compensator outside of the hopper to determine the constant as fueling is in progress.
The FQSC adjusts the capacitance reading of the fuel quantity probes using the dielectric constant reading from the compensator sensor. There isn't anything published about the accuracy of the fuel gauges.
Of course we made that up but it seemed safe. The G has many more probes and a dedicated computer to keep tabs on everything.
Most of us use 3, lbs total as an absolute minimum. So on a tank we are only as good as plus or minus 0. Each compartment is formed by the rear wing spar, the rib at the centerline of the fuselage, an outboard wing rib and a wall located forward of the rear spar.
The outboard wing rib and the forward wall have baffles directing fuel into the hopper, and drain holes for residual fluids. Electric fuel quantity indicators are more common than mechanical indicators in modern aircraft. Most of these units operate with direct current DC and use variable resistance in a circuit to drive a ratiometer-type indicator. The movement of a float in the tank moves a connecting arm to the wiper on a variable resistor in the tank unit.
This resistor is wired in series with one of the coils of the ratiometer-type fuel gauge in the instrument panel. Changes to the current flowing through the tank unit resistor change the current flowing through one of the coils in the indicator. This alters the magnetic field in which the indicating pointer pivots.
The calibrated dial indicates the corresponding fuel quantity. Figure 4. A DC electric fuel quantity indicator uses a variable resistor in the tank unit, which is moved by a float arm. Digital indicators are available that work with the same variable resistance signal from the tank unit. They convert the variable resistance into a digital display in the cockpit instrument head. Figure 5. Digital fuel quantity gauges that work off of variable resistance from the tank unit are shown in A and B.
The fuel quantity indication of a Garmin G flat screen display is shown in C. Large and high-performance aircraft typically utilize electronic fuel quantity systems. These more costly systems have the advantage of having no moving parts in the tank sending units.
Variable capacitance transmitters are installed in the fuel tanks extending from the top to the bottom of each tank in the usable fuel. Several of these tank units, or fuel probes as they are sometimes called, may be installed in a large tank. As the level of the fuel changes, the capacitance of each unit changes. As the aircraft maneuvers, some probes are in more fuel than others due to the attitude of the aircraft. The indication remains steady, because the total capacitance transmitted by all of the probes remains the same.
A trimmer is used to match the capacitance output with the precalibrated quantity indicator. Figure 6. A fuel tank transmitter for a capacitance-type fuel quantity indicating system. A capacitor is a device that stores electricity. The amount it can store depends on three factors: the area of its plates, the distance between the plates, and the dielectric constant of the material separating the plates.
A fuel tank unit contains two concentric plates that are a fixed distance apart. Therefore, the capacitance of a unit can change if the dielectric constant of the material separating the plates varies. The units are open at the top and bottom so they can assume the same level of fuel as is in the tanks. Therefore, the material between the plates is either fuel if the tank is full , air if the tank is empty , or some ratio of fuel and air depending on how much fuel remains in the tank.
Figure 7 shows a simplified illustration of this construction. Figure 7. The capacitance of tank probes varies in a capacitance-type fuel tank indicator system as the space between the inner and outer plates is filled with varying quantities of fuel and air depending on the amount of fuel in the tank.
The bridge circuit that measures the capacitance of the tank units uses a reference capacitor for comparison.
When voltage is induced into the bridge, the capacitive reactance of the tank probes and the reference capacitor can be equal or different. The magnitude of the difference is translated into an indication of the fuel quantity in the tank calibrated in pounds. Figure 8 represents the nature of this comparison bridge circuit. Figure 8. A simplified capacitance bridge for a fuel quantity system. The use of tank unit capacitors, a reference capacitor, and a microchip bridge circuit in the fuel quantity indicators is complicated by the fact that temperature affects the dielectric constant of the fuel.
A compensator unit mounted low in the tank so it is always covered with fuel is wired into the bridge circuit. It modifies current flow to reflect temperature variations of the fuel, which affect fuel density and thus capacitance of the tank units. The amplitude of the electric signals must be increased to move the servo motor in the analog indicator. Additionally, the dielectric constant of different turbine-engine fuels approved for a particular aircraft may also vary.
Calibration is required to overcome this. Figure 9. A fuel quantity tank unit and compensator unit installed inside a wing tank. A fuel summation unit is part of the capacitance-type fuel quantity indication system.
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