Measuring and testing – Liquid level or depth gauge – Float
Reexamination Certificate
2000-08-25
2002-09-24
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Float
C073S305000, C073S308000, C340S623000
Reexamination Certificate
active
06453741
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a liquid level indicator such as a fuel level indicator used in airplanes, boats and automobiles, and more particularly to a liquid level transmitter with magnetically coupled rotors that does not introduce electrical energy into the fuel tank.
BACKGROUND OF THE INVENTION
Without limiting the scope of the invention, its background is described in connection to fuel tanks, more specifically aircraft fuel tanks.
Liquid level measurement in aircraft, automobiles, boats and other vehicles has historically been measured by either of two methods, float or capacitance probe. Each of the two methods in common use are discussed below. In each of these techniques the fuel tank and its contents are subject to electrical energy in the measuring technique. Recently there have been serious safety concerns due to unexplained aircraft losses which may have been caused by a spark from the electrical equipment inside the fuel tank. Also, systems are described that have been proposed but either have not been implemented, have been implemented with poor results or have been implemented on a test basis only.
Float indicators were used on early aircraft since the aircraft lacked an electrical system. The initial non-electrical float system was used on various aircraft, the most famous being the Piper J-3 “Cub,” which used a cork with a wire imbedded in it that extended into the view of the pilot. The fuel tank in a Cub is directly in front of the windshield and the tank has a cap with a hole that allows the wire to move up and down. The wire has a bend near the top end so that the end of the wire cannot fall into the fuel tank. Additionally, the bent wire is more visible to the pilot than a straight wire. The float. and wire indicator operate on a simple principle: lots of wire showing, lots of gas; no wire showing, no gas.
With the advent of aircraft and cars with electrical systems the float is connected to the arm of a variable resistor whose electrical leads are brought through the wall of the tank. The fuel quantity gauge is connected to the resistor leads and to the vehicle's electrical system. Typically, when the float is on the bottom of the tank, the resistance sensed is low and when the float is high, the resistance sensed is high, on the order of 30 Ohms. This causes the needle on the fuel quantity gauge to deflect as the float height varies thus indicating the quantity of fuel in the tank. For odd shaped tanks, particularly a flat tank in a wing with dihedral, the resistance floats may be connected-in series to cover this longer sloped tank. Techniques exist to calibrate the readings on the gauge, which may be either digital, indicated by a discrete number, or analog, indicated by a needle position.
A resistance float device is used on most, if not all, automobiles, all piston engine aircraft, and some turbine aircraft. This system has been given very poor reviews over the years. If the resistance float is poorly designed and constructed, the gauge is poorly designed and constructed, the gauge poorly marked, the damping not suitable for aircraft use, or if the system is poorly installed and calibrated, the criticism is deserved. This system also introduces electrical energy into the fuel tank.
Capacitance probes are another method used to measure liquid level, especially in fuel tanks. This system uses two concentric tubes arranged in the form of a probe inserted into the top of the fuel tank. Since the dielectric constant of fuel is radically different from that of air, a measure of the height of the fuel level can be made by measuring the capacitance between the two tubes. This system is used on most turbine aircraft and is convenient to install on deep tanks.
The spacing between the tubes can be used to provide a linear output in an odd shaped tank. These systems are generally expensive since the interconnecting wiring must be coaxial cable and some sort of a processor must be used to sum and linearize the probe outputs. Since the dielectric constant and density of turbine fuel varies with temperature, and since the pilot of a turbine installation would like to know the mass of the fuel in the tank rather than the volume, the capacitance probe must have a compensator probe built in to compensate for the dielectric constant to provide an output representing mass, . . . i.e. pounds of fuel. This system also introduces electrical energy into the fuel tank.
In the early days of aviation, an air pressure technique was used to measure the quantity of fuel in a tank. Basically, this method consisted of an air pressure gauge and an air pump connected to an air outlet at the bottom of the fuel tank. When the pump pressurized the gauge and the plumbing connected to the air outlet, the reading on the air pressure gauge increased until the air pressure exceeded the head pressure of the fuel tank and air began to bleed from the air outlet stabilizing the pressure in the system. At this point the reading on the air pressure gauge is equal to the head pressure of the fuel and is indicative of the mass of fuel in the tank. The pressure pump had to be properly designed to avoid excessive flow and thus an erroneous reading.
There has been an attempt to bring this method into the modern age. Systems have been built that used a differential pressure transducer measuring the head pressure in excess of atmospheric pressure. This output is then compensated for acceleration of the fuel mass in a dynamic environment, that is, at a varying distance from the surface of the earth to produce a “stable” indication of the mass of fuel in the tank. This technique suffers from the problem of attempting to measure a small reading which is the difference between two relatively large numbers. In this situation the errors generally overwhelm the attempted measurement.
There has even been an attempt to measure potable water by weighing the container, subtracting the bare weight of the container, and correcting for acceleration. This is arguably a more accurate method since the weight of the contents is much smaller than the weight of the container, but the problem with this method is that the container must be isolated from the structure while it is on the “scales.” This method works for potable water since the container is generally removed from the vehicle for filling, but is not a practical solution for fuel tanks. These techniques do not introduce electrical energy into the fuel tank.
There are a number of schemes based upon internal reflection of light in a polycarbonate rod in the tank. This optical method is most often used for low fuel and low oil measurements and is in wide use in the aviation market. Another common application is the “magic eye” on certain automotive batteries to indicate the level of the electrolyte in the battery. These systems function as a “Yes/No” reading. When the liquid covers the end of the probe the magnitude of the reflected light is radically different than when the end of the probe is in air. In order to use this technique to measure a continuously changing liquid level, the sensor must either have “X” discrete sensors where “X” is the resolution desired or the probe must be designed to internally reflect a varying quantity of light with a varying level of liquid. The multi probe method is not practical in a moving aircraft, while the variable reflectivity method has calibration and long term stability problems. In general, these techniques do not introduce electrical energy into the fuel tank.
Magnetic methods of liquid level measurement utilizing a Hall Effect semiconductor device are discussed in the Honeywell Solid State Sensors Catalog. Determining the height of a float is one method of measuring the level of liquid in a tank. A linear output Hall Effect transducer in placed outside of the tank while a magnet is placed inside the non-ferrous metal tank, and moved by the motion of a float arm. As the liquid level moves up or down, the magnet moves relative to the transducer, causing a change in transducer output vo
Goodwin Gruber, P.C.
International Avionics, Inc.
Navarro Arthur I.
Williams Hezron
Wilson Katina
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