This disclosure generally relates to systems and methods for measuring a level of liquid in a reservoir, such as a storage tank or other container. More particularly, this disclosure relates to systems and methods for liquid level measurement using an optical sensor.
A need to continuously measure the level of a liquid exists in many commercial and military applications. For example, liquid-level sensors are commonly used in the fuel tanks of aircraft, automobiles and trucks. Liquid-level sensors are also used to monitor liquid levels within storage tanks used for fuel dispensing, wastewater treatment, chemical storage, food processing, etc.
Many transducers for measuring liquid level employ electricity. The electrical output of such transducers changes in response to a change in the liquid level being measured, and is typically in the form of a change in resistance, capacitance, current flow, magnetic field, frequency and so on. These types of transducers may include variable capacitors or resistors, optical components, Hall Effect sensors, strain gauges, ultrasonic devices and so on.
Currently most fuel sensors on aircraft use electricity. For example, existing electrical capacitance sensors require electrical wiring inside the tank, which in turn requires complex installations and protection measures to preclude a safety issue under certain electrical fault conditions. This electrical wiring requires careful shielding, bonding and grounding to minimize stray capacitance and further requires periodic maintenance to ensure electrical contact integrity.
For new airplanes with large fuel tanks incorporated in composite wings, the numbers of fuel sensors is large. Using electrical fuel sensors adds more weight to the airplane not only because of the electrical sensor and electrical cable weight, but also because the metal standoffs and harnesses that support the electrical cables and sensors inside the fuel tank add more weight. And more importantly, with large fuel tanks in composite wings, electromagnetic interference (EMI) and lightning can be a challenge for electrical fuel sensors.
Other approaches involve the use of fiber optic fuel sensors which require one or two fiber optic sensing elements to be placed in the fuel. Any change in fuel density causes changes in the index of refraction of the fuel. This in turn causes a change in the intensity of the light transmitted from one fiber optic sensing element to the other. The problems afflicting fiber optic fuel sensors can include fuel temperature variation, icing in the fuel, and fungus and fuel residue deposit on the fiber optic sensing elements that blocks the light transmission and renders the sensor useless over the lifetime of a commercial airplane.
Other, more complicated, optical methods have been studied. One such method connects fiber optics to a capacitance sensor and converts light to electricity to operate the capacitance sensor and then converts it back to light coming out of the sensor so there are still active electronics within the fuel tank. Some have proposed the use of light-leaking fiber for fuel level measurement but these all employ the principle of refraction and require the fiber to be in contact with the fuel to modulate the light transmission angle at the cladding layer due to the different refractive index of the fuel.
There is room for improvements in systems and methods for sensing properties (such as level, density, temperature and chemical composition) of liquid fuel in a fuel tank.