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How Does a Fuel Level Sensor Work?
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How Does a Fuel Level Sensor Work?
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How Does a Fuel Level Sensor Work?

    Sensors are a crucial part of most vehicles, especially when determining the fuel level in automobiles and aircraft. Whilst running out of fuel might be inconvenient and costly when driving an automobile, in an aircraft it could have dire consequences. In this article, we look at how these fuel level sensors work.


   



    Fuel level sensors, also known as fuel gauges, allow drivers to monitor fuel consumption and help them to determine when to refill the tank. They consist of two main components: the sensing system itself (also known as the sender) and the indicator (also commonly referred to as the gauge).


   



    Fuel gauges work by measuring the voltage across a variable resistor within the sensing system, to determine the level of fuel; which is then relayed to the driver via the indicating system. Several components work within the sensing system, enabling it to detect how much fuel is in a tank, including the float switch, a variable resistor, and a wiper. The sensor system is relatively simple compared to other sensors currently produced, although newer sensor systems can also utilize microprocessors for faster and more accurate measurements.


   



    The sensing system is located in the fuel tank and consists of a float—usually made of foam and connected to an actuating metal rod—attached to a variable resistor. The variable resistors used in fuel levels sensors are often composed of a resistive material, where one end is attached to the ground, with a wiper (much like a very small windscreen wiper) that moves over the resistive material as the float moves. When the float moves due to changing fuel levels, the wiper moves across the resistor, causing a change in voltage. The orientation of the wiper means that the highest resistance is experienced across the resistor when the tank is empty. At this point, the wiper is also as far away as possible from the ground end of the resistor. The change in current is then passed on to the indicator which in turn changes the reading.


   



    However, fuel level sensors in automobiles can often be inaccurate, especially when driving with a full tank. In this scenario, the float will rise to the top of the tank, with the wiper returning to the ground end of the resistor, resulting in a small resistance and a high current passing through the sensor. As the float drops in height, the resistance changes; but the gauge will often remain on 'full' for some time. This is because when the fuel tank is full, the float cannot position itself on top of the fuel, as it is blocked by the tank or is limited by the reach of the actuating rod attached to it; meaning that the float becomes submerged when the tank is full. This leads to inaccurate readings until the fuel drops to a level where the float can sit on top, allowing the resistance to change.


   



    Similarly, when fuel is low, the rod often does not extend to the end of the tank, causing the gauge to indicate an empty tank when actually some fuel remains.


   



    Aircraft Fuel Level Sensors


    Some smaller aircraft use the same sensing mechanisms as automobiles in their fuel level sensors; however, in larger aircraft, a number of sensors are used to cover the increased size of the fuel tank. It is imperative for aircraft to avoid the inaccuracies present in some automotive fuel level sensors, as the stakes are much higher—loss of fuel could have catastrophic consequences. Additionally, sensors used in aircraft must be able to cope with changes in altitude; therefore, they are more sophisticated than automotive ultrasonic fuel level sensors. Aircraft typically rely on either ultrasonic or capacitance sensors.


   



    Ultrasonic sensors measure the height of the fuel in the tank by sending out ultrasonic signals, which are measured on the other side of the tank via another sensor. On the other hand, when capacitance sensors with driver fatigue monitor are used, the fuel passes through special vents when it consumed, causing the capacitance to change in the sensors, enabling the level of the fuel in the tank to be determined. This information is then passed on to the pilots via the onboard computing system.


   



    The level fleet management sensor in a vehicle’s fuel tank is actually a combination of three components; a float, an actuating rod and a resistor. This combination of components sends a variable signal to the fuel gauge or an electronic device -- a “little black box” -- that actuates the fuel gauge. The sensor assembly is often referred to as a sender. It is a relatively simple system once the function of each component part understood.


   



    Float


    The float can be visualized by thinking of the ballcock in a lavatory cistern. The buoyant float -- a sealed composite or metal ellipsoid, or a foam solid -- is typically oval rather than circular and rests on the surface of the fuel. It is attached to a pivoted actuating rod.


   



    Actuating Rod


    As the level of the gasoline or diesel in the tank changes, the float moves up and down with the fuel’s surface. It is attached to a thin metal actuating rod, one end of which moves with it. The rod is pivoted at some point along its length, then the opposite end is attached to a grounded variable resistor.


   



    Resistor


    12-volt power is supplied to one end of the resistor from the vehicle’s battery. A wire from the resistor runs to the fuel gauge. In some vehicles, the wire runs directly to the gauge, and in others it runs to a stepper or an electronic device that interprets the signal and actuates a mechanical gauge or a digital readout.


   



    How It Works


    Inside the resistor, a device with personal tracker that resembles a tiny windshield wiper is moved over a strip of resistive material by the movement of the actuating rod. The farther along that strip from the grounded end of the resistant strip the wiper is, the less electricity is conducted to it by that material. The wiper is oriented so the most resistance is encountered when the tank is at its emptiest, and the least when the tank is full. The maximum signal -- the unmodified 12-volt current -- makes the needle in the fuel gauge swing over to “full.” As the fuel level decreases the float drops, the actuator rod causes the wiper to move across the resistant strip away from the ground, and less current is passed to the gauge. The needle shows a decreasing reading. When the tank is empty, the float is at its lowest and the wiper is at the far end of the resistant strip from the ground, so very little current is sent to the gauge. The needle doesn’t move far, thus reads “empty.”


   



    Inaccuracies


    Often a float will reach the fullest extent of its mechanical travel before the tank is entirely full or entirely empty. This explains why many cars have gauges that stay on “full” for a long time before starting to drop after a fill-up, and why some cars can run for many miles on what appears to be an empty tank.


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