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 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 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.
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