Editorial Feature

Optical Detectors - How Much Fuel is in Your Tank?

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All cars are fitted with a fuel gauge, indicated by the scale on the dashboard. Drivers are able to know how much fuel is in their tank by simply noting the position of the arm on the scale.

This is possible due to the ‘sending unit’, which is conventionally a floating device connected to a potentiometer. As the fuel from the tank is used, the tank empties and the float descends. As it does, it slides a moving contact along a resistor, thereby increasing its resistance. The higher the resistance, the emptier the tank. Once the resistance reaches a certain threshold, the indicator shows that the fuel tank is empty, and the driver can refuel the car.

Inaccuracies of Conventional Fuel Level Indicators

While there tends to be some inaccuracy with this method, usually due to unusual tank shapes and dynamic movement of the liquid when the car is in motion (aka the sloshing effect), automobile manufacturers have taken certain precautions to allow for this. In most cases, a level sample is automatically taken every second and reading are averaged out every five minutes. In addition to this, most manufacturers build in a reserve in order to allow time for the driver to refill before the car stops. If both of these failsafe’s do not work, a car will not be able to run.

While this is good enough for most cars on the road, accurate fuel gauges are vital for vehicles such as aircraft where under-fueling could be fatal. Airplanes, therefore, use multiple sensors, in order to gauge an accurate reading.

Pros of Optical Liquid Level Sensors

One of the most popular fuel gauge sensors are optical liquid level sensors. This is because they are compact and contain no moving parts. They very accurate and are able to cope with high pressures and temperatures without losing accuracy or sensitivity.

While it is true that they are mostly used for stable tank level measurement and leak detection, optical sensors are also used in aeronautical engineering and in some automobiles. There are numerous names for the technology, including optical prisms, electro-optics, single-point optical and optical level switches.

They are simple to operate. In most cases, the sensor is enclosed with an LED and a phototransistor. The sensor uses infrared light which is reflected back to the detector when the sensor tip is in the air. If the sensor tip is covered by the liquid, the infrared light is refracted, thereby reducing the amount of energy received by the detector.


Optical liquid level sensors are perfect for point level sensing applications. They are a compact, reliable, solid-state device which can be applied to any tank size. However, there are a few cons with using this type of fuel gauge.

One of the disadvantages to the technology is the fact that the sensor is invasive and requires contact with the liquid. In addition to this, the sensor requires a power source and is not suitable for extremely dense liquids which can cling on the sensor tip and cause inaccurate readings by coating the prism.

Sources and Further Reading

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