What were this wonderful simple times when in the VW Beetle float in the tank and fuel gauge (pictured below) were so close together that they could be connected by a small Bowden cable. The fuel gauge then showed besides the fuel level even gratis by motion, curves, acceleration and braking, later it was then damped stronger.

That was already the luxury version, because the Beetle has yet witnessed the times of reserve cock. Began the car to jerk due to lack of fuel, you had to actuate it and knew exactly how far you could still drive without refueling. Of course, today it would no longer possible with the tank rear. Should even be present a reserve light, an electrical contact is necessary. But surprisingly, that one remained largely at a mechanical-electrical solution in the automotive sector.

But there are solutions that are less susceptible to interference. Should, for example, only one switching operation be performed, it is sufficient to direct the current to the base of a transistor via a suitable resistor and two pins, the immerse from a certain liquid level and trigger a switching operation. Thats how works e.g. the warning against excessive water level in diesel fuel filter. An additional circuitry prevents permanent on/off. However, such a solution requires relatively uniform resistance of the liquid.

With no moving parts, and regardless of the soiling of the medium to be measured is the measurement with ultrasound, which e.g. there are also waste holding tanks of boats. It should cope even deposits at the sensors. In contrast, of course, fuel is a much cleaner substance. For the come into consideration still a magnetically inductive and capacitive solution (next video).