I wonder if you've ever opened the toilet tank at home? There is a plastic ball inside, which is connected to a plastic rod. This interesting looking plastic ball is called a float. It can measure and control the water level in the water tank. When there is no water in the water tank, the float position is low. Open the water inlet valve by pulling the plastic rod; When the water tank is filled with water, the float position is high, and close the water inlet valve by pushing the plastic rod.
There is usually a float in the car oil tank. The difference from the toilet water tank is that the lever in the oil tank is not connected with the valve, but with a sliding rheostat. By using float, lever and sliding rheostat together, we can measure the gasoline in the car's fuel tank.
How do the float, lever and sliding rheostat in the car oil tank work together? It's not complicated. When the oil in the oil tank becomes less, the float sinks, and the lever connected to the float is also linked with the rheostat on its side, resulting in an increase in the resistance of the rheostat. A fixed voltage is applied at both ends of the rheostat. According to Ohm's law, when the voltage is fixed, the increase of resistance will lead to the decrease of current. Therefore, we only need to detect the current to know the sinking degree of the float and the height of the liquid level in the oil tank.
In order to provide warning in case of insufficient oil, when the current decreases to a certain value, the measuring circuit will also light up the warning light of "insufficient oil" on the instrument panel. In order to prevent the failure of the circuit system from causing the oil gauge to still display oil when there is no oil, the automobile oil gauge has a simple but very important setting: once the circuit is disconnected, the instrument panel always displays the state of no oil. This seemingly insignificant design can help people avoid the embarrassment of running out of gas on the way.
An electrified wire in the fuel tank always makes people feel uneasy. What if the current changes too fast one day and the circuit sparks in the fuel tank? What if gasoline slowly corrodes the rheostat, resulting in inaccurate reading of the oil gauge? In order to solve these problems, people invented the pressure type oil tank detector. The detector measures the hydraulic pressure at the bottom of the tank to obtain the volume of oil in the tank. By improving the quality of measuring materials, the effects of corrosion and sparks can be greatly reduced.