On 28th March 1979, at Three Mile Island nuclear power plant in USA, part of the core melted in the # 2 reactor. The TMI-2 reactor was destroyed. The cause of the accident was the little malfunction in the secondary cooling circuit which allowed temperature in the primary coolant to rise. This caused the reactor to shut down automatically. This situation developed because the level controls turned off the coolant to the reactor when they detected presence of cooling water near the top of the tank. The water at the top was not because of the tank got completely filled, it was because the water was too little in the tank that it got boiled and swelled up to the top of the tank.
The incident is an example signifying the importance of fluid level sensors and their proper functioning. They are important not only in nuclear plants but in lot many applications. Every car, truck and motorcycle is equipped with a fuel level sensor to measure the amount of gasoline left in the fuel tank. In addition, there are sensors for level measurement of engine oil, brake / power steering fluid, cooling water, windshield cleaning liquid, etc. Industrial applications include liquid level sensing in water treatment tanks, transport and storage tanks, in the petrochemical industry for liquids such as petrol, etc. Liquid level measurement is important in household applications for devices such as automated coffee machines, water dispensers, juice squeezers, water evaporators, steamers, fridges and freezers, boilers, heating systems, dishwashers, washing machines, steam irons, etc.
In short, level sensors are one of the very important sensors and play very important role in variety of consumer/ industrial applications. As with other type of sensors, level sensors are available or can be designed using variety of sensing principles. Selection of an appropriate type of sensor suiting to the application requirement is very important.
WHAT IS A LEVEL SENSOR
Wide spectrum of sensors is available in the market and commonly, they are classified based on the specific application of the sensor. Sensor used for measuring humidity is termed as humidity sensor, the one used for measurement of pressure is called pressure sensor, sensor used for measurement of displacement is called position sensor and so on though all of them may be using the similar sensing principle. In a similar fashion, the sensor used for measurement of fluid levels is called a level sensor.
Quite obvious from its name, level sensors are used to measure the level of the free-flowing substances. Such substances include liquids like water, oil, slurries, etc as well as solids in granular/powder form (solids which can flow). These substances tend to get settled in the container tanks due to gravity and maintain their level in rest state. Level sensors measure their level against a pre-set reference.