What type of sensor is activated by a float that sits on top of the liquid level being monitored?

The correct answer describes a contact level sensor, which is designed to detect the level of liquid by utilizing a float mechanism. In this system, the float is positioned on the surface of the liquid. As the liquid level changes, the float moves accordingly. This movement is then translated into a signal that indicates the liquid level—whether it is too high, too low, or at a desired set point.

Contact level sensors are commonly used in various applications because they provide reliable and straightforward measurements of fluid levels. They can work with a variety of liquids and are essential in managing processes where maintaining a specific liquid level is critical. This method of sensing is direct and involves physical interaction with the liquid.

The other types of sensors mentioned do not utilize a float mechanism to measure liquid levels. Pressure sensors, for instance, measure the pressure exerted by a liquid column but do not provide a direct float-based indication. Temperature sensors are focused on measuring the thermal conditions of a liquid, while ultrasonic sensors use sound waves to detect the level of liquid without physical contact. Each of these serves different purposes in monitoring and measuring conditions but does not function in the same way as a contact level sensor with a float.