Which of the following is an example of on-off control?

On-off control refers to a control strategy where a device is either completely on or completely off, without any intermediate states. This method is commonly used in systems that require maintaining a certain set point, such as temperature or pressure, where the control only needs to activate or deactivate based on that set point being reached or exceeded.

A thermostat-controlled forced air furnace is a perfect example of on-off control. The thermostat measures the ambient temperature and when the temperature drops below the set point, it activates the furnace to produce heat. Once the desired temperature is reached, the thermostat turns the furnace off. This cycle of turning the furnace on and off based on temperature illustrates direct on-off control.

In contrast, the other options do not fit this definition. A pressure relief valve operates continuously to maintain specific pressure levels and will open or close based on pressure, but it does not strictly adhere to on-off control; it modulates according to pressure fluctuations. A flow control valve, likewise, is designed to adjust flow rates rather than operate in an all-or-nothing manner. Continuous level gauges provide ongoing readings and do not operate like a binary system but instead reflect varying liquid levels continuously.