Which of the following is NOT an application of control relays?

Control relays are widely utilized in various automation tasks, but certain functions fall outside their typical application scope. In the context of the choices presented, measuring humidity levels represents a function that control relays are not designed to perform.

Control relays primarily serve roles in controlling operations rather than acquiring data or measurements. They are integral in automating systems like conveyor belts, regulating fluid levels in tanks, and managing the start and stop sequences of motors. Each of these applications relies on the ability of the control relay to execute commands and manage electrical circuits based on predetermined conditions or inputs.

On the other hand, measuring humidity requires specialized sensors capable of detecting and quantifying moisture levels in the environment. While these readings can then inform control relays when to act, the relays themselves do not engage in measurement. Therefore, the exclusion of measuring humidity levels from the list of control relay applications highlights the distinction between control operations and data acquisition functions in automation systems.