Differential air pressure switches detect which of the following?

Differential air pressure switches are designed to monitor and respond to differences in pressure within a system. These devices can detect both positive and negative pressures because they are equipped with mechanisms to sense and compare two separate pressure points. This dual functionality allows them to effectively monitor a variety of conditions in processes such as HVAC systems or gas-fired appliances.

For instance, in applications like the safe operation of a furnace, a differential pressure switch can determine if there is adequate airflow, detecting when the pressure increases (positive pressure) or decreases (negative pressure) in the venting system. By being able to detect both types of pressure, these switches ensure the system operates safely and efficiently, making the correct answer the recognition of their simultaneous ability to handle both conditions.

The other options focus on restricting the functionality to only one type of pressure or to leakage, which does not fully encompass the capabilities of these switches.