What is the primary purpose of the sensing bulb in a thermostatic expansion valve?

The primary purpose of the sensing bulb in a thermostatic expansion valve is to measure the temperature of the refrigerant. This bulb is typically located on the evaporator or near the outlet of the evaporator coil and is filled with a thermally responsive fluid. As the temperature of the refrigerant changes, the fluid inside the bulb expands or contracts, activating a diaphragm or piston that adjusts the flow of refrigerant through the expansion valve.

By accurately measuring the refrigerant temperature, the sensing bulb plays a crucial role in maintaining the desired temperature levels within the evaporator, ensuring efficient system performance and preventing issues such as freezing or inefficient cooling. Effectively, this means the sensing bulb helps control the system's refrigerant flow based on the thermal conditions of the evaporator, which is essential for achieving optimal system efficiency and comfort.

The other options do not align with the primary function of the sensing bulb. While pressure detection and airflow regulation are important in HVAC systems, they are not the direct responsibilities of the sensing bulb in a thermostatic expansion valve. Similarly, controlling humidity levels falls under different components of the HVAC system, rather than the function of the sensing bulb.