What changes inside a sensing bulb in response to temperature variations at the evaporator outlet?

The pressure of the refrigerant inside the sensing bulb is directly influenced by temperature variations at the evaporator outlet. As the temperature increases, the refrigerant within the sensing bulb absorbs heat, causing it to vaporize and increase in pressure. Conversely, when the temperature decreases, the refrigerant condenses back into a liquid, resulting in a decrease in pressure.

This pressure change is crucial for the proper functioning of the HVAC system, as it allows the sensing bulb to relay information to the thermostatic expansion valve or similar device, regulating the flow of refrigerant through the system. The sensing bulb essentially acts as a feedback mechanism to ensure that the system operates efficiently by responding to the evaporator's temperature changes.

The other options don't relate directly to the sensed behavior of the bulb. The quantity and weight of refrigerant in the bulb would remain relatively constant, while the focus here is on the pressure change that occurs due to temperature fluctuations. This pressure variation is vital for the effective thermal regulation in HVAC systems.