The operation of the sensing bulb depends upon the ________ pressure of a volatile liquid.

• A. Absolute
• B. Vapor
• C. Gauge
• D. Atmospheric

Answer: (B)

The sensing bulb works on the vapor pressure of the volatile liquid inside it. As the bulb’s temperature rises, the liquid vaporizes more, increasing the vapor pressure. That pressure is transmitted through the capillary to the actuator, causing it to respond by adjusting the valve. When the temperature falls, the vapor condenses, reducing the pressure and reversing the action. This pressure is specific to the refrigerant and its temperature, not the ambient atmospheric pressure or a gauge reading, which is why vapor pressure is the correct concept here.