The charge in a TEV sensing bulb is?

• A. Ambient air
• B. Similar to refrigerant in system
• C. Identical to refrigerant in system
• D. Pure nitrogen

Answer: (B)

The TEV sensing bulb works by using a volatile liquid inside the bulb whose pressure changes with temperature. When the evaporator warms, this liquid boils and increases the bulb’s pressure, which is transmitted to the expansion valve to adjust flow and maintain the desired superheat.

The charge is chosen to be similar to the refrigerant in the system so the bulb’s pressure–temperature behavior tracks the refrigerant’s behavior. This similarity ensures the valve responds in the same way the refrigerant does as load and temperature change, giving accurate, stable control.

Using ambient air or nitrogen wouldn’t provide the correct pressure response to evaporator temperature changes, and while using the exact same refrigerant as the system might work in theory, practical practice uses a similar liquid to achieve the needed volatility characteristics reliably and safely.