Which component is NOT part of the three lines of defense against refrigerant leaks in a service valve?

The three lines of defense against refrigerant leaks in a service valve are designed to ensure safety and reliability in refrigeration and air conditioning systems. The correct answer highlights that a pressure gauge is not a component of these defenses.

The back seat, packing gland, and protective cap all play integral roles in preventing refrigerant leaks. The back seat provides an additional seal to prevent leaks when the valve is closed. The packing gland is designed to contain the refrigerant within the valve and provides a seal around the valve stem, which is crucial to prevent leaks during operation. The protective cap is used to cover the valve and adds an extra layer of safety by minimizing the risk of accidental opening and exposure to the refrigerant.

On the other hand, a pressure gauge, while essential for monitoring the refrigerant pressure within the system, does not contribute to the physical sealing of the valve itself. It serves a different function by indicating system performance rather than preventing refrigerant loss. Therefore, the pressure gauge does not fit into the framework of the three lines of defense against leaks in a service valve.