Question 45 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is B **Explanation for B (Correct Option):** A gauge manifold set is fundamentally a block of brass with internal passages controlled by valves. The valve labeled "G" (usually the high-side or liquid-side valve) connects the high-side pressure port ("H") to the common vacuum/charging port ("J"), and also allows communication with the high-side gauge ("A"). When valve "G" is closed (turned clockwise, typically), it physically blocks the internal passage connecting port "H" (high side) to the port "J" (center service/vacuum port). This isolation is the primary function of the valve when it is in the closed position, which is required during reading pressures and/or isolating the system from the center port. **Explanation for A (Incorrect Option):** The valves labeled "G" and "C" (service valves) are typically **open** (turned counter-clockwise, or loosened) to connect the ports (H and I) to the center service port (J). However, to **read** the system pressures on gauges "A" and "B," these valves must be **closed** (seated) to isolate the high and low side from the center service port. If the valves are open, the gauges read the pressure of whatever is connected to the center port (e.g., a vacuum pump or refrigerant tank), not necessarily the stabilized system pressures. Therefore, they must be closed to read system pressure accurately. **Explanation for C (Incorrect Option):** Manifold valves are not typically designed to be "back-seated" like traditional globe valves on refrigeration systems; they are usually designed to fully close one path (H to J) and fully open another (H to A). When the valve "G" is fully opened (unscrewed), it connects port "H" to port "J" (the center port). In this fully opened position, the gauge "A" is **not** isolated; it is connected to both "H" and "J." Isolation of the gauge from port "H" only occurs if the manifold is a specialized type (like an isolation valve manifold), but in standard gauge sets, the gauge is always connected to its corresponding side when the valve is open or closed, unless the valve is specifically designed to isolate the gauge upon back-seating. In most standard manifolds, the gauge (A) is always connected to the corresponding side (H). **Explanation for D (Incorrect Option):** When the valve labeled "G" is closed, its primary action is to isolate the two main ports ("H" and "J"). The design of standard manifolds ensures that the gauge "A" is typically always connected to its corresponding port "H" (the high side) so that the technician can monitor the system pressure regardless of whether the center service port is being used or not. Therefore, closing valve "G" isolates H from J, but **not** H from A.