The middle port on a gauge manifold is typically connected to which device?

• A. Calibrated pressure gauge
• B. Liquid line sight glass
• C. Vacuum pump
• D. Service valve

Answer: (C)

Gauge manifolds use three ports to manage both evacuation and charging. The center port is the common line attached to the yellow hose, and it’s reserved for the vacuum/charging path. When you’re evacuating a system, you connect the vacuum pump to that center port so air and moisture can be drawn out through the manifold and pumped away. The two end ports connect to the system’s low- and high-pressure sides via the service valves, letting you isolate and monitor pressure while the vacuum pulls through both sides.

Other components aren’t meant to be fed directly from this center path—the calibrated pressure gauges and the sight glass serve separate roles in measuring or observing the system, and the service valves are the shutoff points on the system lines. The center port’s job is specifically to provide the vacuum/charging line, which is why the vacuum pump is connected there.