During a vacuum service test, after the engine is shut off, the mercury column should fall no more than 10 inches in five minutes.

During a vacuum service test, after the engine is shut off, the mercury column should fall no more than 10 inches in five minutes.

• A. 5 inches in 5 minutes
• B. 10 inches in 5 minutes
• C. 15 inches in 5 minutes
• D. 20 inches in 5 minutes

Answer: (B)

A vacuum service test checks how well the sealed water supply system holds vacuum after the engine is shut down, by watching a mercury manometer. The important point is the amount of vacuum the system can lose in a short, defined period. The specified limit—allowing the mercury column to fall no more than 10 inches in five minutes—means the system is not leaking significantly; it can maintain nearly its full vacuum for that interval, which is essential for proper priming and steady suction when the system starts or runs. If the mercury column falls more than 10 inches in five minutes, that indicates a leak or loss of seal somewhere in the line, valve, or fittings, which could lead to air entering the system and compromising performance. The 10-inch limit is chosen as a practical threshold that catches meaningful leaks without being so strict that normal, tiny outgassing would fail the test.

A vacuum service test checks how well the sealed water supply system holds vacuum after the engine is shut down, by watching a mercury manometer. The important point is the amount of vacuum the system can lose in a short, defined period. The specified limit—allowing the mercury column to fall no more than 10 inches in five minutes—means the system is not leaking significantly; it can maintain nearly its full vacuum for that interval, which is essential for proper priming and steady suction when the system starts or runs.

If the mercury column falls more than 10 inches in five minutes, that indicates a leak or loss of seal somewhere in the line, valve, or fittings, which could lead to air entering the system and compromising performance. The 10-inch limit is chosen as a practical threshold that catches meaningful leaks without being so strict that normal, tiny outgassing would fail the test.