Which condition indicates a vacuum gauge's measurement?

A vacuum gauge is specifically designed to measure pressure levels that are below atmospheric pressure. This is important in various applications such as in vacuum systems, where it is crucial to monitor the pressure to ensure that it is effectively maintaining a vacuum state. When the pressure falls below atmospheric levels, the vacuum gauge provides a reading in terms of the degree of vacuum present.

The correct answer focuses on the fundamental characteristic of a vacuum gauge: it is made to indicate how much lower the pressure is compared to the surrounding atmospheric pressure. Therefore, understanding that a vacuum gauge functions within the negative pressure range is key to its application and significance in instrumentation.