Which type of gauge indicates both pressure above and below atmospheric pressure?

The correct answer is the compound gauge. This type of gauge is specifically designed to measure both pressures that are above and below atmospheric levels. It can provide readings for positive pressure (above atmospheric) as well as negative pressure (vacuum conditions).

Compound gauges are commonly used in various applications such as refrigeration systems, as they can monitor both the pressure of the refrigerant and the vacuum levels within the system. By having a single gauge that can display a range from negative values (indicating a vacuum) to positive values (indicating pressure), operators can more easily assess the operational conditions of a system without needing multiple gauges.

In contrast, a standard pressure gauge only indicates pressures above atmospheric pressure, and a vacuum gauge is solely for measuring pressures below atmospheric pressure. The Bourdon gauge, while a type of pressure gauge that can measure various pressures, does not specifically cater to both vacuum and pressure readings in the same way that a compound gauge does.