What type of instrument would you use to measure absolute pressure?

The choice of a barometer for measuring absolute pressure is correct because a barometer is specifically designed to measure atmospheric pressure, which is a key component of absolute pressure measurements. Absolute pressure is defined as the pressure measured relative to a perfect vacuum, meaning it includes atmospheric pressure as part of the measurement.

A barometer typically uses a column of mercury or aneroid mechanism to quantify atmospheric pressure, thus providing an accurate reading of absolute pressure. This characteristic is essential in various applications, including meteorology and altitude measurement, where understanding the total pressure is crucial.

In contrast, other instruments like vacuum gauges are intended for measuring pressure below atmospheric levels and are used in applications requiring a specific range of low pressures. A pressure gauge generally measures gauge pressure, which is the pressure relative to atmospheric pressure, and therefore does not account for the atmospheric pressure itself. Similarly, a manometer can be used for gauge pressure measurements, especially in liquids, and it typically compares pressure differences rather than measuring absolute pressure directly.

Thus, the barometer stands out as the correct instrument for measuring absolute pressure.