Final answer:
A compound pressure gauge used on the low side of a system measures gauge pressure, which is positive if above atmospheric pressure and negative if below it, indicating a vacuum condition. Manometers and aneroid gauges are other types of devices used to measure various pressures.
Step-by-step explanation:
A compound pressure gauge, typically used on the low side of the system, measures pressure that may be either above or below atmospheric pressure. These gauges are designed such that they ignore atmospheric pressure, thus they read zero when exposed to it. This concept is known as gauge pressure, which is the pressure relative to the atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure and negative for pressures below it, which would indicate a vacuum or a suction condition, typical in applications like a vacuum chamber in physics experiments.
It's important to note, that while pressure gauges like the one on a scuba tank are calibrated to read zero at atmospheric pressure, other devices such as a manometer or a mercury barometer can be used to measure atmospheric pressure or the pressure of gases in settings like a laboratory. The pressure due to the weight of a fluid in a manometer is given by the formula P = hpg, where P is pressure, h is the height of the fluid column, ρ (rho) is the fluid density, and g is the acceleration due to gravity. An aneroid gauge uses a flexible bellows connected to a mechanical indicator to also measure pressure.