What unit is commonly used to measure low pressure and vacuums?

The unit commonly used to measure low pressure and vacuums is millimeters of mercury (mmHg). This measurement originated from the use of a mercury barometer, which is a device that measures atmospheric pressure by balancing the weight of mercury in a column against the weight of air above it. The height of the mercury column is directly indicative of the pressure exerted by the atmosphere, making millimeters of mercury a practical and historically significant unit for expressing low pressures and vacuums.

In practical applications, especially in laboratory settings or specific industrial processes, mmHg is often used to specify pressures that are significantly lower than atmospheric pressure, such as in vacuum systems. While other units like pascal can also measure pressure, they are more commonly used for general pressure measurements rather than specifically for low pressures and vacuums.