What is a risk of using a manometer to measure an unknown vacuum?

Using a manometer to measure an unknown vacuum poses the risk that the gauge liquid may be drawn into the system. This occurs when there's a significant vacuum pressure, which can create a difference in pressure that pulls the manometer liquid, often mercury or water, into the system being measured. This can result in contamination of the system, as the liquid is not typically designed to be part of the fluid being measured. It can also impair the functionality of the system by altering the fluid dynamics or introducing foreign substances.

The nature of a manometer involves a column of liquid that equilibrates with the pressure being measured; if the pressure is sufficiently low (in the case of a high vacuum), the manometer may not be able to withstand the draw of that pressure, leading to this potential issue. This is especially critical in systems where the properties of the fluid interact with the measured environment, which can have significant consequences for both operation and safety.