The presence of moisture in a system that uses a vacuum pump can increase the pump down time. This is because the vacuum pump must work harder to remove the additional molecules of water vapor present in the system. Water vapor is considered a gas load in vacuum systems, and the pump must remove all gases to achieve the desired vacuum level. The more moisture present, the more work the pump has to do, and the longer it will take to reach the required vacuum level.

Moisture can enter the vacuum system in various ways, such as from the materials being processed, from leaks, or from outgassing of internal components. To minimize the effect of moisture on pump down time, systems are often pre-baked to drive off moisture, and materials are pre-dried before being placed in the vacuum.

Additionally, the type of vacuum pump used can be affected differently by moisture. For example, some pumps, like dry pumps, may be less tolerant to moisture than others, like oil-sealed pumps, which can handle moisture but may suffer from degraded performance or require more frequent maintenance if exposed to a lot of moisture.