Have you ever wondered why pump performance is even worse on humid days? It’s all because of the water that condenses in the pump. When highly agitated, oil will emulsify with water, so at times, there’s so much water that the oil turns milky. On humid days, you might not be able to get under 100 microns even with a blower.
The water in question is pumped out of the vacuum chamber and piping when drawing vacuum in the chamber, condensing in the pump as the pressure of the water vapor is raised to atmospheric pressure near the exhaust of the vacuum pump.
A single milliliter of liquid water will vaporize to form approximately 1.24 liters of vapor at standard temperature and pressure (STP). Governed by the ideal gas law, when pressure goes down, the volume occupied by vapor goes up. If the pressure drops from a million microns (atmospheric pressure) to one micron, then 1.2 liters of water vapor turns into 1.24 million liters of water vapor. (For reference, 1 mL of water is roughly the volume of the end of your finger under the nail.) Ask yourself how many mL of condensed water are inside your piston pump at any given time, laying in the bottom of the oil reservoir or being thrashed about inside the pump while operating.
Every time your pump goes through a compression cycle, there’s a vacuum that builds up behind the piston. When the pressure becomes lower than the flash pressure, some liquid will turn into vapor within your pump. This vapor can consume up to half of your pumping speed, and it’s ultimately what’s stopping you from achieving the published pumping performance.