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Originally Posted by Craig
If I understand the question, I don't believe you will see a significant increase in pressure at ambient temperatures. Lets assume the temperature is 70F, at that temperature water turns to vapor (boils) at 0.36 psia (that's a vacuum of about 14.3 psig, or 29 in of Hg). Therefore, if you pump the system down to 29 inches some of the water will turn to vapor until the partial pressure of the water vapor reaches the equilibrium pressure. At that point the water should stop turning to vapor until the system is pumped down again. IMHO, the only way the pressure would continue to increase would be to increase the temperature. Is that what you were asking?
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There were a couple of posts suggesting that 15 minutes wasn't long enough to get rid of all the water in the system. In my post, I was trying to politely suggest that although that is almost certainly true, I didn't think that water could, by itself, account for the pressure increase that
R Leo observed.
At 70 degrees Fahrenheit, the vapor pressure of water is about 0.74 inches of mercury. Randy saw the pressure rise by 11 inches of mercury.
His reading after letting it sit overnight - 18 inches of mercury - is nearly 40% of atmospheric pressure!