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Hi Randy, Jeff is correct. It's best to look up such properties in a table made for that purpose. For water, the appropriate table is called the "steam tables". There's a version on-line here. Scroll down and input your pressure in absolute terms and the calculator will tell you what the saturation temperature is at that pressure. The saturation temperature is the same as the boiling point. For example with your 18" vacuum (I am assuming you mean 18" mercury vacuum): Normal barometric pressure at sea level is 29.9 " mercury absolute or 14.696 psi absolute
18" mercury vacuum = 29.9-18 = 11.9" mercury absolute.
18" mercury vacuum = (11.9/29.9)x14.696 = 5.849 psi absolute Entering 5.849 in the correct location in the calculator (situation 2) yields 168.97 degrees F. Therefore water boils at 169 deg F when the pressure is 18" mecury vacuum. The boiling temperature will change with even small changes in pressure. It's the reason why water will boil easier on a mountain top than at sea level and why it boils easier when the barometric pressure falls because of a storm. For other materials you are on your own :o).  Rod
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