Boiling at Minus 0.8 Bar

Hackaday's latest vacuum-drying installment puts a self-regulating heater, about 65 °C at the surface, inside a jar pumped to "at least -0.8 bar". That is a gauge reading: 0.8 bar below the air outside. Here is what it does to the boiling point of water, and how close the heater comes.

The curve is the Antoine equation for water, from Wikipedia (A = 8.07131, B = 1730.63, C = 233.426, valid from −20 to 100 °C). The dots are the CRC Handbook values from Wikipedia's vapour-pressure table, which sit on the curve. Water boils where the curve meets the chamber pressure. Water held inside a desiccant is bound more tightly than water in a cup, which is the whole point of a desiccant, so this is the lowest temperature that could work, not the one that will.

The numbers

At −0.8 bar the 65 °C surface is 3.5 degrees above the boiling point, so free water in contact with it boils. Everything a few millimetres away, warmed only by radiation, may not. The gauge is relative, too: on a 980 hPa day the same −0.8 bar leaves 18 kPa in the jar, and water boils at 58 °C.

"Primarily the infrared"

The article says it is "primarily the infrared radiation" in sunshine that warms our skin. In energy terms infrared is about half. Wikipedia's Sunlight gives two splits for sunlight at the ground. One, calculated from NREL's reference spectrum, is 52–55% infrared, 42–43% visible and 3–5% ultraviolet. The other, citing Fondriest, is 49.4% infrared, 42.3% visible and 8% ultraviolet, and its 8% does not square with the 3–5% a paragraph earlier. As a cross-check, a blackbody at the Sun's 5,772 K puts 50.2% of its output beyond 700 nm. Visible light warms skin too.

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