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Monday, October 19th, 2009

Sauna Physics

Categories: [ Science ]

We discussed sauna physics during this morning's coffee break, especially whether one should throw cold or hot water on the stones.

Here are the physical values:

In other words, you need 355 J to heat 1 g of water from a cold 15 °C to 100 °C, and another 2,260 J to evaporate it; the heating part represents only 14% of the total required energy. If you use hot water (60 °C), you need 167 J plus 2,260 J; the heating here represents 7% of the energy. The relative difference in required energy between cold water and hot water is less than 8%; accoring to my former chemistry teacher, if it's less than 10%, it's negligible.

Additionally, if we consider that 1 kg of burning wood produces roughly 10 MJ (from Wikipedia's Wood fuel article; it depends very much on its moisture content and the efficiency of the furnace, but in ideal conditions you can get 16 MJ out of it), we need about 25 g of wood to evaporate 0.1 L of cold water. Given that I put about 5 kg of wood into the furnace and throw well below a litre of water to the stones, the temperature of the water won't really matter.

The sudden cooling down of the surface of the stones from 300-350 °C to 100 °C when pouring water on them may however have an impact on their capacity to absorb and regulate the heat.

[ Posted on October 19th, 2009 at 17:30 | no comment | ]

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