Saturday, 18 January 2014

How much water is in the air? A worked example

Here's a practical physics problem. I have a leaking ventilation system with a bowl catching the drips.  How often do I have to empty the bowl?

Assume it's 20 degrees inside, with 40% relative humidity, and a little below freezing outside. Assumptions are approximate. All the best ones are. Save the precision for what you know.

A cubic metre of saturated air hold a little under 20 ml of water at 20 degrees C. A little under 5 ml at freezing. The ventilation system is set to shift 150 cubic metres per hour, so that's something like 450 ml of water per hour. The bowl holds 2.9 litres. It's going to fill up in about 6 hours.

Oh no, better go and empty it!