A humidity test chamber must be supplied with water to create humidity. As the water is heated to create vapor, and hence, humidity, it leaves any impurities behind. Supplying straight tap water will leave a lot of deposits on the humidity heater. Thus will increase maintenance upkeep for cleaning. Moreover, heating efficiency will also be reduced. Therefore water supplied to a humidity chamber must be purified.
On the other end of the spectrum is water that is too pure. Some electronics companies use high-purity water for cleaning, and think that they can supply this water to the chamber. The water is so pure that it is trying to refresh itself of the minerals missing in it, which is why it makes a good cleaner. Used in a chamber over time, it will etch holes into the stainless steel material the chamber is made of. A hole in a chamber is disastrous because moisture can now fill up the insulation space.
