tomcat,
There are a couple of schemes to get moisture into the air from a Scuba tank. Some use direct addition from an in-line canister mounted at the first stage end of the low pressure hose - a more elegant solution is used in the Sherwood Oasis reg, because it is so simple and doesn't require any per-dive maintenance. In the second stage there is a stainless steel plate - that plate gets cold from the air entering the second stage, so when you exhale, moisture from your breath condenses on it. When you inhale, the moisture evaporates, humidifying the dry tank air somewhat, returning some of your exhaled moisture to you. Simple and effective, and if you are one who suffers from "dry mouth" to the extent it interferes with your ability to enjoy your dives, you might want to check it out. I think Sherwood may have put the plate in a higher performance reg as well, but I can't recall the name.
Rick