T&T: How does a "densometer" work?

[Bob McLeran]

Have a question about how a "densometer" (those seemingly magical 
devices used to detect moisture in boat decks and elsewhere) actually 
work. Do they really detect moisture, or do they simply sense the 
density of the material? Is there a way to differentiate a change in 
density caused by moisture from a change caused by other materials of 
varying density in the structure?

-- 
<><><><><><><><><><><><>Mozilla Thunderbird<><><><><><><><><><>
Bob McLeran and Judy Young                  Manatee Cove Marina
M/V Walkabout (soon to be "Sanderling")     Patrick Air Force Base
Defever 41 Trawler                          Melbourne, Florida