Lots of specs written for '19%' - but with a standard meter you'll only see that to about 1" of depth. What happens beyond that is anyone's guess.
I'm not sure how useful it would be beyond 'That looks drier than that timber'... especially working in green material, or even conventionally kiln dried material - where you can only get the timber (unless it is narrow / small) to give up a portion of its moisture on the outside.
The RF folks out west have a pretty sophisticated method of seeing inside timbers:
http://www.fraserwoodindustries.com/index.php?action=products.heartdry 
