As a lot of our bales got wet during the very wet August we experienced, we borrowed a bale moisture probe to find out how far into the bales the moisture penetrated.  Bales that have got wet in the centre will probably not dry out before they rot.  The meter gives a moisture reading between 1 and 99.9.  Anything below 16 is dry enough to stay, any bales with a reading greater than this will need to be removed from the walls.