BBT & +OPK

So like clockwork every month I seem to ovulate based on bbt, CM and opk's on CD 15, (with a + opk on CD 13).  This month my positive opk's came a bit earlier at CD 11 (2:00pm) the neg on CD 13 (8 pm). My question is how soon after a positive opk should I see a rise in bbt?  I normally get at least a full degree rise.  And is it true that little or no rise means no ovulation?   Sorry if this is confusing :(