don't pay attention to blow back affecting the sub troll speed reading. I've had a sub troll since they first came out, got to be getting close to 20 years now. Every few years I take it back in to have it serviced or just checked out. I was talking to Duane & Hank (owners) and he explained their testing on blow back. What happens is due to the fact that the ball is so close to the probe it pulls the probe level, I had my doubts on that but he told me they did some kind of testing to prove it. Anyways like said get to trust it and pay attention to it and you won't leave home without it.