Others on this forum are way more knowledgeable on this than I am, but I'll try to help. Others can jump in with more if needed. This is in the CTX manual:
NOTES
In Auto Sensitivity the detector is able to operate at a higher sensitivity level than you are able
to achieve for the same noise level.
Most Beaches are non‐mineralized, therefore greater depth can be achieved using the highest
stable Manual Sensitivity.
From what I can tell, minelab pretty much recommends auto sensitivity in most cases. They claim that the CTX can get better depth with more stability using auto than you can using manual. Many users seem to get good results using manual. My guess though is that they are pretty experienced.
It would be my opinion that a beginner should use auto. Become familiar with the machine, the sounds it makes, the visual indications, and the reliability of the detections. Meaning, when it detects something, is there really something there? (read about halo effect) When you become familiar, you may then want to experiment with manual and see if you can do better and get greater depth.
I'll tell you what I remember about how auto and manual work. I may not have it completely correct, but it's pretty close. By unstable, this mainly means that the detector starts to read false signals. There's nothing there.
The CTX sends three frequencies down into the ground for depth. In auto, the CTX can adjust these three frequencies independently. If frequency one gets unstable at 15, the CTX can set it at 14. If frequency two gets unstable at 17, it will be set at 16. But, perhaps frequency three can get all the way to level 26 where it begins to get unstable and be set at 25. The result being that you get good depth and good stability.
In manual, when you adjust the level, all three frequencies get adjusted at the same time to the same level. You can't change that. So, if only of those frequencies gets unstable at say 17, then you have a slightly unstable detector a 17. And it will get more unstable with higher settings.
That is why minelab says that the CTX auto setting can do better than you can with manual. I'm guessing that experienced users can sometimes tell the difference between a false signal and a good one. Plus, perhaps they're willing to dig some empty holes in order to get more depth in manual.
As for a higher sensitivity to junk; Yes, higher sensitivity to junk, higher sensitivity to good items, and higher sensitivity to false signals. Setting your sensitivity higher gives you greater depth, but might make your machine crazy enough with false signals that it may become basically unusable.
As for a good setting for manual, I think others could give you better instructions on how to find a good setting in manual. But, as I said, I think a beginner should stick with auto for a while before pushing it with manual. And that's what a higher manual setting is, you're pushing it.
