Well, usually I am running iron mask around -10 so I am hearing iron falses when in high iron, and if its intermittant nulling I know its iron. Most times if its mineralization then the threshold is nulling everywhere at the site, and I can switch to auto it generally returns if its not iron, then I back off the threshold to around 22, I might not still get a threshold, but thats about as low as I hunt in manual because of mineralization, just hunt a bit slower..not too many areas here I null because of mineralization though..did get it in virginia red soil, but that might have been iron mineralization, and auto seemed to work best there, if you run into strage conditions the best thing to do is experiment. If you get a deep or semi deep signal try different sens settings and see where the signal seems to come in best and leave it there while hunting that site.....Jim