I also have the older Surfmaster PI ...the model just before the PI Pro ...and I cut my teeth on that machine and fell in love with it. I could actually tell the coin denominations from the tones...crown caps were easy...bobby pins also very easy. It had a bad calibration because it ignored all gold under 2 grams but I found lots of gold over 2 grams.
My new Dual Field is just different enough that I'm still learning the tones and consequently digging a lot of iron. I'm only at 50% recognizing bobby pins...sometimes they sound so good. On my best days I usually dig about an equal amount of trash and keepers. Today though...no gold and my trash exceeded keepers by at least 400%. Still..it was a beauty of a day...93 degrees at home...maybe 85 at the beach. I got tuckered out but it was a good tuckered...and I know there will be gold in my scoop again..one of these days... and I really wonder what it will look like!