After dozens of hours behind the stick and in most kinds of weather and soil conditions my ACE 250 finally met a situation it couldn't handle. I was detecting a private athletic field with my ACE and immediately started getting false hits just every few steps. It was the type of signal for example that would show a dime sized coin at 2", I would dig and find no coin. Then I would run the coil back over the same spot and it would show the same signal at another two inches and so on, but nothing to be found. This happened about 90% of the time so I quit hunting and went home to get my GTI 2500.
What I discovered was that the athletic field had been graded and backfilled with slag from a large foundry that once operated here in town. It was having trouble getting rid of the slag piles due to the trace metals in the slag so it was giving it away free to various contractors to use for fill. Under about 4 to 6" of topsoil was a solid bed of slag. Since the ACE has a set ground balance, it could not contend with the changing conditions of the slag. Or at least that's what I finally narrowed it down to? The GTI 2500 on the other hand has a auto-track feature that automatically adjust the ground balance as you go, or you can set it manually. With the GTI 2500 I dug up over $3.00 in coins in a small 10X10'area in about an hour near the concession stand. I counted six dry holes I had dug with the ACE the day before! The 2500 would sound on the slag often too but I could tell from the tone it was not a good signal as it was usually broken, kinda' off key and non-repeatable. A lesson well learned. Jim
What I discovered was that the athletic field had been graded and backfilled with slag from a large foundry that once operated here in town. It was having trouble getting rid of the slag piles due to the trace metals in the slag so it was giving it away free to various contractors to use for fill. Under about 4 to 6" of topsoil was a solid bed of slag. Since the ACE has a set ground balance, it could not contend with the changing conditions of the slag. Or at least that's what I finally narrowed it down to? The GTI 2500 on the other hand has a auto-track feature that automatically adjust the ground balance as you go, or you can set it manually. With the GTI 2500 I dug up over $3.00 in coins in a small 10X10'area in about an hour near the concession stand. I counted six dry holes I had dug with the ACE the day before! The 2500 would sound on the slag often too but I could tell from the tone it was not a good signal as it was usually broken, kinda' off key and non-repeatable. A lesson well learned. Jim
I was running it on jewelry at first and then switched over to custom and tried to discriminate out the slag, but it wouldn't work. Didn't try all metal Mode. There was all kinds of slag including copper and brass, zinc, iron and steel, lead and possibly some containing just a little silver, and probably some coke....no, not that kind. 
Jim
You know when Coca Cola first came out many years ago ( was invented by a pharmacist ) it had cocaine in it. That's where the name Coca comes from.