Need help with coils

[cowboyup]

I have delta 4000 with 8in coil what is different with the other coils and what do they do

 

[Ahab8]

As a rule the larger coils will go deeper and obviously you can cover more ground at a large site, a field or what not. The small could are great for detecting in area that have a high concentration of iron or trash. They will be better at separating a good target from the iron and trash because they aren't picking up as big an area due to the small size. I have a 5" coil for my T2 and it's great in the trash and iron.