Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com> We usually set the depth at 80% x dia. of copper tubing (OD). It can go as high as 100% if a flat plate is attached across the turns of copper tubing. This would apply especially if you intend to operate the coil in an inverted position (keeps the copper tubing from falling out). Dr. Resonance