Terry: You are off to a good start on this video.
I would not use the term knock out iron as it is misleading, in that you know when the coil is over iron as you loose the treshhold tone. The reason why multifrequency VLF detectors are more expensive is that it is more costly to build wide band amplifiers and NEEDED higher capacity power supplies.
I would like to see more on what differentiates these detectors from each other E.G.
Excalibur Nulls, Multivariable tones
CZ21 - conclusive, have three tones, able to cover more ground as it allows you to swing your coil faster.
White Dual Field inner six inch coil has higher sensitivity to gold, 12 inch coil allows you to cover more surface.
Sea Hunter - flexibility. Allows you to change coils and head phones to meet beach conditions.
Sand Shark - Best price point, life time warranty.
I liked your objectivity Terry, I hope you keep it this way.
The scoops should be in a different video.