If you watch carefully, air tests in fact actually do give out some vital information. They can give you a maximum potential depth reading. Further comparisons in ground will give you an idea of how the detector punches through real soil conditions. Using various conductive targets, will highlight those targets representing those you may pursue. Gold comes to mind. If gold targets suffer in air tests, they most certainly will suffer in real conditions. Air tests also identify VDI numbers or icons, and how they act in the fringe air tests also gives you an idea on how well the overall function of the detector locks on targets.