I doubt that air testing any metal detector will produce real world results, due to the way the soil affects and reflects electrical signals.
This would be especially true with an excalibur and other detectors that discriminate and try to balance out the background mineralization.
Without soil surrounding or underneath the target, the target might be seen as background mineralization.
Of course if all your targets are suspended three feet in the air, then air testing might be valid.
But if your targets are in the ground, test by burying the targets in the ground.
Bill