Lots of things can affect air tests. There is variation detector to detector and coil to coil of the same model. Environmental noise; either interference to the detector or noise that keeps you from hearing the detector as well. Sweeping the target directly across the center (deepest point of detection) of the coil is important. If you are off center a little, you can loose distance.
The sensitivity setting on the Compadre can make a couple inch difference on the air test results. When the sensitivity is set high, it can also make it unstable in some hunting conditions.
When I do the air tests, most of my detectors do not do as well inside my house. I need to go outside and get away from the house and the power lines. I also use headsets that cut out a lot of outside sound. The Compadre chatters loudly when it is turned on inside my house, even with the sensitivity set fairly low. Sensitivity on the Compadre is an internal adjustment.
Over 7 inches is decent depth for a Compadre. It may be possible to adjust it with the internal potentiometer for a little more depth in air tests. You have to understand what the internal adjustments are that you are setting or you can make performance worse rather than better.
It is interesting that the pull tab gets the best reading in your tests. The Compadre I have does a little better on depth with the higher conductive range targets. My Cibola and Tejon get better depth on mid-conductive range targets than on the high conductive range. The pull tabs are normally in the mid-conductive range. You just might have a little gold magnet with your Compadre.
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