I don't use air tests to compare characteristics of different makes and models of detectors. I don't use air tests for the purpose of determining depth of detection for any specific detector. And, I agree that air-testing is not a perfect practice. After all, how many coins do you find suspended in the air? However, for me, it does have a purpose. I use air tests to help me compare separation characteristics of various coils on a given detector. And, I think air tests provide accurate numerical information for target ID. If you look back at some of the posts made over a year ago on this forum, you will find several of us addressed the matter of TID in our initial Field Test reports. In mine, I mentioned that different coins of the same denomination may have totally different readings. For example, I have IH cents that read from 24 up to 32. But if I have one that reads 30, for example, it will always provide a reading of 30, (give or take a notch segment) no matter how many times I sweep the coil over it. And, I mention that some coins will read differently after they are dug, as compared to what they read in the hole. But, I believe that is more due to the angle of the target and adjacent targets than because it was simply under ground.
I can't think of why the numeric readings you get from passing your coil over a target laying on the ground should be any different than if the target is in the air. The only variance would be due to Ground Balancing the detector, in an effort to neutralize the soil. Ground balancing a detector should be thought of as the "First Responder" to hunting a site. Setting the ground balance on the detector is simply discrimination of the mineralization at that site. Properly setting the ground balance is an effort to neutralize the readings of the ground, so the detector don't see it. Kind of like if the detector was hunting in air. (see air test comments above) Ground signals tend to stay fairly steady as the coil is moved across the ground. If they do change, it is very slowly, compared to the quick rise that a coin will produce. As well, after the coil passes over the target, the target signal drops off sharply. By looking at the rate of change of the receive signal, rather than looking at the receive signal itself, detectors are able to separate ground mineralization from targets. With the discrimination circuitry of motion detectors, further identification of targets can be determined, by the variances in their signal patterns.
Phase angles.
My point is, whether you lay the coin on the ground or pass it under the coil while suspended in the air, TID for a specific coin is the number you get consistently. The charts I have posted in the past have been based on air tests. If GB is thought of as discriminating out the minerlization of the soil, and adjusting the discrimination (up or down) of your detector does not impact on TID readings. Then IMHO, the TID numeric readings you get from an air test should not be any different than from a coin laying on the ground, passing over it with a properly ground balanced detector. With that said, coil design, improper GB or sensitivity settings or inconsistent sweep speed may cause different readings. If your TID readings are different in the air than they are while the target is laying on the ground, and all other factors are the same, I don't understand why they would vary.
HH Randy
