Is air testing an accurate way to measure the depth your detector can get? The answer is yes and no. Let me explain...
Air testing a machine is a controlled environment. Assuming the area you are testing is free from interference (emf), an air test is as deep as your machine can get from a recent drop. It cannot get deeper than an air test for an object that was recently dropped and yet to begin the corrosion process.
However, once the halo effect occurs to that object, you can get deeper than the air test. This halo effect can occur for two reasons and sometimes they happen
together 1) because the ground around the object is more compact and
condensed which allows one to easier to detect the target (think of a conductor) and 2) The
item can rust around the object which can cause a greater field of
detection and it to actually gain weight (rusting causes an increase in mass). Either or both of these instances can cause what we perceive
as the halo effect. Much of this depends on the object, its environment,
and how long it's been in the ground. But either of these instances can cause an item to be detected deeper than what we get in the air or recent drop. This is why a quarter in a test garden that has been sitting there for a year might be detected deeper than what one can get with another quarter in an air test.
Now the biggest caveat in the air testing debate - you can't really compare one detector vs another. Why? The issue can be seen in the following example: Let's say detector A air tests a quarter at 10 inches and detector B air tests the quarter at 15 inches. The problem with saying detector B is deeper is that once you add ground (mineralization) the detectors will respond differently. This means that detector A might still get 10 inches in soil vs detector B which might only get 5 inches in that particular soil. Meaning, even though detector A air tests worse, it actually detects deeper than detector B. Thus making it very difficult to compare machine vs machine in an air test.
Does this make an air test meaningless? Not at all. Air tests are helpful to tell you if your machine is working, how it responds to various metal, and what max depth its going to detect recent drops. These are all very useful pieces of information. Air testing is one of many tests. But its not the end all test for metal detectors by any means.
So overall what does my post tell you:
1. An air test is as deep as a machine will detect a recent drop
2. The halo effect can cause a machine to detect deeper
3. You cannot use air tests to compare machine A to machine B