This blog entry is motivated by all the videos I see posted online that compare one detector against another with the goal of determining which is “best”. Far too many of these rely on air test – in other words testing items above ground with the influence of the ground either completely removed. In my opinion air tests are completely worthless for evaluating in ground depth performance of metal detectors. In fact, I believe that when used to draw conclusions between two detectors air tests can be extremely misleading.
Air tests only serve a few purposes in my opinion. They allow a person to learn basic things like what the target id numbers and sounds are for various items under perfect conditions, and they tell you what a detector will not do. If a detector cannot detect a small gold nugget in an air test, it is unlikely to do so in the ground. Air tests reveal maximum possible performance under ideal low to no mineral conditions. I do not expect detectors to do better in ground than in an air test. Air tests can reveal how well detectors deal with adjacent trash targets. Air tests can also reveal by comparing two of the exact same model of detector if one is possibly malfunctioning, but even then just because one air tests better than the other it may not mean what people think. It could be the one that air tests better is the one that is malfunctioning!
Certain forums obsess over air tests. Yet I do in ground tests on a regular basis that completely reverse the air test results people are getting so excited over. A high frequency detector will often air test better than a low frequency or multi-frequency detector, with opposite results in ground. A VLF can easily do better in air tests than a PI detector, with vastly opposite results in ground.
The key to all detector performance is ground handling capability. Removing the ground from the equation removes the single most important thing people should care about, and that is evaluating the efficiency of the ground balancing method the detector employs. This ties into target ID accuracy, which also can only be evaluated in ground.
In fact, I can easily misadjust a detector’s ground balance to make it perform better in the air, while that very same adjustment will make it perform worse in the ground. I have seen people take detectors with factory preset ground balance settings, and attempt to get better performance by setting it themselves. They usually do so by using air tests to set the internal pot to get the best air test possible. They are then usually surprised to find out the in ground performance actually got worse. Well of course - you can only ground balance a detector over the ground! The ground balance setting that works best for mineralized ground will often hurt performance in an air test. That is why a Pulse Induction (PI) detector air tests so poorly compared to a VLF - a VLF has far less inherent ground handling capability than a PI and that ground handling capability is what a PI is all about. It does not make them air test well - BUT WHO METAL DETECTS IN THE AIR?
Air test videos work best for people with low mineral ground, and so are halfway valid for turf hunters or white sand beach hunters. The guys in the eastern U.S. love air tests. For nugget hunters or anyone hunting bad soil conditions, hopefully they know better. VLF detectors in my ground get about 50% of the depth or less than all these air tests that get published all over the place as meaning something. Machines that air test the best are often the absolute worst detectors to put in really bad ground conditions.
There are many videos that compare detectors that have a factory preset ground balance to a detector that has an adjustable ground balance control. In low mineral ground conditions such videos are halfway valid. However, the simple lack of a ground balance control means that in bad ground a detector with a preset ground balance can be seriously out of adjustment and there is nothing you can do about it. Most air test videos only compare detectors in discrimination modes, ignoring that for prospecting the true threshold based all metal is far more powerful than most discrimination modes.
Another common error of is comparing two detectors with vastly different coils. Concentric coils will overload more easily and misidentify non-ferrous items as ferrous in bad ground more readily than a DD coil. This DD advantage of course is completely lost in air tests and in fact concentric coils will usually outperform DD coils in air tests. I hate to have to bring it up, but it is incredibly easy to manipulate detector tests in videos to get a desired result. Always question who is doing comparison videos when two different brands are being shown one against the other, and what their motivations might be.
The best videos are those that show a single detector and show a user how to get the best performance out of it. Nearly all the worst videos are those air testing two or more detectors seeking to determine which is best. Videos of that type must be done in the ground with a great deal of effort expended to explain the conditions and settings and also the inevitable caveats involved in the testing. The main caveats being actual ground conditions relative to where the actual end user is and what types of targets it is that they are seeking. I can show well why one detector perfect for Florida is a poor choice for Arizona, and also the exact opposite. It is all about the ground conditions where a person hunts and that changes from location to location.
As far as I am concerned if people are interested in true detector comparisons the only ones that matter are in ground tests on found targets. That is how I test when I get serious about it. I use air tests and contrived buried item tests to reveal certain basic facts (can this detector detect a one grain nugget?) but for serious testing I have to haul two or more detectors into the field, go find targets, and compare the detectors on the found targets. Most top of the line detectors will find 90% plus of found targets just as well, so it takes a lot of time and effort to find the small percentage of targets that reveal true differences between the best detectors. And even then those results are only valid for me in my ground conditions and must be taken with a grain of salt.
Original version posted 20 December 2015 - 08:33 AM at http://www.detectorprospector.com/forum/topic/1472-minelab-x-terra-705-vs-garrett-ace-250-do-metal-detector-air-tests-have-any-value/?p=17193
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