Monday, January 19, 2015

1/19/15 Report - Some Problems With Air Tests: Physical & Psychological. Calm Surf and Sandy Beaches

Written by the Treasureguide for the exclusive use of

Fresh Cut Found On Treasure Coast Beach Yesterday.

Everybody knows that an air test won't tell you exactly what you can expect in the field.  An air test doesn't have to deal with things like ground mineralization, but in my opinion there are much bigger concerns and limitations.

When you test a detector you are not just dealing with one system.  The detector is an electromechanical system, but when you use a metal detector there is actually a much more complex and important system involved - your nervous system, including your sensory system.

Before I address those systems and how they play a very important roll, I want to talk a little about air tests.  There is the problem of generalizability.  An air test is not like a field.  There will normally be some difference in how far your detector will detect an object in air and how deep it will detect an object buried in the ground.  Most everybody knows that.  But those differences are not due ony to the differences in the ambient environments.  Some differences are due to how air tests are conducted.

I'm sure that most of you have conducted an air test and have watched others conducting air tests.   Carefully observe the human behavior involved. 

Usually a person conducting an air test will start close to the coil where a distinct signal is achieved and move the object farther from the coil until the signal is no longer distinguished.  One problem is that during an air test, the object is usually moved right over the sweet spot of the coil.   Most often that does not happen in the field.   Usually the object will be off one direction or another.  During an air test, people usually seek the sweet spot until they get the absolute best signal.

In the field during a hunt, an object will only fall under the sweet spot a fraction of the time.  In more cases than most people think, the object will not pass under the coil at all.  In the field many objects are missed by inches, and when the coil does pass over them, it is just as likely that the object will be at the edge of the sweep, and only occasionally under the sweet spot.

Secondly, and no less important. the sweep speed is typically varied during an air test until the optimal signal is obtained.  I think that more often than not, people are not using the optimal sweep speed in the field. 

I've mentioned this before, but you should test your sweep speed in the field on a sample target, and then either speed up or slow down your sweep speed.

Thirdly, during air tests, the object is kept in a very narrow area just under the coil.  Most signals may be attributed to the object - not always accurately.

It is more difficult to detect a signal in noise, such as you might get in a field environment.  In an environment with noise, your nervous system will adapt and begin to ignore marginal signals such as those that you would might attribute to the target (correctly or incorrectly) during an air test.

Thirdly, as I stated above, the detector is not the only system involved.  Your nervous system, including your brain and sensory systems are also involved.  Those systems are very important.  They are very flexible and may adapt in ways you don't realize.

Here is one fun example.  Which of the following horizontal lines is the longest?  Don't include the arrow heads.

Maybe you've seen this classic demonstration before.  The three horizontal lines are the same length.  Measure them if you need convincing.

The point is that your nervous system interprets sensory data.  It can be fooled and sometimes it is wrong.

When you do an air test, not only do you listen but you also see when the target is under the coil.  Try this experiment.   Close your eyes and pass the object under the coil.  You might find that you can not distinguish the auditory signal quite as well when you are not also seeing the object under the coil.  That will be even more true if you are in a noisy environment.  Not using the visual information will make the auditory data more difficult to interpret.  During a hunt you will not see the object when you hear the signal, that means that a more distinct auditory signal will be required for it to be correctly interpreted.

Don't get me wrong.  I'm not saying that air tests are no good.  I'm just saying that they can be misleading, and you have to be cautious about drawing too many conclusions. 

I think most people should probably do air tests more often.  They should not, however, attempt to evaluate a detector based upon air tests results, and should be cautious about generalizing from an air.

I hoped that by pointing out some of the problems with air tests, both physical and psychological, you will be able to conduct more meaningful air tests and draw more accurate conclusions.


We are back down to a one or two foot surf on the Treasure Coast.  Really smooth.   That will make water hunting easy.  Also easy to do the low tide area.

The surf will not be increasing hardly at all for the next few days.

The cut shown above was not visible from any beach access.  You'd have to walk a ways to find it.

Happy hunting,