Comparison of Three Different Coils Graphs and discussion on how well a Garrett GTI 2500 works with the three different coils it can use
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The
Garrett GTI 2500 can be used with three different coils: a 4.5"
non-imaging coil for trashy areas or looking for small shallow
objects, the standard 9.5" imaging coil, and a 12.5"
imaging coil for deep objects. There is also a 12.5" non-imaging
coil, but I don't have one of those.
This graph shows the detection envelopes for all three coils. The targets were clad quarters and sensitivity was set at 10.
The most interesting feature on this graph is that for coin-sized targets, the large 12.5" coil doesn't reach any deeper than the standard 9.5" coil. However, for large targets like cans the 12.5" coil can spot targets twice as deep as the 9.5" coil.
The detection envelopes in discriminate mode showed similar relationships.
The 4.5" coil's performance in the test garden reflected the reduced reach shown in the graph. On almost every target the return was weaker and the accuracy of the ID poorer. However, in the case of very small objects in the top 1" of soil, like the gold nugget, large staple and steel BB, the small coil did better than both larger coils. The small coil's main virtue is that in areas where targets are closer than 8" to each other, it's capable of separating them whereas the two larger coils have so much range that they respond to several targets at once. The result is that they can't provide accurate pinpointing or IDing in such conditions.
The inaccuracy of ID values for all targets was greater for the 12.5" coil than either the 4.5" or 9.5" coils. Additionally, the 12.5" coil tended to predict coins as being shallower and smaller than they really were. Very small targets, like the BB, gold nugget and staple were missed completely. I have to say that the 12.5" imaging coil has such inaccurate imaging capabilities that it is worthless in that capacity. What it can do very well is spot larger targets beyond the reach of the 9.5" inch coil. The 12.5" coil is a real bear to handle. My arm aches after only half an hour using it.
Great Link about Coils!!! http://www.ionet.net/~tgomez/coil.html is an outstanding page explaining the design and operation of metal detector coils! Be sure to check it out as well as the rest of Mr. Gomez's metal detecting pages.
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