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  Advertising And In Air Tests

With the introduction of the magazine Detector User, came a major step forward. For the first time a serious attempt was made to compare detectors on the basis of in soil performance. Previously, the information given in magazines was in the form of meaningless air tests. The in air tests were, and are, a ridiculous means of assessing detector performance. They do not take into account the huge reductions in efficiency caused by mineralisation in the soil and mineral levels in salt wet sand, and also ignored the drastic effect that junk contamination can have. Excellent depths of 14" for a large coin in air can be transformed to 5" or 6" in mineralised soil in the all metal mode and completely missed at 3" or 4" when TR Disc is used. This is why the Field Test reports in Detector User were so valuable. Sadly, this magazine is no longer published. However, there is something I feel sure the ex-editor of Detector User would like you to be aware of. Although every effort was made to make sure the results would be accurate, there was one factor he could not control, and that was the weather.
Each set of tests were carried out at monthly intervals. Obviously, it would have been better to do all the tests at the same time but this was impossible. This was due in part to time restrictions as sometimes he would not receive the test unit a week before the test deadline for that issue. An editor's life is very busy! And, I can asure you that detector tests are very time consuming when undertaken properly.

Anyway, why was the weather such a contributing factor? It is, because variations in the moisture content of the soil plays a significant part in determining detector performance. Therefore, the results may have been too favourable for one unit, because, the soil, was, at, it's optimal level of dampness while another test may, not, have shown how excellent another detector was due to the soil being a lot drier. In any case, these "in soil" tests were still a great deal more meaningfull than those other "in air" tests.

To demonstrate the importance of the soil's moisture content two sets of identical tests were carried out on consecutive days. The sun had been beating down for more than a week and the ground was very dry. The only difference was that the rain had fallen in the intervening night. The same tests were carried out, in the same spot and with the same machines. On the first day a 10p coin gave no signal at silver paper reject when buried at 5". It read at 4". On the second day the 10p gave a positive signal not only at 5" but 6" as well, this was of course at the same discrimination level. The maximum GB Disc "depth" in air was 7". A depth of 6" was the maximum in soil depth in day two whereas it was 4" in day one.
Another example of how misleading the in air tests are come in the form of an advert that appeared quite recently. It was supposedely comparing one of their top machines against those of rival companies. The claimed depth range was just over 12" for a 2p coin when the TR Mode was set to reject iron.
This is possible in air but will not be approached on the sites we search. A depth of 6" is quite usually a very good depth for a coin the size of a 2p when rejecting iron.

Tha magazine Detector User (no longer in print) was the first serious attempt to assess detector efficiency on the basis of in soil tests. In air tests are usually misleading and a poor indicator of a machine's real performance capabilities.
Iron particles in soil--and wet salt in beach sand--reduce the performance that our detectors can acheive. The mositure content of the soil as well as the prevailing level of junk contamination are other factors that can significantly affect performance.

The use of rejection, as a means of "speeding up" the process of finding desirable items amongst junk is a very useful and widely used tool in metal detecting. However, if you use high levels of rejection (say above silver paper) then there is a good chance of missing objects with small surface areas and/or low conductivities. This includes many gold, platinum and white gold rings as well as many hammered coins.

GARY BURT ( RIP)
Edinburgh
Scotland

 




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