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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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