On the Minelab Owners forum we often see questions concerning Minelab’s flagship FBS metal detector, the E-TRAC.
A couple of regular questions are: Do I need a threshold? How fast should I swing my coil?
The answer to these questions is related, and in this blog I’ll explain why.
Threshold
The E-TRAC’s threshold feature is one of the most powerful tools available to users. To turn the level down until it stops is a serious mistake. Not only will you miss deep targets, you will lose your “feel” of your soil environment. With no threshold you become blind to bad areas of a field, where habitation may have existed.
So the answer to the first question “Do I need a threshold?” is YES!
On page 58 of the E-TRAC instruction manual you can read how to adjust the threshold to optimise audio responses from the deepest faintest targets. It also explains that rejected iron targets will null the threshold, silencing all audio response from the E-TRAC.
It’s these nulls that we will ultimately use to determine the optimum coil sweep speed.
Sweeping the coil
In the E-TRAC user manual (page 28) it states an average sweep speed of four seconds from left to right to left. This average will only be achievable if the iron trash content of the soil will allow it.
As you sweep the coil over an iron target, the E-TRAC will discriminate it, nulling the audio. From this null the E-TRAC will quickly try to recover back to threshold. This time from null to threshold is called the recovery rate.
How fast should I swing my coil?
The iron content of the soil you are searching should determine coil speed. If you pass your coil over the ground and there’s only the occasional iron target (e.g. one piece every few swings), then you can swing briskly at the recommended average.
If however you are detecting an ancient habitation site, heavily contaminated with multiple iron targets, the nulls are likely to never recover if you swing at the average. So you will need to slow down until you hear the threshold recovering in-between all the nulls.
On my best Roman fields I have the worse iron contamination imaginable, it’s not uncommon to have sweep speeds as long as thirty-seconds from left to right to left.
So to recap...
If you turn off the threshold, you will lose one of the E-TRAC metal detector's most useful features… So don’t!
Try to maintain a threshold by altering your sweep speed.
If you sweep too fast over iron-infested soils, you will lose desirable targets in the nulls… so slow down!
It’s far better to sweep too slow than too fast.
Gordon Heritage
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