Ferrous-Conductivity readings for Non-Ferrous targets

21 Oct 2016

Recently we had a Facebook comment about the Target ID difference between our older Explorer SE detector and the CTX 3030:

“One thing I've noticed on the CTX is that an all copper penny sounds just like a silver dime and both will read 12 43. On the Explorer SE coppers would show as 7 21 and silver 03 23. But it's a small sacrifice to pay to straighten out the graph curve into the 12 line.”

So, it's was worth revisiting the rationale behind evolving the Explorer FE/CO discrimination method into the one used in E-TRAC and CTX 3030…

The FE/CO numbers (in Explorer) provided a very good level of discrimination to targets based on their ferrous and conductivity characteristics. Based on this premise, similar types of objects should group within the same regions of the ‘FE/CO space’.

Considering non-ferrous coins; depending on their physical properties (e.g. thickness, diameter and type of metal) the conductivity will vary. Small thin coins will generally have a lower conductivity while larger thicker coins will have a higher conductivity. The type of material the coins are made of also affects this; e.g. silver coins will be more conductive than nickel-copper coins. However, the ferrous characteristics will be generally consistent across all coins. Additionally, the conductivity factor of a target can be determined more accurately than its ferrous factor (especially with targets in mineralised ground) and it is also of more benefit to the user in separating different coins from each other. This means that all coins should have a similar FE number, with a varying range of CO numbers which can be used to identify ac specific target coin. However, in the Explorer detectors, the FE/CO numbers for coins have resulted in an S-shaped characteristic curve that varied in BOTH ferrous AND conductivity. This curved nature was particularly noticeable for highly conductive coins. In this region the CO numbers become compressed (which explains why different coins can have the same CO number) and the FE numbers become expanded (which explains why ferrous characteristics can be used to differentiate the coins). Essentially, what is happening is the discrimination between the coins, which is determined by their location along the curved S-shaped characteristic (i.e. the red line) is divided between FE and CO. This transform is shown in the diagram below. At the top of the curve, any movement along the red line (green and yellow dots) is mostly shown as ferrousity; whereas in the middle of the curve, any movement (pink and orange dots) is mostly shown as conductivity.

In the E-TRAC and CTX 3030, all of the discrimination (for coins) is projected onto the conductivity numbers and none onto the ferrousity. Please note, the amount of discrimination has not changed, that is, two coins that were previously separated will still be separated by about the same amount, only the discrimination is along the conductivity axis. The diagram below shows the transform of the FE/CO numbers in the high conductivity region.

As an example, we can measure some US coins (shown below, diamonds before straightening, squares after). These coins were chosen because most of them are in the high conductivity region, which is the most curved section of the S-shaped characteristic.


Therefore the E-TRAC / CTX 3030 practical conductivity range is expanded; compared to the Explorer detectors. This extra resolution and increased dynamic range means that targets with very low or very high CO numbers, that were previously giving the same target ID in Explorers, due to clipping at the top of the range, now have distinct target ID’s. The three higher conductivity coins above (Kennedy 50c, Silver Quarter, Roosevelt 10c) have the same FE/CO numbers of FE=0 and CO=28/29 on the Explorer SE, but have different values of 12–49, 12–46 and 12–41/42 respectively on the E-TRAC / CTX 3030.

So, the newer discrimination mapping method should be more beneficial for uniquely identifying a wider range of highly conductive coins from each other. Whether you still use an Explorer, swear by your E-TRAC, or love the CTX 3030, all of the Minelab FBS technology detectors have earned their reputation as ‘silver magnets’ and continue to produce the finds on a regular basis!  


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