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Relation Among Target Surface Area, Target Weight and CTX 3030 CO Number
Posted by: LawrencetheMDer
Date: February 21, 2018 09:14AM
In a continuing investigation into the relationship between CTX 3030 CO number and target characteristics, I have previously shown that CTX 3030 CO number is highly dependent on target weight; as target weight increases so does CO number. The relation between CO number and target weight has found to hold for lead (fishing weights), aluminum foil and gold (rings and coins).

However, in these investigations when weight was increased there was a natural increase in size (surface area) as well. Lets face it, when a target of the same composition increases in weight it also increases in size (I also know this from personal experience with pants size!). Any ways, to uncouple the effects of target weight vs target size on CO number, I decided to record CO number with different sizes (surface areas) of aluminum foil while keeping the weight of the target (1.96g) the same by using the same piece of foil for all sizes.

I took an 8" x 8" single sheet of aluminum foil while holding distance constant from the coil at about 6" and measured the CO number with the CTX 3030. I then folded the single piece of foil to match the listed surface sizes on Table 1. The largest size of aluminum foil was 8" x 8" (64 sq in) and the smallest size was 0.75" x 0.50" (.375 sq in); I couldn't fold the single piece of foil smaller. The left-hand side of Table 1 shows for 1.96g the size of the aluminum foil surface area facing parallel to the 17" coil and corresponding CO number. Note that as target surface area increased the CTX 3030 CO number decreased and finally plateau around a CO of 20 for the 1.96g of aluminum foil at 16 sq in.

Table 1

Figure 1 solid line shows the relation between surface area and CO number while weight was constant at 1.96g across all surface areas. Note that the CO number didn't change much if at all after surface area increased greater than 4" x 4" (16 sq in) for the approx 2g sheet of foil. At the smaller sizes from .375 sq inch to 16 sq inches there is a near perfect inverse linear relation between target size and CO number (R= -0.99, almost perfect negative correlation); as foil size increased CO number decreased proportionately.

Figure 1

Next I took a 4" x 4" piece of foil that measured 0.46g and basically repeated the experiment with the smallest size being 0.5" x 0.5" (.25 sq in). The right side of Table 1 shows the foil size (sq in) and the corresponding CO number. Again note that as foil size (surface area) increased, while holding weight constant, CO number decreased. Graph 2 shows the results of the 0.46g foil study and the relation between foil surface area and CO number is best fitted by the solid line with almost perfect fit. Again, there is a systematic decline in CO number as surface area increased.

Figure 2.

Graph 3 shows the results of the 1.96g (2g) and 046g (0.5g) foil weights on CO number with change of surface area. Weight shifts the function up (more weight) and down (less weight) along the Y-axis and these results are consistent with my previous studies showing the there is a direct relationship between weight and CO number; as weight increases so does the CO number. The fitted lines reflect a log function and show the effects of surface size on CO number; as surface area increases CO number decreases. But there seems to be a limit of about 16 sq in ( for the 17" coil) where surface area no longer affects CO number. This appears to be the case for both the 2g and 0.5g foil weights. For both weights, the CO response flattens out at 16 sq in.

Figure 3.

The results of numerous studies on the effect of target weight on CTX 3030 CO number show that as target weight increases, CO number increases proportionately (for aluminum foil, lead, gold rings, gold coins). I have also demonstrated that as size increases, while weight is held constant, CO number decreases. Normally, when a target increases in weight it also increases in size (surface area). The increase in target size due to an increase in weight would serve to somewhat decrease the CO number given the finding that CO number decreases with an increase in size. But the negative CO size effect is more than off-set by the positive CO weight effect and, thus, as weight (and size) increased the CO number also increased.

Figure 4.

Figure 4 helps to illustrate the relationship among target weight, target size (surface area) and CTX 3030 CO number. The up-down arrows on the right illustrate the effects of target weight on the whole fitted solid line; as target weight changes the whole CO function moves up or down the Y-axis depending on whether weight increases (function moves up) or weight decrease (whole function moves down the Y-axis). Heavier targets yield higher CO numbers regardless of size and lighter targets yield lower CO numbers regardless of size.

The left-right arrows in Figure 4 illustrate the effects of size on the CO number. A change in size causes the response to move along the solid (regression) line and the effects of target size is the opposite of that reported for weight (negative correlation); As size increases the CO number decreases along the solid function line, but only for relatively smaller targets (<16 sq in). At larger foil target sizes, the CO number remained constant regardless of size.

In conclusion, target weight and target size both play a significant role in the generation of the CTX 3030 CO number. Target weight is the main driver of the CTX 3030 CO number and determines the height of the CO response function. Size serves to move the CO number along the CO weight function. As a consequence, the CO axis on the CTX 3030 can be considered a measure of target weight, to the first approximation, and lighter targets will fall at the lower end of the CO axis and heavier targets will fall toward the higher end of the CO axis.

One conclusion from my studies of target characteristics and CO nunber is that gold can fall anywhere along the CO axis with the primary factor being weight of the target. From a practical point of view, I am a Metal Detectorist by-the-way, these data strongly suggest that to maximize gold recovery one should dig all targets on the FE 12 line (I use FE 9-15 to cover environmental influences) regardless of CO number. Unfortunately, situations exist where time for digging every good target is not an option (e.g., low tide hunting, night hunting, too many target and limited time). In these situations, CO numbers corresponding to pull-tabs and push-tabs (e.g., 12:15-23), for example, could be filtered out or ignored when necessary but with the full realization that some gold rings of particular weight will fall within the filtered or ignored CO numbers. In the present case, for example, if one were to ignore 12:15-23 CO numbers because of excessive pull-tabs and push-tabs, one would have missed 15% (4/27) of the gold rings I had analyzed earlier when assessing the effects of gold ring weight on CO number. Gold rings appeared at 12:16, 12:17 (2) and 12:18 and would have been missed because they're in the ignored pull-tab and push-tab CO range. Beware and Happy hunting

Re: Relation Among Target Surface Area, Target Weight and CTX 3030 CO Number
Posted by: Johnnyanglo
Date: March 19, 2018 12:03AM
Good work there Lawrence. From my study of the CTX, the CO number varies proportionally to the ability of the target to generate eddies. A large sheet of aluminum with a low op freq detector (such as the CTX which uses with its base freq at 3.125kHz) is ideal for generating deeper eddies in metal. Some data I have indicates that at 3kHz the skin depth might be about 3.0mm deep (0.12 inches). Your smallest folded piece of aluminum was probably at least that thick. With conductivity and permeability held constant (in your experiment), the increasing foil depth increased the strength and depth of the eddies, which reflect a stronger secondary magnetic field from the target, which is recorded as a higher CO number. Another factor related to current flow (in eddy currents) has to do with the length of their circuitous route in the metal surface. The longer the eddy path, the greater the resistance, the weaker the secondary field. So, by decreasing the effective radius as the sheets are folded smaller, the eddies flow in smaller radii with a shorter path to generate a larger CO value.

With real-world ring targets, there is some correlation between ring weight (which infers thickness), but other factors also dominate. Thus, a relatively thin 1.8-gram sterling silver ring with a 17.3mm inside diameter (ID) with a smooth surface (hospitable to eddy formation) can have at 46CO. But a less conductive 14K gold ring at 1.98-grams with 17.5mm I.D. can manage only a 01CO. The difference in conductivity of alloys, the roughness of the surface (due to design patterns or embedded stones, etc.), thinness, and the orientation in the ground lead to widely differing CO values.

My database shows that most rings fall on the 12-13FE line, but not all. Most rings with lower FE values (< 12) are often heavily embellished with design patterns or inlays. Despite producing low FE values from 01 to 10, they all produce relatively high CO values from 44 to 48. They are also typically sterling silver (not white or yellow gold). They share similar FE/CO pairings as silver dollars (01FE, 42CO).

The CTX (and E-TRAC) display the CO values in a more normalized fashion (than the older Explorer) so that coins will purposely fall along the 12FE line. However, with high conductors, like silver, despite the attempt to bring coins in-line to FE12, there still remains a tendency for the FE value to drop towards lower FE values (as the Explorer detectors would). In other words, the CTX will still see the FE values decrease when dealing with highly conductive targets, perhaps not as much as the Explorer would. The goal in the CTX was to avoid the crowding of CO values when in the high conductivity region. Instead of the Explorer showing two targets as 08FE-30CO and 06FE-31CO they are re-mapped on the CTX as 12FE-35CO and 12FE-38CO (the CO values gain separation).

Anyway, that's the way the CTX and E-TRAC were designed. When dealing with high conducting silver rings (or silver coins) the mapping will cause the FE value to drop below 12, often bottoming out at 01FE. It tries to normalize to the 12FE line but is not entirely successful across the entire spectrum of CO values.

Re: Relation Among Target Surface Area, Target Weight and CTX 3030 CO Number
Posted by: LawrencetheMDer
Date: 3 hours ago
Hey Johnny, fascinating discussion and truly appreciate the background material. My field findings on the FE variability of heavier silver objects matches perfectly with what you discuss on a more theoretical level.
I'll have to meet this Eddy guy sometime. :lmfao:

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