Factors That Affect VDI Readings of Non-Ferrous Targets, page 8

Operation Frequency Change (Formula), Mixed VDI Value, High Mineralization, and High Salinity Effects

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proficiency level: Expert, date created: ;

Even though this article is included into a section on features and modes of the XP Deus metal detector, information presented on this page is the "must-know" for ALL metal detectorists using other brands of advanced metal detectors.

1. Effect of Operation Frequency Change on VDI Readings of Coins (Formula) and Other Non-Ferrous Targets

If you plan to use different detection frequencies in your search programs, it is extremely important for you to understand this: a VDI (Visual Display Indication) value of a coin (or any NON-FERROUS target) VARIES from one detection frequency to another, and this phenomenon is reflected by the following SIMPLIFIED formula:

Formula of Metal Detected Target's VDI Value Proportionality to Detection Frequency, Target's Conductivity and Dimensions

where:

  • F - operation frequency
  • σ - coin's conductivity value
  • D - diameter of coin
  • T - thickness of coin

According to this formula, the coin's VDI value is proportional to OPERATION frequency times coin's conductivity value times coin's diameter times coin's thickness. It is obvious from this proportionality that the lower the frequency, the lower the VDI number if other variables remain unchanged. This relation can be observed when the operation frequency is changed both during actual metal detecting and during an Air Test. For example, a clad dime (US 10-cent coin, 17mm in diameter) is indicated with a VDI number of 91 at 18 kHz, 88 at 12 kHz, 83 at 8 kHz, and 73 at 4 kHz.

After other coins of different sizes and conductivities, I noticed almost the same difference between any coin's VDI values obtained at 18 kHz and 4 kHz frequencies - near 20 points. This difference may not be critical for high-conductive and large-sized coins and other non-ferrous valuables, but it certainly is "fatal" for low-conductive, small-sized specie and non-ferrous targets.

For example, if a 14k gold ring has a VDI value of 43 at 18 kHz, its VDI would change to 29 if the frequency is switched to 4 kHz. If you set a second tonal zone ranging from 10 to 30 for low-conductive non-ferrous junk targets and assign a low "iron" tone (200 Hz) to it, the ring with a "newly attained" VDI value of 29 will be most likely ignored by you because an audio response to the coin will sound "iron".

If you do not lower the Threshold T2/T3 to 25 after selecting the 4 kHz frequency, you will pass up all gold rings that had VDI values below 43 at 18 kHz.

Of course, this is an extreme example because only an unwise enthusiast would use the 4 kHz frequency for detecting the low-conductive gold rings and small silver coins. However, the example describes exactly what may happen in reality: thin gold chains and their fragments as well as other tiny valuables simply "drop out" of the third tonal zone and "fall" into the second zone. And they may not even get visual indication when current detection frequency is changed to the 4 kHz.

The worst situation may occur when the Multi-Notch mode of Conventional Discrimination is utilized. For instance, if one switches from the 18 kHz to the 12 kHz frequency to increase the Detection Depth Range (increase of up to 2 inches is possible) for certain coins (or other targets), some coins with VDI values near the notch threshold(s) may end up within the adjacent rejecting notch(es), and the Deus' responses to them would be muted.

In the same case, if you use the 3-, 4-, or 5-Tone modes of Tonal Discrimination to designate tonal zones for various NON-FERROUS targets, you should change the operation frequency with caution. Just like in the above-described case, desirable coins with VDI's occupying the Threshold-adjoining area may turn up in the lower conductivity zone and, therefore, "sound off" like the unwanted targets when the frequency is changed form 18 kHz to 12 kHz. This may be confusing and counterproductive. This is probably another good argument in favor of using the 2-Tone mode of Discrimination.

To sum everything up, if you search for small, both shallow and deep, valuables, using the Multi-Tone mode of Tonal Discrimination and/or Multi-Notch mode of Conventional Discrimination, it is recommended to operate your Deus on the 18 kHz frequency and do not switch for lower frequencies unless you really have to.

The best way to deal with the Frequency Change effects is to run an Air Test (Bench Test) with coins to be sought and write down their VDI readings for each operating frequency. If you memorize them all before searching for the coins sought, you would not have any of the above-described problems during the search.

If it is problematic for you to air-test various targets due to not having them at hand, you might want to use my , in which you will find the VDI values for common US targets that were air-tested under all four operation frequencies.

Operating Frequency Change Has Opposite Effect on Some Iron Targets

Due to ferromagnetic properties of ferrous targets, the operating frequency change does not affect them in the same way as it affects the non-ferrous targets. In fact, the effect is quite opposite! Large/medium ferrous targets and targets made of thin sheet iron, such as beer bottle caps and fragments of roofing, are registered with the higher VDI numbers (95-97) at 4kHz than at 18kHz (83-95) as shown below, even after they get dug up!

XP Deus VDI Example Chart for Some Iron Targets

Although VDI readings of the dug small- and medium-sized rusty square nails are always in the lowest zone of the Conductivity scale, the VDI readings of the undug iron square nails, which are obtained during real metal detecting, not air-testing, might reach the mid-90s. Two US coins with the highest VDI values, a Silver Kennedy Half Dollar and a Silver "Peace" Dollar, are placed into this example chart to show where the most conductive silver coins are positioned on the Conductivity scale in relation to a rusty Bottle Cap and a rusty fragment of roofing (you can see its picture on ). You can see that a VDI readout of the latter is higher than the silver coins' VDIs, but, unlike the coins' VDIs, it is positioned at the opposite end (4kHz) of the 4-frequency range.

This is just another phenomenon related to the operating frequency change, which could be a manifestation of the . However, many detectorists worldwide found a good use for this phenomenon. The "Switch To 4kHz Frequency" technique is based on it and used for quick identification of the rusty bottle caps, rusty fragments of roofing, large square nails, railroad screw-nuts, horseshoes, axe heads, other large- and medium-sized iron objects, etc. Unfortunately this technique does not work for the small- and medium-sized square nails that were machine-made in the 19th century. They might full you! This technique is described in my article "Switch To 4kHz Frequency" Method To ID Bottle Caps and Large Iron Objects (will be posted soon).

2. Mixed VDI Value Effect

"Mixed VDI Value Effect" is another negative factor that affects VDI values of "good" targets. This effect is a major cause for accidental "rejection" resulting in passing up many coins and other valuables by all brands of metal detectors operated at hunt sites with high concentration of iron junk. This is what usually happens when a buried coin is in close proximity to a rusty nail or any small iron target: the coin's high VDI value gets "mixed" with the nail's low VDI value, and a resulting VDI for the coin shown on a display is lower than it is supposed to be.

If the resulting VDI "lands" into an adjacent tonal zone on the Conductivity scale, and this zone is assigned a lower audio tone, not the one to pay attention to, an operator ignores it and... passes up the coin. The same happens when the rejecting notches of Conventional Discrimination are set up on the Conductivity / Discrimination scale, and the coin's "mixed" VDI value "drops" into one of them. The worst case is when ample Discrimination is utilized, and numerous VDI's of "goodies" fall into a vast rejecting zone, the good targets are ignored and left for the more experienced detectorists.

Below is an example chart of a few non-ferrous targets' VDIs registered during air-testing these targets both without a nail and with it (pink cells). All targets were placed next to a rusty nail head (shown on ). Since there are no two identical dug square nails in reality, the resulting VDIs in the pink cells are relative and would be slightly different if the targets were tested with different nails.

Part of Target VDI Chart Showing Mixed VDI Values of Assorted Good Targets

One should notice that, in this chart, targets of either a small size (a clad Dime) or low conductivity (a Nickel and, therefore, the medium-sized gold rings) have a wider "gap" between their normal and "mixed" VDIs. This explains why such targets, even if not rejected by the Conventional Discrimination settings, may be easily "rejected" by the Tonal Discrimination settings and, therefore, are hard to find at the "pounded" hunt sites littered with square nails.

3. High Mineralization Effect

This effect takes place in areas that contain high mineralization, both natural and man-made, in soil. Because the High Mineralization Effect negatively affects both the metal detector's Penetration Depth ability and VDI readings of the non-ferrous targets, it is considered a major cause for accidental rejection of coins and other non-ferrous valuables during metal detecting in the above-mentioned areas.

Metal detectors with less potent Discriminate and Visual Target ID circuits usually suffer the most from this effect, if not disabled by it at all. But even for the most advanced metal detectors (excluding Pulse Induction (PI) metal detectors which generally are not affected by mineralization), analyzing and processing responses to non-ferrous targets through heavy mineralization can be quite challenging. Cases when the target's VDI value turns out to be 10-20 points lower than a "normal" readout are not as bad as the cases when the non-ferrous target's VDI reading can not "bail out" of the iron range on the Conductivity/Discrimination scale. As a result, such low-conductive valuables end up being rejected.

Obviously, to avoid leaving coins and other non-ferrous "keepers" behind in the high-mineralized areas, one should run a "Mineralization/Penetration Depth Test" (a link will be posted here soon) and factor the test results in while creating a reject/accept pattern on the Conductivity/Discrimination scale in one's search program. And this concerns areas with both relatively uniform mineralization (naturally mineralized ground) and sporadic or diverse mineralization (resulted from former human ctivities in the past) - hot rocks, slag, pottery and brick fragments.

4. High Salinity Effect

Not being as common as the "Mixed VDI Value Effect", the "High Salinity Effect" nevertheless is another negative factor affecting the non-ferrous target's VDI value. On a beach, in wet sand of high salinity, the target's depth can affect a VDI reading big time! The clad dime lying on the wet sand surface would have a VDI value of 91. If you bury it 4" deep, the VDI read-out would be in the 70's. And at 7" depth, the dime's VDI number would drop to 30! Other coins were tested in the same manner, and similar effects were observed.

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