Well, it's a rainy day here, messing up my plans to go play at the park, so since I can't detect I decided to look into my Fors CoRe VDI numbers to try to understand better what I've been seeing. I have had a couple occasions where I dug a target because the sound was good and the VDI was unusual only to find multiple coins in the same hole or a coin and a non-coin target. A single coin with its face towards the coil will give a sweet, clean sound and a solid, stable VDI number. BUT, if the coin is on edge or touching another coin or touching another conductor or is very near another conductor, you may not get that sweet, clean sound and your VDI numbers will not be what you would expect.
To check this out, I found a small cardboard box, mine is 2 1/2 inches on all sides. I wrapped several rubber bands around the box. The bands will hold the target against the side of the box. This simple contraption can now be used to air test single targets, multiple targets on the same plane, and multiple tagets on different planes. Simple and cheap, just like me! I would strongly encourage you to put one of these together and play with different combinations of targets and different orientations of the box to the coil. I promise you can learn a lot.
I am gonna be lazy and not type out a bunch of data, as I have done in the past. Instead I will just summarize what I found:
1. A single target can give different VDI numbers depending on the orientation of its greatest surface area plane to the coil. Example: A quarter can give a VDI number anywhere from 71 to 92, depeding on the relationship of the face of the coin to the plane of the coil.
2. When two targets are on the same plane and not touching each other, the VDI number will correspond most closely to the last target the coil sees.
3. When two targets are not on the same plane, the VDI number will be biased to the target closest to the center of the coil. Note that this means that the further from the center of the coil the two targets are (heel or toe of coil), the less separation they have in relation to the center of the coil and the more likely the VDI will fall somewhere between the VDI of each separate target.
4. The most reliable, stable and accurate VDI number will be obtained by making sure the center of the coil is swept over the target, and that the sweep speed is consistent.
5. When two different targets are on the same plane and they are touching, the VDI will fall between the VDI of each separately. Example- Apenny with VDI of 82 touching a pull tab of 60 will give a VDI of somewhere between 60 and 82. The VDI will be biased towards whichever target is closest to the center of the coil.
I only spent about thirty minutes playing with this. I am sure there is much more to be learned about VDI numbers, if one so desires. It certainly does illustrate to me how much one can be missing when they just cherry pick specific VDI numbers. It also helps in understanding jumpy VDI numbers. I know I will still use VDI numbers as part of my decision making but I will be a little less likely to reject digging just because the number isn't "right". Hope you try this for yourself. It is worth the time and trouble.
To check this out, I found a small cardboard box, mine is 2 1/2 inches on all sides. I wrapped several rubber bands around the box. The bands will hold the target against the side of the box. This simple contraption can now be used to air test single targets, multiple targets on the same plane, and multiple tagets on different planes. Simple and cheap, just like me! I would strongly encourage you to put one of these together and play with different combinations of targets and different orientations of the box to the coil. I promise you can learn a lot.
I am gonna be lazy and not type out a bunch of data, as I have done in the past. Instead I will just summarize what I found:
1. A single target can give different VDI numbers depending on the orientation of its greatest surface area plane to the coil. Example: A quarter can give a VDI number anywhere from 71 to 92, depeding on the relationship of the face of the coin to the plane of the coil.
2. When two targets are on the same plane and not touching each other, the VDI number will correspond most closely to the last target the coil sees.
3. When two targets are not on the same plane, the VDI number will be biased to the target closest to the center of the coil. Note that this means that the further from the center of the coil the two targets are (heel or toe of coil), the less separation they have in relation to the center of the coil and the more likely the VDI will fall somewhere between the VDI of each separate target.
4. The most reliable, stable and accurate VDI number will be obtained by making sure the center of the coil is swept over the target, and that the sweep speed is consistent.
5. When two different targets are on the same plane and they are touching, the VDI will fall between the VDI of each separately. Example- Apenny with VDI of 82 touching a pull tab of 60 will give a VDI of somewhere between 60 and 82. The VDI will be biased towards whichever target is closest to the center of the coil.
I only spent about thirty minutes playing with this. I am sure there is much more to be learned about VDI numbers, if one so desires. It certainly does illustrate to me how much one can be missing when they just cherry pick specific VDI numbers. It also helps in understanding jumpy VDI numbers. I know I will still use VDI numbers as part of my decision making but I will be a little less likely to reject digging just because the number isn't "right". Hope you try this for yourself. It is worth the time and trouble.
