[size=medium]Considering that iron is technically supposed to show a VDI of -95 to 0 (please correct me if I am wrong), why does it regularly show up in the high 80s and 90s? I'm sure that this topic has been covered exhaustively, but it doesn't make sense to me that iron would jump around so much on a detector's VDI, while other metals seem to stay in a general acceptable range. I know that I have dug rusty square nails with very high VDIs, but I would never expect to dig a -60 and find a quarter or a pull-tab. Does this also occur in detectors with "iron masking" abilities?[/size]
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VDI question
- Thread starter stikman
- Start date
floridason
New member
Hey stickman, now this is JMO and from my area here in FL. I have that happen to me all the time. I think (jmho) that the amount of rust buildup on the iron has a lot to do with this. And thats the
same with pulltabs are the same way. Here where I tect at, the alum tabs down to 3 or 4 inches with no coating on them show up a VDI mostly 22 to 24. Then the tanic acid gets to them at about 4" and they turnup at 18 to 20 (mostly18VDI) which is what 99% of my nickels are at
as most of the ground here is what's called sandy loam, or black dirt. And it also changes a lot when the ground is moist or dry. I use mostly my 8" DD EX from KellyCo or the Whites 950 and it happens with both. I'm still having a hard time between iron bottle caps and quarters as bottle caps (when rusty show 25c. When they are new shows iron. Again . I find that I can eleminate a lot of iron and pulltabs using C/J trigger forward and that helps too. I've been over this same place for almost 2 years now and having picked up a lot of trash all along the way it is getting better and I'm finding more coins (deeper). I found a dime at approx 8" down in a totlot I've been over weekly for the past year. Just goes to show, you never find it all, and getting thru the trash is where you find it. hope this helps. What did I say, I 
HH KC
floridason
retired
V3, MXT, DX-1, bullseye, + 4 coils
same with pulltabs are the same way. Here where I tect at, the alum tabs down to 3 or 4 inches with no coating on them show up a VDI mostly 22 to 24. Then the tanic acid gets to them at about 4" and they turnup at 18 to 20 (mostly18VDI) which is what 99% of my nickels are at
as most of the ground here is what's called sandy loam, or black dirt. And it also changes a lot when the ground is moist or dry. I use mostly my 8" DD EX from KellyCo or the Whites 950 and it happens with both. I'm still having a hard time between iron bottle caps and quarters as bottle caps (when rusty show 25c. When they are new shows iron. Again . I find that I can eleminate a lot of iron and pulltabs using C/J trigger forward and that helps too. I've been over this same place for almost 2 years now and having picked up a lot of trash all along the way it is getting better and I'm finding more coins (deeper). I found a dime at approx 8" down in a totlot I've been over weekly for the past year. Just goes to show, you never find it all, and getting thru the trash is where you find it. hope this helps. What did I say, I HH KCfloridason
retiredV3, MXT, DX-1, bullseye, + 4 coils
Horsesoldier
New member
You won't be digging quarters and pull-tabs in the iron range but you will be digging "big iron" in the qarter and pull-tab range. Why? Because the VDI is based on signal strength, conductivity (at a given frequency) and SIZE. Don't confuse VDI with discrimination. The VDI target ID or "number" is based on a consistent size and range of conductivity ( ie. a quarter is a quarter sized object with a specific conductivity, etc.). That's why pull tabs and some rings and coins fall in the same VDI range. You start throwing large iron objects in the mix then you have an object that, because of its size, will give you the same "characteristics" as a good conducter. Due to the mass of the large iron object it will give you a good signal. Also, depth of the target is a factor. A large deep target may sound as like a shallower good target and a shallow bad target can sometimes sound like a good deep target. The bottom line: "If in doubt, check it out". The VDI is simply a tool that you use to decide if you want to dig or not. It will not tell you 100% what the target is but it will certainly help eliminate guess work if applied properly.
Hope this helps.
"Keep Your Coil To The Soil"
Be sure to watch "The Relic Hunter's Journal" available only on YouTube.
Hope this helps.
"Keep Your Coil To The Soil"
Be sure to watch "The Relic Hunter's Journal" available only on YouTube.
I'm still having a hard time between iron bottle caps and quarters as bottle caps (when rusty show 25c. When they are new shows iron.
The MXT discrimination and sensitivity is such that many bottle caps (the crimped steel ones) will ID as a good quarter or dime signal (depending on how rusty they are) when your sweeps are centered over them. The easiest way to identify them before digging them, is to take another pass over them in discriminate mode, but this time do it with the edge of the coil, not centered on the target. "Rimming" the target this way will let the MXT show them up as a ferrous target. Igot this years ago from Rudy.
I find that I can eleminate a lot of iron and pulltabs using C/J trigger forward and that helps too.
I don't see how this eliminates iron. It will eliminate some of the older pull tabs but not the new ones. It will also eliminate many of the gold rings and for me some Indian heads etc which fall into the range it notches out. Rob
The MXT discrimination and sensitivity is such that many bottle caps (the crimped steel ones) will ID as a good quarter or dime signal (depending on how rusty they are) when your sweeps are centered over them. The easiest way to identify them before digging them, is to take another pass over them in discriminate mode, but this time do it with the edge of the coil, not centered on the target. "Rimming" the target this way will let the MXT show them up as a ferrous target. Igot this years ago from Rudy.
I find that I can eleminate a lot of iron and pulltabs using C/J trigger forward and that helps too.
I don't see how this eliminates iron. It will eliminate some of the older pull tabs but not the new ones. It will also eliminate many of the gold rings and for me some Indian heads etc which fall into the range it notches out. Rob
stikman said:
Stikman,
Try the rimming or edge pass rejection technique whenever you are in a crowned bottle cap ridden area and you won't be digging them anymore.
Now to the why this happens. It is because items such as crowned bottle caps, steel washers, etc. have both magnetic and conductive properties. The VDI number
is assigned based on the phase difference between the transmit signal and the response signal from the target. Bottle caps, washers and many other items with significant
magnetic properties will react very differently to the transmit pulse, depending on the location of the coil relative to the suspect target. That is, the instantaneous VDI is changing
dramatically as you sweep the coil.
Motion discriminator circuits in VLF detectors are set to filter out slow changing signals, such as those coming from the ground matrix, and allow fast changing ones through (ie. signals
coming from a small target). As the coil passes over the bottle cap there is a wide swing in the VDI as the coil sees the bottle cap from different angles. These are the signals going into
the detector's filters and the portion exhibiting a high AC VDI gets through and it is detected..
This is why "rimming" the suspect target will make it decloak and show itself as ferrous.
Thanks guys. I'll try the "rimming" method next time I'm out. Most of the time, it seems that my gut feeling kind of helps out when coming across a good signal. I've always dug though, because I have never really trusted my gut feeling. Whenever I think "square nail" I'm usually not too far off. Maybe I need to start thinking "silver half, silver half". In doing this, I might someday find my first silver. Surely it'll happen this year.
Jason
Jason