Enough researching for the day on meters but figured I'd post a little overview of the Sovereign's VDI and what makes it so special, along with a few VDI response tips to judge targets by...
First some unique things about it...I'm not aware (?) of any other machine on the market that has an adjustment to tune it's VDI to various coils used on a detector. If anybody knows of another machine that does this let me know? Anyway, the perks to being able to do that is that when changing coils there is a risk of a machine's VDI being slightly "off" in terms of the proper VDI # for a specific target. By tuning a Sovereign meter to 180 over a dime or quarter you've fine tuned the machine to give the same ID on targets with each coil you use. That's one of the reasons why I trust the VDI on the Sovereign more than any machine I've personaly used.
Second, some believe the meter reads the VDI from the coil. It does not. The Sovereign processes the target's signal and then sends a voltage based on the conductivity of the target back down via the coil cable to the meter. I think this is based on a 2 volt scale or so. Because this VDI isn't processed heavily for say display in unique ways on a screen like on some other detectors with heavy processing, I also trust what this machine is saying to me more than any other for that reason too. It's a more "pure" report of the target's conductivity to me and nothing is lost in translation through layers of software or special display processing.
Another perk to this is that the VDI is also very "instant". What you hear you instantly see the VDI for on the meter. There is no "lag" between hearing the target and then seeing it's potential ID on the screen. Some peoples have so much software or electronics layered between the audio and VDI processing that the output of audio and VDI is out of sync on them. Usually the VDI lags behind the audio, so for that reason on some machines they will tell you to ignore the VDI and listen to the audio because the VDI can be saying one thing and the audio another. Paying attention to the audio first and foremost is still a good idea on the Sovereign, but not for the same reasons. For instance, if you hear a whisper of a high coin tone mixed in trash it might be so quick that even the great instant VDI on the Sovereign can't stabilize the number on the screen and show it long enough for you to catch that whisper of a high tone mixed in the junk. As fast as the VDI is on the Sovereign, much faster than even some "fast" response machines I've owned and in fact probably more "instant" than any machine I've owned, audio should always be your first and foremost hint to a target. Yes, this is a slow machine but you'll be shocked at how "instant" the VDI response is on a Sovereign.
You'll find that foil usually (from memory) starts around 40 to 60 or so on the 180 meters. Iron will read in the negative number range. In terms of resolution another thing that makes the Sovereign's VDI so special is it's got super high resolution from foil all the way up to copper pennies. I'm not aware (?) of any other machine on the market with such a high resolution in that conductivity range, making it real handy for say digging gold rings or relics while avoiding specific common trash numbers at a site like a specific tab or tabs present there while digging all other numbers. Combine this high resolution with the excellent telling audio of the Sovereign (very long and drawn out) and the numerous alert tones, and you can see why it's a favorite among some gold ring hunters. This machine (like other Minelabs) is not very sensitivity to fine gold like thin chains, but in terms of gold rings (even super thin ones) it's pretty much as deep as it gets in a VLF machine for finding them. The excellent analog-like detailed audio also is a good way to avoid a bunch of trash. Anyway, back to the VDI thing...There are some machines that have "2D" discrimination, based on a ferrous and a conductivity scale. However, the conductivity range is rather limited. On one machine's 0 to 50 conductivity scale copper pennies start at about 38, so you can compare that to the roughly 40 (or so) starting point for foil on the Sovereign all the way up to about 178 or 180 for a copper penny and you can see a lot of fine detail resolution in that range of conductivity on targets to really split some hairs on stuff. For example, if you note and avoid a few pesky specific tab numbers on the Sovereign and dig everything else you won't be blocking out a wide range of potential good targets, where as on a machine with less resolution in conductivity you would be blocking out a more broad range of targets but avoiding a few tab numbers.
I feel this is one of the strongest strengths of the Sovereign and it's VDI for ring or relic hunting, or for certain old coins that range down the scale. You can dig targets 1 digit off of that pesky tab present at a site and might find a lot of good stuff that way. Not only that, but this machine is also the most "sure bet" I've owned for nickles. They are distinctly several digits below the start of the lowest tab range (rare exceptions to this, but very rare), so you can easily avoid tabs and most other junk and still dig a ton of nickles. Also handy for old coin hunting where you want to avoid most trash but are willing to take a chance on an old nickle as it's probably a sure thing, and in fact might come up with a gold ring even though you were just out for coins. I've dug a ton of nickles, old and new, at pounded out sites with this machine. More so than any other I've owned. And, the nickle sound is very distinct and lower than tabs, so by sound alone it will get your attention. The VDI resolution is just icing on the cake for nickle hunts. Also, because the nickle zone is so well defined with high resolution you'll avoid most junk that reads a hair or more below it, where as on other machines with less resolution you'd have much more lower reading junk like foil and higher reading junk like tabs that are included in that machine's wider "nickle zone" due to it's less distinct resolution.
When I'm being particular about what to dig one of the VDI tricks is to watch the VDI was you short sweep or wiggle (which should be done to derive the best ID out of the target) over something. Is the VDI # changing by more than 2 digits? If not, change your angle by 90 degrees and short sweep or wiggle over it again. Has it changed from the original one or two VDI #'s that the first angle gave? Then chances are it's not a round object, and thus is not a coin, ring, old button, token, or some other round good find. When I'm being particular I will wiggle or short sweep over it from one angle and if it doesn't change by more than two digits, and also doesn't from another angle, then I'll dig it. Not saying some round good finds can't change by 3 digits or more, but that more often than not those targets are odd shaped trash by my experience. Also, let's say it reads 144 and 145 from one angle, then reads 145 and 146 from another. Even though it didn't change by 3 digits or more from any two angles, it did change by 3 in total from those two angles...144, 145, and 146. Here again it's *probably* odd shaped trash but I usually dig those. It's when it roams by 3 digits or more from one angle I'll walk when I'm being picky, but even still if I'm extra picky if it changes by a total of 3 digits or more when the two angles are summed up then I'll walk on those too. Hope you see what I mean here...If it says 144, 145, and 146 or more from one angle I'll walk. If it says 144 and 145 from that angle but doesn't read 144 OR 145 from 90 degrees, but instead reads like 145 and 146 from the next then I'll still bet money it's junk, but it could be a nickle or a ring. You'll get an unspoken feel for what target IDs vary too much from various angles to find out what is usually trash or what is round. Also, you've got to continue your short sweeping or wiggling over the target to try to stabilize the ID to judge this. If you are being lazy and not constantly wiggling or short sweeping then by default the ID can get unreliable. You are trying to pull the best ID out of it and if it fights you and wanders by 3 or more, or changes from the first 2 numbers to 2 others when you switch 90 degrees then bet on it being trash probably.
Far as all coins above copper pennies being 180 on the meter, I don't think of that as a big deal to me. I've owned machines with high coin resolution and they can make a coin look junky due to the unstability of the machine trying to reason out which percise conductivity number they fall on as the differences are very close on many coins from copper penny and above. Strangely, despite the high resolution of the Sovereign in the low and mid range, I don't find it's VDI "floaty" or unstable at all. Very rock solid, but in terms of high resolution on coins they are so close that on other machines I've owned it can make them seem like bad targets. Also, when I am old coin hunting if the coin is deep or even shallow but in trash then I just want to know it's a coin, because those two factors could make one coin read like another and make it harder for me to decide it's really a coin as it's fighting the depth or trash and ranging wildy. Besides, I've dug plenty of silvers that read like clads or even as wheats for some odd reason. Depth, nearby trash, being on edge, the coin being worn a bit, the ground being real dry...All of these things can make one coin read like another. A bigger net catches more fish in a sense, so I just want to know it's a coin when out for the old stuff. I feel that wider "net" catches more coins that I might otherwise pass on thinking they are too unstable and thus junk or are hiding as looking like some other coin. The only coins I really want to ID are zincs to avoid those when say clad hunting and trying to rack up fast clad totals. The Sovereign can easily ID zincs as they read below 180, so that's all I really care for when wanting to be particular about what coins I'm digging.
One final VDI related tip...A trick I use on the Digisearch meter is to notice the negative number the machine displays when it resets. This is easy to notice as the machine will reset it's threshold to the stock tone. You can often make it do this by thumbing the coil on the ground and it's easier to do with sensitivity running high. Hold the coil still and look at the negative number displayed. At least on my Digisearch meter if it's calibrated to go 180 on a dime or quarter the negative number will be -506. For this reason I never use a coin to calibrate mine. I just adjust the negative number to -506 when the machine resets. If I look down and see that number after it resets I know it's still set right to go 180 on a coin above a copper penny.
for various reasons, I'll next try to dig up the analog meters that were sold at one time for the Sovereign. Some guys who cut their teeth on analog machines of yesterday prefer the analog meters you could get for the Sovereign. I prefer digital myself. Analog needles have a lag time while waiting for them to move when working heavy trash, and not as much resolution as a digital meter where you can see a one digit change in targets where that's a hard thing to do with an analog meter.