Every VLF metal detector gives you some form of numeric or visual readout estimating what a detected target is likely made of, before you ever break ground — usually called a Target ID or VDI (Visual Discrimination Indicator) number. Learning to read this number accurately, and knowing when to trust it versus when to dig anyway, is one of the highest-value skills in the entire hobby, and one that separates detectorists who consistently find good targets from those who spend most of their time digging trash.
What the number actually represents
Target ID / VDI numbers correlate roughly to a target's electrical conductivity, typically displayed on a scale (commonly 0–99, though the exact range varies by brand and model). Low numbers cluster around iron and foil; high numbers cluster around copper and silver; a wide middle band covers everything from pull tabs to nickels to most gold jewelry. The number isn't a certain identification — it's a probability estimate based on how the target's conductivity compares to reference values the manufacturer has calibrated the machine against, built from thousands of test targets during development rather than a fixed physical law, which is worth remembering the next time a borderline number tempts you to skip a dig.
Why gold jewelry is the hardest case
Gold's conductivity often lands right in the same numeric range as pull tabs, foil, and other common junk — which is exactly why gold rings get discriminated out and lost by detectorists who trust the number too literally. This is the single most-cited reason experienced detectorists recommend digging more borderline signals than feels efficient, especially on any site where jewelry loss is plausible (public beaches and swimming areas being the classic example, though old parks and picnic areas carry real potential too).
Single-number vs. 2D Target ID
Older and simpler detectors show a single Target ID number representing conductivity alone. More advanced multi-frequency machines, including Minelab's Manticore, add a second axis — ferrous content — displayed as a 2D grid rather than a single number. This resolves a lot of the ambiguity a single number can't: two targets with an identical conductivity-based Target ID can show very different ferrous-content readings, letting you distinguish, for instance, a rusted bottle cap from a worn coin that happen to share a conductivity number on a simpler, single-axis machine.
How to actually use Target ID in the field
- Learn your specific machine's number ranges, not a generic chart — every brand and model calibrates slightly differently, and the exact numbers that mean "probable coin" on one machine won't transfer directly to another.
- Cross-reference with audio tone, not just the visual number — most detectors assign different tones to different conductivity ranges, and learning to recognize tones by ear lets you make faster decisions without staring at a screen.
- Dig borderline signals more often on promising sites — a documented old home site or a beach with real jewelry-loss potential justifies digging more junk-range signals than a picked-over modern park would.
- Watch for signal repeatability — sweeping over a target from multiple directions and getting a consistent Target ID each time is a stronger signal than a single inconsistent read, which more often indicates trash, a mineralized rock, or two overlapping targets confusing the reading.
When Target ID becomes less reliable
Deep targets, targets near iron trash, and targets sitting at an unusual angle in the ground can all return a distorted or unreliable Target ID, even on an excellent detector — the phase-comparison technology behind Target ID needs a reasonably clean signal to work with, and depth or nearby interference degrades that signal before it reaches the display. This is part of why experienced detectorists treat Target ID as a strong decision aid rather than a guarantee, especially on deep or ambiguous signals, and why the field techniques below matter as much as the number itself.
Tone-based discrimination as a complement to numbers
Many experienced detectorists eventually rely on audio tone as much as, or more than, the visual Target ID number — a machine set to multi-tone audio assigns different pitches to different conductivity ranges, letting you make a fast initial read by ear before ever glancing at the screen. This matters practically because looking down at a screen while swinging breaks your rhythm and can cause you to miss the exact spot where a signal peaked; a well-trained ear can make that initial judgment call without interrupting your swing at all. Building this skill takes time and deliberate practice — try consciously noting the tone alongside the number for your first several months, rather than only glancing at the display.
Check price on Amazon Check price on eBayBuilding your own reference over time
Because Target ID calibration varies by specific model, the most valuable reference chart you can build is your own — note the Target ID and tone for every confirmed dig for your first few months (a small notebook or phone note works fine), and patterns specific to your exact machine and typical hunting ground will emerge faster than any generic published chart could give you. Over time, this personal reference becomes far more useful than any general guide, including this one, since it accounts for your specific detector's quirks and the specific trash and target mix typical of the sites you actually hunt.
This habit pays off fastest for detectorists working the same handful of sites repeatedly, since the trash-versus-treasure Target ID patterns at a specific old home site or park will repeat consistently once you've logged enough confirmed digs there to notice them.
The bottom line
Treat Target ID as a strong first opinion, not a verdict — it narrows down what's likely down there and helps you prioritize your time, but the machines and situations covered above (deep targets, iron-adjacent signals, and especially gold jewelry's overlap with junk) are all real reasons to dig more than the number alone would suggest, particularly on ground with genuine promise.
A note on notch discrimination
Many detectors let you "notch out" specific Target ID ranges entirely, silencing the tone for anything falling in a chosen band — commonly used to skip pull-tab-range signals automatically rather than evaluating each one individually. This is a genuinely useful time-saver on heavily trashed modern sites, but it comes with the same tradeoff as aggressive discrimination generally: notch out the wrong range on a site with real jewelry-loss potential, and you'll silently skip exactly the signals worth digging. Use notching deliberately and site-specifically, not as a permanent default setting across every location you hunt.
Frequently Asked Questions
What does a VDI number of 40 mean?
It depends on your specific detector's calibration, but on most machines, numbers in the 35–45 range commonly overlap pull tabs, some foil, and — critically — a meaningful share of gold jewelry, which is why this range is often worth digging rather than skipping.
Why did my detector give a different Target ID on the same target?
This usually happens with targets near iron trash, at unusual depth, or approached from different swing angles — try re-approaching the signal from multiple directions for a more reliable read before deciding whether to dig.
Should I trust Target ID enough to skip digging low numbers?
On most sites, skipping the lowest, most confidently iron-range signals is reasonable time management — but be more conservative about skipping mid-range signals, since that's exactly where gold jewelry commonly hides.