One of the first things most detectorists want to know is how to stop digging rubbish.
If your metal detector can recognise iron, foil, pull tabs and other unwanted targets, surely you can simply tell it to ignore them and concentrate on the good stuff?
Well... sort of.
That’s exactly what discrimination is designed to help with. But there’s an important catch: your metal detector doesn’t actually know whether the object beneath your coil is rubbish or treasure.
It only knows how that target responds.
And sometimes, the targets we desperately want to find can respond remarkably similarly to the ones we'd rather leave in the ground.
So, what exactly is discrimination, how does it work, and how much should you use?
What Is Discrimination on a Metal Detector?
Discrimination is a metal detector's ability to accept or reject targets based on their electromagnetic response.
On many modern detectors, the Target ID scale is divided into individual numbers or groups of numbers. These correspond to different target responses detected by the machine.
Depending on your detector, you may be able to choose which sections of that range you want to hear.
If a target falls within an accepted range, the detector produces its normal target response.
If it falls within a rejected range, the response may be silenced or suppressed.
This allows you to create a discrimination pattern.
For example, you might reject part of the ferrous range while leaving the non-ferrous range open.
Sounds perfect.
But there's a reason experienced detectorists don't simply reject everything except the numbers where coins normally appear.
Your Detector Doesn't Know What It Has Found
This is probably the most important thing to understand about discrimination.
A metal detector isn't identifying objects.
It doesn't see:
IRON NAIL
and decide not to bother you.
Instead, it analyses the target's electromagnetic response and places that response somewhere within its identification range.
Different metals tend to behave differently, which is why discrimination works at all.
But different objects can produce similar responses.
And the response isn't determined by material alone.
It can also be affected by:
- Target size
- Target shape
- Depth
- Orientation
- Ground conditions
- Mineralisation
- Nearby targets
- Corrosion and composition
This is why discrimination can never provide a perfect division between good targets and rubbish.
The detector doesn't know which is which.
What Does "Notching Out" a Target Mean?
You'll often hear detectorists talking about notching certain targets or numbers.
Notching is essentially selective discrimination.
Rather than rejecting a large continuous section of the Target ID range, you can reject a particular section while continuing to accept targets above and below it.
Imagine a simplified Target ID scale:
0 — 10 — 20 — 30 — 40 — 50 — 60 — 70 — 80 — 90
You might decide that a particular piece of rubbish regularly appears around one section of that scale and reject those IDs.
When your detector encounters a target within that rejected "notch", it suppresses the response.
The problem?
Something you do want might fall into exactly the same range.
Why Can't Metal Detectors Just Discriminate Out Pull Tabs?
This is a perfect example of the limitations of discrimination.
If you've spent any time detecting parks or recreational ground, you'll know how convincing some aluminium rubbish can sound.
Unfortunately, aluminium also occupies conductivity ranges where desirable targets can appear.
Gold jewellery is the classic example.
Depending on its size, shape and composition, a gold ring can produce a response remarkably similar to aluminium foil, ring pulls and other modern rubbish.
If you aggressively discriminate against those targets, the machine will happily silence them.
It may also silence the gold ring.
The detector can't tell the difference simply because one target is valuable and the other isn't.
Why Iron Discrimination Isn't Perfect Either
Iron is where discrimination becomes particularly interesting.
Rejecting obvious ferrous targets can make detecting much more pleasant, especially on land containing huge quantities of nails and other iron objects.
But iron can also interfere with desirable targets.
Imagine a small hammered silver coin lying close to an iron nail.
When the coil passes over them, the detector may not receive two beautifully separated signals saying:
NAIL + HAMMERED COIN
Instead, the responses can interact.
Depending on the detector, coil, settings, target position and sweep direction, the iron can partially or completely mask the non-ferrous target.
If you're using heavy discrimination, you may never investigate the signal.
This is one reason iron-contaminated sites can continue producing good finds long after they've supposedly been "detected out".
Discrimination vs Iron Bias: What's the Difference?
These two settings are sometimes confused, but they aren't the same thing.
Discrimination determines which target responses you choose to hear or reject.
Iron Bias helps influence how the detector classifies difficult or mixed ferrous/non-ferrous responses.
So, broadly speaking:
Discrimination:
"I don't want to hear targets within this range."
Iron Bias:
"When a difficult signal contains both ferrous and non-ferrous characteristics, how strongly should the detector favour identifying it as iron?"
Both can help make difficult sites easier to detect.
And both can potentially affect whether you investigate an ambiguous target.
We've covered this in more detail in our Iron Bias Explained guide.
Does Using Discrimination Reduce Depth?
This one needs a little nuance.
Simply rejecting a Target ID doesn't necessarily mean your detector suddenly loses raw detection depth across every other accepted target.
The detector may still detect the rejected object — you're simply asking it not to give you the normal audible response.
However, heavy discrimination can affect what you effectively recover in the real world, particularly where desirable and undesirable targets are close together.
A deep or partially masked target may already produce an imperfect response. If part of that response falls into an area you've rejected, you could make an already difficult target even easier to miss.
So rather than thinking:
"Discrimination reduces depth."
It's more useful to think:
"More discrimination means I'm asking my detector to make more decisions about which signals I don't want to hear."
Sometimes those decisions won't be the ones you would have made after digging the target.
Should You Discriminate Iron?
For general detecting, rejecting at least some obvious iron can make perfect sense.
Nobody wants to spend an entire day excavating every tiny nail from a heavily contaminated field.
But there is a difference between using enough discrimination to make a site manageable and trying to eliminate every imperfect signal.
On an old habitation site, for example, lots of iron can actually be a clue.
Nails and other ferrous material may indicate previous structures or concentrated human activity — exactly the sort of place where desirable non-ferrous objects could also have been lost.
Sometimes hearing some of the iron helps you understand the site you're standing on.
When Should You Use Less Discrimination?
There are several situations where opening up your detector and hearing more of what's beneath the coil can be useful.
On a productive historical site
If Roman, Saxon or medieval finds are coming from a relatively small area, you probably don't want the detector making too many decisions for you.
When searching amongst iron
Less discrimination can give you more information about what's happening beneath the coil and help you investigate questionable signals beside ferrous targets.
When looking for small targets
Tiny hammered coins, fragments and other small non-ferrous objects don't always produce textbook responses.
When searching for gold jewellery
Gold overlaps with plenty of modern rubbish on a detector's identification scale.
Unfortunately, digging more rubbish is often part of searching for gold.
When Is More Discrimination Useful?
Discrimination isn't bad.
It's a tool.
If you're detecting an enormous modern site full of unwanted targets and have limited time, selectively rejecting some responses can make your search much more efficient.
It can also make detecting more enjoyable for beginners who would otherwise be overwhelmed by constant signals.
The important thing is understanding the trade-off.
Every target range you reject is a range you've decided not to investigate.
If you're comfortable with that decision, discrimination is doing exactly what it's supposed to do.
So, What Should You Discriminate Out?
There's no universal discrimination pattern that works perfectly everywhere.
A better approach is to consider where you're detecting and what you're hoping to find.
On relatively clean pasture, you might choose to hear considerably more because there aren't enough targets to become frustrating.
On an iron-infested habitation site, hearing some ferrous information could help you pick desirable signals from amongst the iron.
On a modern park where you're specifically searching for lost jewellery, rejecting foil and pull-tab ranges could be completely counterproductive.
And on a rally where you've only got a few hours on productive historic ground?
You might decide you'd rather dig a few questionable targets than discover afterwards that you discriminated out the find of the day.
Don't Let Your Detector Make Every Decision
Modern metal detectors are remarkably good at giving us information about what's beneath our feet.
Target IDs, tones, discrimination, Iron Bias and other features can all help you decide whether a target deserves investigating.
But none of them can tell you with absolute certainty what's beneath your coil.
The more discrimination you use, the quieter and potentially easier your detector can become to use.
The less you use, the more information you're allowing the detector to give you.
Finding the balance between the two comes down to the site, the targets you're searching for and, ultimately, how much rubbish you're prepared to dig in pursuit of the good stuff.
Because sometimes that horrible little signal really is rubbish.
And sometimes it isn't.