UK METAL DETECTING

loftedfalcon · Joined: 06 Nov 2007 Posts: 11 Location: Grantham

Does anyone know if there is such a thing as a table of readings?

What I mean by this is an approximation of conductivity associated with certain metals, i.e:

+30 = Tin foil
+80 = Gold

etc. I reckon this would help me to learn my XLT quicker!

kered · Joined: 29 Sep 2007 Posts: 97 Location: NW Spain

Don't know the XLT but if you dig all, but note the reading beforehand you will end up with a table for your machine rather than a generic list(there maybe small variations between machines) but i think the list would be endless because different makes of foil or ringpulls have different amounts of the varios metals and would read differently even if only a fraction, also that way nothing should get left behind by mistake.

quickcuff · Joined: 25 Aug 2007 Posts: 30 Location: Lancashire

The progs I use on my XLT, coins are roundabout a good +70, foil type rubbish is approx +20 and those blummin' ointment tubes are approx +45.
But saying that I've had silver coins at +50 !

However, listen to what noise your XLT makes and you'll soon get the hang of it.

Jagged , harsh tones are usually rubbish.
Smooth tones are usually good targets.

Unfortunately the best way to learn is to listen to the tone, look at the meter and dig the target.

Best of luck Smile

bluefish_121 · Joined: 06 Oct 2007 Posts: 28 Location: Derbyshire

Have you tried doing a bench test? get various items you have been finding, or general items, ringpulls, cans, foil, foil wrappers, nails, coins, rings, necklaces etc etc.

Put the machine on a non metallic surface, and test each one and make notes. This may help a little.

loftedfalcon · Joined: 06 Nov 2007 Posts: 11 Location: Grantham

I was doing that for an hour or two last night bluefish, combined with coins on edge and so on. I'll get there, I was just looking to see if there was a shortcut but as ever there's no substitute for experience!

UK Brian · Joined: 08 May 2007 Posts: 431 Location: Wales

This is a bit of a worrying topic which is ignoring the fact that all meters are probability in the manufacturers view but possibility is a more accurate statement.

The reading depends not only on the machines design but also the size/thickness of the item, the heat treatment its received and the ground mineralisation of the ground.

So quickcuff has had silver coins at 50+ and I've dug a hammered silver penny this week at -2.

If you look at a Whites/Garrett/Compass etc analog metered machines you will see they have two sweeps of colour full width across the meter. One for silver, one for gold. This is to indicate that they could show up anywhere.
Same applies to all the other metals but they normally have a much smaller range that they I.D. in.

Then add mineralisation. Most machines, as minerals increase, display a lower reading. So foil may drop to a good reading causing you to dig it, and a good item drop down into a 'bad' reading leading you to ignore it.
A very few machines work the other way and the readings rise.
The partial answer to the mineralisation readings movement is to check what the ground mineralisation is (the XLT's ground phase gives some idea) and allow for it. If you have a machine with a knob to 'balance' ground effects you can see where it has ended up being set. One way balances positive mineralisation, the other negative. The further its turned the worse the ground and the more allowance to be made.

With all the variables most more experienced detectorists don't bother I.D.ing items but accept there is so much overlap that on older sites its not worth the risk. Meters can still be of use. The Spectrums/XLT do an iron smear across the meter which I.D.'s iron that many machines can't and its Signagraph 'bars' indicate target width with most coins being one or two bars wide.

I like the XLT which has good target 'lock on' ie doesn't bounce readings all over the place and most consider its got excellent discrimination as you don't dig much iron with it. Many of the older Fishers on the other hand are considered 'iron magnets' . You spend an awful lot of time digging deep iron.
So which is the best machine ?
Well for ease of detecting you can't beat the XLT, while the Fisher has you returning with a pocket full of iron (though there are ways round this).
Why buy the Fisher then ?
The answer is at the design stage a decision has to be made. Items at depth, at the fringe of the machines discrim. abilities can be guessed at. Fisher decided its best to indicate the 'iffy' target as good, so you dig it and its sometimes good, sometimes bad. Whites took the other tack. If its not almost certainly good, I.D. it as bad. With this approach most of which you dig is good items and it SEEMS to have the better discrimination but only because you missing possible good targets.

So ends the sermon for today. Use I.D. meter or tone but use with care and really in this country its only ferrous that we should be trying to I.D. to avoid on older sites. I'll ignore I've iron arrow heads, spears, foot patterns, cannonballs etc.

loftedfalcon · Joined: 06 Nov 2007 Posts: 11 Location: Grantham

Very good point Brian and taken on board.

You're obviously knowledgeable so I'll ask you this next question.

Mineralization, It keeps cropping up and I'd like to know more about it. Can you help?

How does it affect a machine?

What soil types have more?

Is there "good" mineralization?

UK Brian · Joined: 08 May 2007 Posts: 431 Location: Wales

Good question.
There's positive or negative ground mineralisation and at some point in between 'good' ground.

All detectors create an electromagnetic field around the coil (or in the case of Pulse machines transmit a signal, then switch off and machine listens for the returned signal to come back). These fields are disturbed by natural salts in the soil/beach or by ferrous oxides.

A buried coin or ring is hit by the electromagnetic field which creates an electric eddy current on the metals surface. This is picked up by the receiver part of the coil which passes it on to the control box to be messed with and the results displayed on meter or by audio.
Unfortunately the signals from the ground minerals are also picked up and mixed in.
There's a lag in the speed the different signals return to the coil which gives the opportunity to build references into the ground balance circuit to allow the unwanted ground signal to be 'balanced out'.

Modern motion machines use filters to apparantly get rid of the ground effect (just because you don't hear it doesn't mean its not there !). Low or medium mineralised ground needs two filters, higher mineralisation four filters.

Finally getting to your question, doesn't matter if its positive or negative, all mineralisation is bad. Iron, salt, even chemicals sprayed on crops cause problems. With non motion machines if you lifted or dropped the coil as you swept it the audio of the detector would either increase or go silent . Both would lead to the loss of finds. Same happens with motion machines but you don't hear it.

Mineralisation is one of two reasons that its often better to go to a smaller than stock coil rather than a larger one. Large coils to some degree lose sensitivity, though attempts are made to work round this, and they all, because they have more square inches to respond to the minerals pick up more ground effect.
So if bad ground has one grain of iron per square inch and the coil has a surface area of 30sq. inches you have thirty grains picked up. A silver penny might be 40 grains of silver. Take one from the other and you have a signal based on 10 grains which you should just hear. Go to a bigger coil, say 50sq inches and you have -50 less +40 = -10 so you don't hear the coin.
This is part of the reason for the popularity of double D/widescan type coils. The live part of the coil is a narrow band down the middle which sees much less of the ground underneath.

So mineralisation is bad, cutting depth and affecting accurate discrimination but manufactures have provided methods to minimise the worst of its effects. We have to help by (1) Using any adjustment they provide correctly (2) Selecting a coil size and type to suit the ground conditions (3) Using the correct sweep speed to allow the filters to work and (4) Keeping the coil level and at the right height (which might not be low).

Sorry its longwinded, hope it helps

Brian

Rabstar · Joined: 20 Nov 2007 Posts: 6

Isn't there a table of readings printed on the control box? That should give yot some idea of where certain targets will register. The only others you might need to know are £1 (63) and £2 (66).