Monday, January 01, 2007

Metal Detectors

In the case of recovering metallic treasures there is an overwhelming array of metal detectors to choose from. If you already own a metal detector, then you have probably made a good choice and frankly any metal detector worth its name will perform the task reasonably well. For those of you who are not already in the metal detecting hobby I will make a few suggestions but the final choice of what to buy must be yours. I’ve lost count of the number of times I’ve heard the question: "Which metal detector is best?" The question should really be: "Which metal detector is best for me?" For the answer depends very much on you and your requirements. How fit are you? Do you want to search beaches, rivers, farmland, underwater?
The vast majority of metal detectors are designed for finding coins, jewellery and similar sized artefacts in the top few inches of ground on inland sites while discriminating out the undesirable contaminants: iron and aluminium foil, for that is what most participants of the metal detecting hobby want. Iron is a major contaminant on farmland and aluminium foil abounds on beaches and recreational areas. Most popular machines work on a Very Low Frequency, Transmit/Receive system, discriminate audibly and/or visually and use the motion system of ground cancelling. Ground cancelling nulls effects from minerals in the ground and the motion system requires the machine to be kept moving otherwise desirable objects are also cancelled out. The system actually works a lot better than might be imagined. To pinpoint a target there is usually a selectable non-motion all metal mode although it is easy enough to pinpoint in motion mode by passing the head over the target in a cross pattern. Machines at the lower end of the market may be non-motion and may have little or no discrimination although by nature, these types are fairly insensitive to iron but very sensitive to aluminium foil.
Within the motion detector range there are choices to be made regarding the desired amount of user control over the machines electronic operation. Manufacturer’s are clearly split between simple ‘switch on and go’ and fully programmable detectors; some manufacturer’s making only one type and some making both types. Logically the computer controlled programmable type will be better able to maximise depth and sort out the trash from the cash but you could spend a great deal of time messing about with the settings trying to achieve perfection instead of getting on with the searching. My own view is that if you are getting at all involved with dowsing then that will more than make up for any advantages of the computer control without the complexities but at the end of the day the choice between simplicity or bells and whistles is entirely yours. White’s have been the traditional UK choice for programmable types but Garrett, C-Scope and Minelab also offer programmable models.
A less popular type of hobby metal detector works on a principle known as pulse induction which is a non-motion deep seeking system. These machines are notoriously sensitive to iron and very few discriminate between ferrous and non-ferrous metals (those that do discriminate tend to reject some desirable objects.) Pulse machines are firm favourites among beachcombers and underwater treasure hunters because of their ability to reach greater depths on most targets, typically twice that of many VLF machines, and to cut through severe mineralisation such as black sand.
There are two types of very specialised machines generally available one being underwater detectors, which are sealed to keep out water and constructed to withstand the pressures encountered in deep water. The other speciality is the so-called hoard hunters, which are usually some sort of ‘two-box’ design, carried like a suitcase, rather than a forearm extension as with conventional detectors. Hoard hunters are designed to find only large objects, the size of a pint (565ml) pot upwards. They do not discriminate between ferrous and non-ferrous metals as treasure may be buried in an iron container (detectors cannot detect through metal) and they are very deep seeking capable of probing several feet into the ground.
The standard coil size fitted to the majority of detectors is eight-inch diameter, which is a compromise to enable the detector to perform reasonably well under a variety of conditions. Most manufacturers produce a range of optional coil sizes typically from 3.5" (89mm) up to 15" (380mm) diameter and these can be employed to improve performance under certain conditions. As a rule of thumb the larger the coil the deeper it will detect but they have their disadvantages too: less sensitivity to smaller targets, more difficult to use on heavily mineralised or iron contaminated ground and less accurate pinpointing. Larger coils are also heavier and more cumbersome to use although the weight can be compensated for by hip mounting the detector control box if the machine has that facility.
In addition to size variation there are two different types of coil construction: concentric and 2D or widescan. Concentric coils, usually fitted to metal detectors as standard, have an inverted cone detection pattern, which achieves maximum depth only at the centre of the coil. Widescan coils have a pudding basin shaped detection pattern and while they don’t achieve as great a depth as the same size concentric coil they do take in a larger volume of ground per sweep. If it’s fast ground coverage you are after, the widescan coil is better and if it’s depth you are after the concentric coil is better. If you have any choice in the matter, widescan is better for typical metal detector searching and concentric is better for locating dowsed targets.
The choice of machine is very much dependent on what you want to do with it. If you just want a basic machine then go for one of the lower priced ones from your own Country. They are better value for money and probably more suitable for your conditions. Typically if you buy an American machine in Britain you pay pound on the dollar and the conditions and even the artefacts which are looked for are quite different in the two Countries. Amongst the higher priced detectors, foreign technology may be superior to your Country’s and there may be less advantage in going for the home produced model. Foreign detectors made in Britain or for the British market have a large following. Laser and Whites particularly with Minelab gaining ground.
If you are serious about the metal detecting hobby then I would suggest you go for a detector in the middle to top price bracket. You can always buy second-hand to keep the cost down and detectors can keep going, or be kept going, for many years. I have a fifteen-year old Tesoro that is still in regular use. If you expect to search mainly inland then a VLF machine will be more suitable. If you want to search beaches then a pulse machine will probably be more suitable but bearing in mind the lack of discrimination on Pulse Induction machines it may be preferable to go for a VLF machine with a good reputation on beaches such as Whites or Minelab.
Anyone who spends a lot of time detecting usually has more than one metal detector. I personally have three – a Laser B1 Hi-Power as my main inland machine, a Minelab Sovereign XS2aPro, which I use mainly for the beach as well as a back-up inland and a Pulsepower Goldscan II for beaches and deepseeking work. I also have a selection of coils for the Laser – 3.5", 8" conventional, 8.5" widescan, 11" conventional, 11" widescan. I use the Laser with the 8.5" widescan coil most of the time. I fit one of the 11" coils for pasture or rolled arable and occasionally use the 3.5" for badly contaminated ground. The Pulse Induction Goldscan, which has an 11", coil I use for beaches and although the general advice is Pulse machines are not suitable inland, I use mine on some fields in conjunction with dowsing which eliminates most of the problems and capitalises on the Pulses depth capabilities. I wouldn’t suggest for one minute that my selection represents the absolute best in metal detecting technology but, in conjunction with dowsing, it does allow me to perform well over a wide variety of sites and conditions.
C-SCOPE UK VLF £130-£600
C-SCOPE UK PULSE £360-£650
FISHER USA VLF £239-£899
TESORO USA VLF £229-£695
VIKING UK VLF £75-£249
WHITE’S USA/UK VLF £185-£849


mlitty said...

I'm just beginning in MD'ing. My dad bought me a Bounty Hunter Tracker IV so I could go out with him.

Looking for more information on tips and techniques, I just wanted to say thanks for your article.

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