Identifying Nails

[Anonymous]

Some PI detectors have the capability to identify a proportion of nails, wire and pins, but this depends on several factors. The foremost is the response speed of the detector, with detectors having the faster response performing better. Nails, being ferrous, give a disproportionally large signal for the amount of metal present. This is because the signal is made up of two components, a conductive one which is due to eddy currents generated in the metal, and a magnetic component which is due to the coil field magnetizing the iron and the magnetization decaying relatively slowly at the end of each field pulse. The magnetic component of the signal is greatest when the lines of flux from the coil are parallel to the axis or length of the nail and a minimum when the flux is at right angles to the nail. This can be tested by passing a nail vertically under the centre of a PI search coil. The signal will be a maximum when the nail is under the centre of the coil in the line of the flux. Turn the nail through 90 degrees and pass it under the coil. the signal will now be a minimum under the centre of the coil but there will be a maximum at each edge where the flux is turning and is again parallel to the nail. In this orientation you get a weak signal under the centre of the coil which is due to the conductivity. The majority of nails etc tend to lie horizontally in the ground with the result that this effect can often be used to identify them. Bury a 2in nail horizontally in the ground at a depth of 6in. Scan the search head over it and listen carefully to the signal. If the signal has two peaks, do a second scan at right angles to the first and listen for a single, but much weaker response. Compare this with a coin buried horizontally, where there is just one single peak whichever way you scan across it. This method does rely on most coins and rings lying horizontal, as also do most nails. It falls down when a nail is vertical, when it will sound like a coin and also for a vertical coin which will sound like a nail. If the detectors response time is rather slow, then it may not be possible to distinguish the two peaks as they will blur into one. However, it will be a broad signal spanning the width of the coil in one direction and a weak one in the other, so with care, identification is still possible but it takes much practice. Coins and rings by comparison always have a sharp cut off and a nice localised response under the coil centre. Below is a sketch that illustrates the effect of a horizontal nail.
Eric.

 

[Anonymous]

G'day Eric and all, firstly congratulations to you and Bill for this highly informative "classroom", which is buzzing along like a Swiss watch on steroids. You are spot on about identifying nails etc, the trouble is that sometimes, rarely, gold nuggets can occur as "wire gold" and will give a very similar response to nails so we just dig every target even if it does sound like a nail. 99.999% of the time it is a nail or a piece of wire but we sure would hate to miss digging out a wire gold nugget. I have a few in my collection, one is nearly 2" long by only 3-5mm wide. It looks man made by extrusion but it's 100% natural. Hey Eric, is this really the best form of discrimination we can expect from a PI ?. We need a PI that can discriminate like the Explorer, or an Explorer that handles mineralisation like an SD, apart from that we're pretty easy to please Cheers, Chris Hake.

 

[Anonymous]

Hi Chris,
Good to see you in the class. I'm surprised that wire nuggets sound like nails. I'd have thought that the signal would simply be longer in one direction than the other, without the double peak. Send me a few samples for R&D;-)
PI could do much better than this very basic form of discrimination. We're working on it.
Eric.