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robby_h

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  1. It seems nobody has noticed that the QED is where the Pulse Devil was years ago, ie, supposedly for sale "soon" in a Whites distributor's shop here in Victoria Oz and any delays are being blamed on the battery, again the same as the PD. Rob.
  2. Sorry but Minelab didn't patent the use of Litz wire in coils, they patented a low eddy current coil. Bruce Candy discovered that excess metal in a coil responds as if moving relative to the coil when the coil passes over magnetic ground or ferrite type rocks. Third party coil makers could in fact use Litz wire or stranded wire and not infringe the patent if the coil contained large solder joints or metal shielding or any other excess metal that would normally give a signal if moving relative to the coil. Some third party coils were hopelessly noisy on some ground and this was wrongly blamed on the detector. To demonstrate this, place the coil flat on the ground away from metal and then place a coin on the ground at a distance from the edge of the coil where it would normally give a response if moving. Then pass a ferrite rod, or piece of soft ferrite over the coin and it will give a response as if moving relative to the coil. You can also try placing a 1 gm nugget on the coil and it will give a response if you pass a piece of soft ferrite over the nugget. And Minelab do not use Eric Foster's ground subtraction method. The late sample in Eric's method must be amplified, which means that any target signal in the late sample is also amplified and then subtracted causing a substantial depth loss. The late sample in ML's method doesn't require amplification thus very little depth loss. A little bit of thought and knowledge of nugget modelling should make it clear why this method works better than first thought. Minelab also patented a method that compares the result of one pulse length with the result of a different pulse length, another first, and they also developed the smooth timings that rely on knowledge only disclosed in ML patents and can cancel two different ground types with one setting of the GB. The new salt mode is also novel. To spend a fortune researching and developing these methods and to not patent them is just plain crazy. BTW, one of the few who actually understands Minelab's patent in that Geotech thread is a guy called Clancy, those who expressed a negative view obviously have no idea at all.
  3. Metal targets are represented as a coil with L (inductance) and R (resistance). The time constant is L/R (large bulky objects have a long time constant and small targets have a short TC). The magnetic properties of iron enhances its inductance, which in turn increases its time constant making it appear to be much larger than it is. For those interested in further reading download the following article by Bruce Candy, page 4. http://www.minelab.com/__files/f/11043/KBA%2000-1%20Metal%20Detector%20Basics%20and%20Theory.pdf
  4. As Reno Chris said, the coil in most pin pointers is a closed loop and this creates a signal in much the same way as the little anti-theft coils inserted into goods to prevent shoplifting. You should be able to stop the interference by shielding the pin pointer. This could be done by lining the holster with alfoil (aluminum cooking foil) although it should also work if you fix a suitable small piece of metal or a ring to the holster, but you would have to find the sweet spot or it wouldn't work. It would be simpler to experiment with an alfoil shield first.
  5. Steve, You seem to have a very good handle on it. I used simple PI GB in my example where the late ground sample is amplified to equal the first ground sample. Minelab use a more efficient method but the high and low tones still follow normal timing convention and pivot around a point determined by the GB setting, but all bets are off with enhance/fine gold/difficult soil timings because the tones can go high for say a 4 grammer in normal and low in enhance etc and then reverse again, this is because the difficult soil algorithms combine more samples than normal timings. Before the GPX it was rather alarming to hear about the number of large nuggets found in half dug holes abandoned by mostly novices because the tone went low. This usually occurred because most gold they normally found gave a high tone and a low tone usually meant junk. It's silly at the best of times but became ridiculous when they walked away from a low tone in ground that had never been disturbed. Jasong mentioned the fact that the ZED has to deal with an additional ground component that the GPX does not. This is the so-called X component but it's separate to the VRM signal (magnetic decay signal) common to both technologies and cancelling VRM is responsible for the tone change. It's rather interesting to note that X discrimination can be reasonably reliable, especially when combined with time constant discrimination. The CTX 3030 uses both. One thing that can occur if the VRM signal is very weak is the VRM GB might settle on a different setting to another detector of the same make and model in the same ground simply because there is little to cancel and this might make a selected nugget respond with a high tone on one detector and a low tone on the other detector. This can actually happen on dry clean golden beach sand where the auto GB settles anywhere and then gets way out of whack when swept over a ferrous target. In this case you would have to carry a ferrite and a VRM ground sample to obtain the same result with two ZEDs or a VRM sample for SDs/GPs and GPXs. Shows why air tests are unreliable.
  6. Simple pulse induction ground balance is achieved by subtracting a late amplified ground sample from an early sample, ie, if the second sample is adjusted to equal the first sample then we have GB, but for most small bread & butter gold the nugget's signal in the early sample will be much higher than it's signal in the late sample so the audio goes high in tone. As the nugget size increases, we eventually get to the point where the nugget's signal in the late sample is greater than in the early sample and the audio then goes low. Size isn't the only determining factor though. In one case a solid smooth 12 grammer might give a low signal but a larger lump might give a high signal if it is made up of smaller bits all joined together or smaller bits joined to a larger mass or if it's a specimen. This is because the small bits increase the amplitude of the early signal relative to the late signal. It should be obvious by this that purity doesn't necessarily determine the tone. The ZED isn't a conventional PI and has to deal with an additional ground signal but the outcome is much the same. This is the basis for the TDI's time constant discrimination but it only works when used to ID solid man made objects such as coins and rings, mainly because irregular shaped nuggets and specimens can easily give the same signal as iron junk. If anyone is interested, Bruce Candy explains it well in a 1996 patent.
  7. Tailing piles usually have a lower magnetic material content because it is removed in the washing process with other heavy material. Rob.
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