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mcjtom

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  1. I realize that there is not a single answer to this, but I just found a local Stainless Steel manufacturer willing to experiment with me. I kind of want a small portable round scoop (to make it easier to make). So perhaps I'll ask for an advice: what would be a reasonable round-hole size for such a scoop? I was thinking between 10 and 15 mm, for wet sand/surf and dry as well, easy to shift sand through but still retain things (maybe not the smallest)? Also figured that 1.5 mm thick plate is plenty? Does it sound reasonable?
  2. Seems like there may be some correlation between the two, perhaps a causation as well? ML in 2014 report appears to also blame unfair competition and borrowing their technology by others for the fall.
  3. Not exactly on topic, but from the graph of the Minelab revenue above, unless I've messed it up, it transpires that in 2014FY there was a $100mln slump that took them some 5 years to recover from. Anybody knows why?
  4. Cheers for that! At one point the author is apparently trying to do something similar to what I tried. He guesses that in the US alone there are some half a million detectors sold each year (is that true?), and that a detector lasts 3 years, and that scanning MD forums is a good way of estimating the ratio of population actively detecting metals, and that people in Florida detect with the same enthusiasm as people in e.g. Minnesota, and therefore there are at least 1.5 million active detectorists in the country (I'm not sure, but I think he's only interested in estimating detecting for relics). A few other countries he considers pale in comparison. It probably needs to be read more carefully to judge the methodology, but I have a feeling that despite a veneer of rigour, it may not be that much better than the order of magnitude estimate method above. p.s. 'Critical' is a gross understatement - he vents his spleen every chance he gets.😀
  5. I took a stab at estimating the answer. It uses 'Fermi Estimation' - a fancy term for breaking up the problem into smaller bits, best-guessing their values, and arriving at the order-of-magnitude range of estimates. The idea is that as long as the guessed values are not all biased in one direction, the estimates will likely even out and possibly give an indication of what a reasonable range may be. In this case I simply used the Codan/Minelab annual reports to find out the revenue from metal detectors sales as a starting point. The rest should be self-explanatory. The actual computations can be found here. You can change the input values in the white cells (possibly to something more reasonable, as mine were mostly wild guesses) and see how the results change and if you can narrow them down. So, it seems to me that there are perhaps some half million active detectorists worldwide (within a factor of ten). Would anybody have a better idea?
  6. How would one go about trying to build MicroVanquish, I wonder? Opps, I just found this link: https://www.ebay.com/itm/373959570273 I imagine that's the most viable option? Don't know how to get it in Thailand at a reasonable cost (shipping and duties will likely at least double the price...) Is it worth it though? I'd love to make the Vanquish smaller when packed. I imagine those shafts are sturdy and well made?
  7. I was recently asked if I knew how popular worldwide metal detecting is, and I realised that I have no idea. Would anybody roughly know or suspect how many people may be involved in metal detecting or how many detectors being used may be out there? I became curious.
  8. This is likely a stupid question, but I'll ask. While I'm not interested in landmines, it seems that some mine detectors fold into those nice-looking and portable packages. Why all(?) civilian detectors that I know of have this standard, less-portable, and more awkward form factor: a coil on a (collapsible) stick with a control box attached?
  9. With '2D Identification Map', x-axis seems to be related to the 'conductivity' (time constant) of the targets. What's y-axis and why would it go both up and down from the middle?
  10. https://www.theguardian.com/lifeandstyle/2022/jun/03/experience-i-found-a-hoard-of-161-roman-coins-in-one-weekend
  11. Out of curiosity: is this coin different from 'the US copper penny'? Which one is the modern, copper-coloured 1 cent coin?
  12. Did anybody publish some approximate ML VIDs for modern bullets and their brass shells? I imagine that if they are not badly deformed, they should have pretty standard VIDs, similar to coins? For example, uncorroded .22 brass shells read around 9-10 on Vanquish - a 9 mm shells at about 17.
  13. Would it be safe to assume that the GB adjustments on MF detector such as the Nox would mostly adjust the location of the ferrous ground type hi-pass filter, as opposed to adjusting the range of the the other filter trying to ignore the non-ferrous salt water ground response, in general? I guess my logic is that, depending on the composition of the ferromagnetic minerals in the ground, the location of their 'time-constant' changes, but the wet salty ground signal time constant is relatively stable - just the intensity differs?
  14. It looks like there are two VID clusters for those rings too (at least in air or if the signal is strong), somewhat blurrily demarcated by an imaginary 'nickel' (or maybe a pull tab in my case) of around 13 to 15. The low karatage of most of the rings didn't seem to make much difference (or actually could contribute to increasing the VIDs due to relatively higher conductivity of low-grade 9K alloy - see the gold alloys conductivity chart above).
  15. What would be the range of possible GB type signal elimination on Nox ID scale? Somewhere within low negative to slightly positive IDs?
  16. Cheers! Would the Relic mode be the worst in your area (which I imagine has the ground leaning toward the magnetite type?), as the Relic GB is probably set more away from it (toward more conductive salty type) than the other two? But then, the Relic mode has the slowest recovery time, which may make it more susceptible to ground noise of any type... All I'm trying to figure out is if the apparent difference in GB settings between Vanquish modes can be used to ones advantage, given different ground types (for instance, would the Relic mode that seems to have GB closer to conductive wet salt, be of an advantage on the beach, as opposed to the other two, which may be closer to the right GB on the iron-mineralised ground?). It may seem trivial, given the Vanquish adjustment limitations (which may be a good thing as fiddling is limited 🙂), but I'm trying to understand my machine...
  17. Would knowing which direction the ground balance is fixed in different Vanquish modes help you any in different grounds?
  18. Yes, cheers! But I'm just wondering what the wet salt beach itself (with no black sand and without any targets), would show as falsing on the Nox (as opposed to magnetite mineralised ground). Would it be in low positive VIDs or still in low negative ones?
  19. If the Nox is not well ground-balanced on ferromagnetic ground, it falses (due to ground) in low negative VIDs, I understand? What VIDs are likely if the ground blance is off on the wet salt beach (or surf)?
  20. It appears that while GB on Vanquish may be 'fixed', it is not fixed equally between modes: in addition to other differences between Vanquish modes (recovery time, default notching, and low/mid tone break), in the Coin and Jewelry modes the GB seems to be set more towards reducing more ferrous type of ground signal (toward magnetite) and in Relic mode the GB seems to be set more towards eliminating more conductive ground signal (toward salt). If the actual ground type differ from the one assumed in a Mode, insufficiently reduced ground signal may creep in when the coil is moved over mounds and depressions or when the ground type changes over short distances, I gather. While I'm still working on convincing myself that the above is the case (to perhaps take an advantage of it once I understand it better), perhaps playing with the Vanquish modes in different ground types could quiet the machine (if the GB is the culprit)?
  21. This is probably nothing new, but often the composition/karatage of the gold alloys is reported along with the size/shape of the item on test VID vs. Target charts. It looks to me that there should be v. little difference in the conductivity of gold alloys between some 12 to 20 karat (50% - 83% of gold), so the VID differences are probably driven by mass, shape, orientation (and depth/ground type) of the object, and not so much by the gold alloy 'karatage' when it comes to jewelry?
  22. Would depth be the third variable, omitted in this chart, which would correlate with lowering the VIDs for any of those rings?
  23. Would at least these general statements be roughly correct? Whether measuring the response phase change in a single frequency or comparing the responses in different frequencies in MF (and also disregarding how such GB scale and its 'zero' or default are implemented in a particular detector), the ground balance objective is to filter out (ignore) the slow-changing responses caused by moving the coil over the ground minerals devoid of valid, smaller/weaker, targets. Rather than targeting all slow-changing signals, the filter tries to decrease only those on the ferrous-conductive 'ID' scale that the particular mixture of minerals in the ground is likely to produce (i.e. preventing unnecessary removal of slow-changing signals in the range outside of the minerals-mixture), but it needs to be shown (set to) what range it should be. Pumping (or even sweeping) the coil over given soil simulates such slow-changing responses and allows the machine to detect the ferrous/conductive nature of the signal due to the ground in order to subsequently aim the filter (automatically or manually) at the most consequential slow-changing signal range to be decreased by the filter and thus best quiet the machine on this ground type. If the ground signal increases when moving the coil towards the ground, the machine GB (the particular range of 'IDs' for slow-changing signals set to be filtered) is off and the filter range is incorrectly set too much towards removing slow-changing ground signals in the ferrous range, best represented by magnetite (at the expense of insufficiently filtering the true ground response which may be in the more conductive range, i.e. towards the salt). If the coil 'sings' when being moved away from the ground, it indicates that the machine GB is also not set optimally, but this time it is trying to filter the higher conductive range (closer to salt) whereas the actual soil response may be in the magnetite range and remain unfiltered (thus producing the false signal caused by spacial changes in soil composition or changing the coil-to-ground distance).
  24. Would my understanding of this be correct then: if pushing the coil towards the ground produces signal, it means that the GB value is set too high for this ground (on Fisher's scale) and too low on Minelab's scale (and when pulling the coil up, the reverse is true)?
  25. It looks like this may be the answer as in 'yes', if Minelab uses similar metaphor:
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