Capricornstreasure

Shep, To my knowledge there is no Vacumb present inside any coil of any make. The process used to form the coil is called Vacumb Forming and involves using vacumb to form the plastic to the mold instead of pressing the plastic or injecting it <filling>. The coil lead protection suggested by Bob was to prevent moisture from entering the coil cavity and causing problems with the coil. Coils "breathe", that is when they get warm they expand and take in air thru openings, like the coil lead or cracks, and when they cool the air is expelled. If humidity is high <dont know what level> this can lead to an intake of moisture and give you a funky coil. Same if you use it in the rain. Bob's suggestion is good, you can also try unscrewing the lead restraint and applying a little White or Red RTV to the orifice and screwing the restaint back down. Mind the coil will attempt to breath still and gaps may eventually form. As long at the coil is not cooked in the sun it should stay sealed. I agree that most sites are not maintaning thier catalogs for most of the eqiupment available. I only speak for myself and I will say they have ALL lost business from Me as a result. Maybe the motivation factor will pick up. I understand that alot of these guys are very Busy, however site maintence should be part of a quarterly review. Just my Opinion and not a very qualified one at that. If you have the bones to spend try an XP, its got some advantages even though its not an E coil and it does some fun things with signals. Enjoy, pz

Interesting problem, JP's analysis looks good. Might be helpful to check the coil resistances when its working and when its not using Req's guide to see if indeed moisture is present. I tried resistance checks against my 11 DD and the results were comperable to yours on the wallaby, excepting different manufactures and sizes the basics are going to be the same. If its still in its warranty period you may want to turn it over to thier techs. It will help to note temp's when you start to when it reacts~ I've seen plenty of temperature related intermitant faults to lend my beliefe to JP's results. Let us know what you find out. Definitly noteworthy. pz

Req, I think you just "Coined" a new term, Nickelitis. *ROFL* (oh thats just to punny) I see where your coming from and I am considering it~ will for the next day or so. As to the new experiment I'm also considering how to test your thoughts. Differents strokes for different machiens indeed. Appologies for the eyestrain~ I felt it on this end to but I could not condense it without lossing alot of meaning~ that and its my hope others can follow it. As to the traces, I'm digesting that to and the Fet info as well. It is repeatable, the ML traces, right? Just have to ask for my own touchy feely of the situtation. That is a HUGE ringing, I would expect something at a much higher frequency but I can speculate that what you say maybe true about the coatings and shielding or there is some sort of capacitive effect that is not being accounted for..... Like I said, I'm digesting it..... Hopefully I wont have to blow another Meg of Robs space in reply. Well moving on for the next bit. Talking soon~ Anyone else want to play in the pond come on in..... the water is fine...... Nickelitis..... BWAHAHAHAHAHAHAAA I need that. pz

Req, My art work is just as bad as yours~ we using the same editor? Take a look let me know what you think. A bit about me, I was trained by the US Navy as an Aviation Electronics Technition and have further training as a Metrologist, Calibrations Technition. 'Trons aren't a minor hobby and I know how to set up experiments and test/make devices. I currently work in my field as an Avionics Technition.... I fix and build things and have a pretty good Idea of what I'm looking at. I bring this up so you understand where I'm coming from. There are two issues in your experiment and I want you to understand from my perspective. One, True Depth is defined by myself as the extent that the output signal is able to penetrate the soil. This has little bearing on Detection Depth which is related to reciever sensitivity and target signal strength. The target signal must overcome the ground signal in order to be interpreted by the reciever. It is not a matter of time delay but signal strength and the recievers ability to cope with all signals presented to it. The analogie is that you cant hear a person wisper in a room full of yelling. Time delay only plays its function in that the ground signal and coil decay pulse must deminish sufficiently to allow the reciever to correctly interpret the remaining signals apparent to it. DD coils aid in this by canceling the ground signal at the coil because the ground decay signal is presented to both coils at the same time after the output pulse and thier phases are reversed to each other. This provides a natural canceling effect and leaves only the coils decay time to deal with. Remaining signals only present themselves on one side of the coil and in the overlap zone are of suffieciently different time (induced by the velocity of the target sweeping under the coil) and power as not to cancel themselves out. Time delay can not cope with induced signal decay caused by large metal targets on or near the coil. Increased delay would only interupt the sample time of the machein to the degree that it would make it useless to very small targets with small signals before the next cycle. Addition of metal to the coil results in the presence of a large decaying signal whose time extends past the time delay and swamps the reciever and by virtue of the induced decay signal. This increase in decay signal strength washes out any signal that is not equal to or greater than the decaying signal. Increased threshold volume is the result~ because the target is not moving it can not induce the power fluctuations that are a result of the target moving thru the coils detection zone and thus does not give the pitch changes that a moving target would provide <no velocity>. The decaying signal will raise the input power available to the reciever from the coil. That power level is directly related to the recievers sensitivity to smaller targets levels of power as related by the analogie provided. The result is loss of Detection Depth and during the experiment was demonstrated quite adequatly for my purposes. A conclusion to my first point : Introduction of metal to a search coil can reduce interference by other sources as a result of decreased sensitivity induced to the reciever. But this also results in loss of Detection Depth. True Depth is not affected, true, but it would take a damn big signal to punch thru. It is easier to adjust the threshold of the machien~ you still retain a greater degree of sensitivity for the loss in Detection Depth than you do by applying metal to the coil. I may experiment with this further as a field aid. My second point is in relation to my speculation of the GP3500 to adjust its time delay to compensate for larger coils. This experiment has no bearing on this point because it is realted to coil size and dampening time. In a fixed system, time delay is fixed, some sensitivity must be sacraficed to allow multiple coil sizes to work on the same machien in order to account for increased dampining times. As an extra "Tweek" if the machien knew what size of signal line it was dealing with I could theoreticaly adjust its time delay to give the best possible results for ANY given coil. Its just a very little amount of variance but it could be suffiencient to increase the machiens sensitivity to very small targets without modifications to the amplifing circuit and signal processing. I can only see this being tested by trying different coil sizes against thier Smallest detection sizes. Its not a mater of micro seconds..... its a much smaller time frame I am guessing but it is an Edge. Heck it might be totally inconsequential if the time frame is small enought but it was just a thought. No knocking the experiment but by reading your reply I have to wonder if you think I performed the experiment and knew what I was doing. I changed from you suggested Mono to DD because I know that is where the true results lay for this type of experiment. I did not agree with your premise of time delay being affected by addition of metal to the coil. My target placement and procedure where ruff but accurate enough for what was being examined. Besides a Mono would falsify the experiment, in my opinion, and is too difficult to account for in terms of truth in results. Placement of the metal on the disk for a mono would be an edge placement and the variable becomes Distance from the reciever. In a DD the overlap zone is much closer to the reciever and the canceling affect of the coil helps to prove that signal timing is not an issue to the reciever. Also I used a GP3000 and my proposed thought concerned a 3500. For the Coil issue under discussion we are talking about somthing that is entirly related to the coil and the machien and outside interference has no bearing exept where noted in the traces on the pictures posted in this reply. I am enjoying this conversation though and pulling a fair amount of info from it. This is not an arguement for me, but a debate. I look forward to your replies. Past time for Dinner now, talking soon. pz

Ok Req, Ran the test and got some interesting results. I ran it using two nickles <had three but I'll get to that> and two 22cal bullets from the target collection. I used masking tape and brought some aluminium foil tape in case I needed to up the masking. It turned out not to be necessary. I ran it using my biggest DD in N for the XP, GP was coil DD and sensitivity both tested in Normal and Sensitive. Tuning was performed twice and the coil was only balanced once for the first run and balanced each time a bit was taped on after the first run. The 22 and nickle sounded off loud from over 6 inches up without the addition of metal onto the coil. After the first nickle went on there was a noticable increase in threshold and it became smooth~ no warbles or pops. The 22 cal bullet immediatly droped in intensity only sounding loud at half the distance, 3 inches, and the nickle target lost about 1 inche. I taped a second 22 cal bullet to the coil and the results did not change much. This changed when I added the second nickle..... I lost the 22 cal target and had to use sensitive mode while SCRAPING the target bullet to get a response. The nickle target was lost at about 3 inches more and the response was weak above that. In sensitive as noted the bullet all but disappeared and the nickle only gained a few inches of hieght. I removed the two nickles and left the 22 cal bullet attached and resumed playing with the other bullet target.... It came right back but was slightly diminished. During the experiment the threshold increased in intensity and smoothed out even a bit with only the 22 cal bullet attached. Even after retuning and balancing I could not get the 22 cal target to come back in normal mode for the GP with two nickles attached to the coil. When the nickles and bullet were removed Man did that thing act funny..... warbled and spit until I retuned and re balanced. Pretty cool. What do I feel this test proved to me..... that its a Bad Idea to stick a nickle on your coil. But really it showed me that the machien was not able to cope with the added metal ringing near the coil and it droped its sensitivity in a most profound way. I suspect if an O'scope where attached it would prove that the increased decay time created by the nickle simulates a bad coil and would indeed cause a steady threshold with no variance and hence make one think they had a dead coil. Kind of like operating a mono in cancle. I also feel it proves that the 3000 does not adjust its time delay in any good way. It does result in loss of depth as hypothesized due to lack of sensitivity by the reciever being over driven by the constant noise of the nickle(s). What you get Rob? Thoughts Req.....

Got the Midas seach coil do ya Rob? Not in any race for the results. I'm working with theory Req with equipment. I dont have access to a lab as I would like and I'm packing my life up for my walkabout beinging this month so I'm real limited on what I can do for controled experiments. Should be interesting results but the time delay for the 3500 hypothesis I proposed may not be readily apparent without electronic aids. It would be nice if someone from ML would chirp up but I dont hold any hope in that. Moving thru the morning... pz

Req, I run a 3000 that alone invalidates the test. Neat idea but here is what I understand of the process. See if you agree. Some explainations are for other readers not familure with electronics so bear with me please. I'll do it however I expect it will only be balanced out .... the taped nickle. By automatic changing of the time delay in a 3500 I was refering to coil size and optimal time to resume sampling and the possibility to catch weaker signals. Larger coils should take more time to calm down and thus affect this delay and consequentialy sensitivity to smaller targets. Coil line length can be measured easily and adjusted for by compairing velocities of fixed signals at turn on to datum stored in a program. TDR's use simular principals. <Time domain reflectormeters> For the 3000 and below the sample time is probly fixed to the optimum for an 11 DD stock coil line length. That any other coil works is because its made to be quiet before this fixed time.(a presumption on my part) I am omitting the facts of increased signal line (how big the coil is physicaly in length) distance to the reciever and powerloss effects due to this increased line lenght as well as the signal to reciever distance ratio, ie the signal will be weaker due to the distance traveled from time of emmision of the target to the time of signal interception by the coil as it passes over. <this is an inverse problem and squared to boot>. The pulse that gets the target ringing will be constant as long as the target is under the coils radiation pattern but the time it takes for the coil to physically sweep over the target close enough for the target to induce sufficient EMF on the coil for a response varies with distance and whether its inside the pattern. Sensitivity would be difined by when the sample began, the target being inside the pattern of radiation and cutting the coil with its emissions and the sensitivity of the reciever to the induced EMF whenever that induction occurs and at what strength. (The inducted signal will still have to overcome the line loss) This affects only very small targets in relation to coil size. Its a matter of resolution to coil size in the problem I propose. By taping a nickle to the coil the math affected by the machien should be a ground signal sample minus ground signal <with or without nickle> and all other signals are good. Because the nickle is not moving relative to the coil it would only be percieved as part of the ground and the detector will raise its level of response accordingly, just like hot ground~ you only lose depth. The response from the second nickle <the one on the ground> should be undiminished. At depth however it would make a big difference. This is my hypothesis. I'll let you know the results when I conduct the test tomarrow. For what I proposed on the 3500 you have to change the coil size. We'd really need two machiens, a 3000 and 3500 should be compairable, and 3 coils, mono's would be best, at 11, 16 and 20 inches with 4 targets, two of equal size of minimum detection range for the 20 inch coil and two for the minimum response for the 11 inch. Strength would be measured on a fixed amplifier in open air by response strength ie what do you hear. I'm not discounting better amplifiers in the 3500 over a 3000 so maybe two 3500 would be better for the analysis of whether the time delay is affected or not. The same coils must be used on all macheins tested however to yeild verifiable results. Air tests might help with that set up on a dual trace O'scope and pick offs at the points you have defined Req, the coil and Preamp and I'd love one at the output stage as well or a sound level meter would work on a fixed output from the machien. Thoughts.... anyone~ your invited Req and I cant have all the fun. This does stray rather far afield from the points of intrest. Pics are still in work. pz post note : That PI machiens do not respond <typicaly> to hot ground is a result of electrical canceling of signals that VLF machiens can not accomplish easily without a drastic reduction in sensitivity. It is a fundimental falicy, in my opinion, to say that PI's are immune to the affects of hot ground when taken in relation to Depth sensitivity. I say this to keep it clear about how small signals at depth can be masked by hot ground and this spiel of mine is in relation to mild ground and shallow targets. I do not even try to take in all the variables in my thoughts on the subject of PI coil depth verse sensitivity verses target size or orientation. To much information.

Req, No worries on the drawings, its my turn to hash them up a bit now.... only it will take me a bit longer. I'm still thumbs with pics~ and video of late blah Now that I am oriented on what I'm seeing I'll get back to ya. I still wish to know if you are using identical coils~ there is a time variance I'm not understanding that does not appear to be induced by the O'scope <to big>. I think your using two sizes of coil. Not that it matters much for the direction of our discussion. Once I lined up the traces an explaintion presented itself for one of my questions to ya~ but I'll address that when I get the pics up. Looking at the file title of the pictures your using your Hammer Head for the demonstration? For simplicity I will be sticking to the sample window starting at a fixed 7us~ I'm basing my part of the discussion on a PI which has no delay adjustment. However your explaination in picture three does point out how that variable would be benificial~ it would aid in operator calibration <minimum sensitivity> for various coil sizes. From a troubleshooting perspective: I disagree that the coil would not work if the decay signal is still present~ its not the coils fault but the machiens start time or sensitivity level. The coil will work even if its still ringing~ it will just take a bigger signal than the coil is putting out to override the false signal~ the result is garbage but it does not stop working. My opinion anyhow. It would be percieved by myself as a malfunction in the machien and most likely result in a blank signal or warble in the headset. (Why not?) An example is found in the use of a detector near EMI sources~ lots of operators in noisy areas by ignore the pops and whistles and concentrate on the big repeatable signals. By that statement I'm relating the pops and whistles to the interference caused by a ringing coil. To big a Flyback, as you call it, could fry the Preamp, I guess, as well and reveal weak components that is also a problem in the machien if the coil is in spec. Insufficient dampining is another problem but I dont know that it is built into the coil or machien~ my money is on the machien as its the most controllable of the two variables ie: Coil or Detector. As a side note, the delay variance can be mathmaticaly accounted for if the machien is able to sence the size of the coil used. The machien would adjust for a theoretical optimum for a given size of coil percieved. No reason why a machien cant do it automaticaly~ I suspect the 3500 has done something like this. I'm stopping here as I'm drifting away from our main line of discussion~ the O'scope signals and what they represent. Like I said I'll get some pics up directly to point out some things I see. Talking soon, pz

Some questions for your one, and a confession of some ignorance on my part. Also comparitive analysis of the siganals I see.... On both pics there are two traces. Please allow me the indulgence of labeling them A and B. A is the one with the Delta T of ~4 micro seconds and B is the ~7 micro second displayed. Both A and B show a large pulse of approximatly 2.5 volts~ what is this pulse? My guess is its the output pulse <are you using a 10x probe? Did you calibrate before measuring?>. The second question is whether the coils are the same size and were they placed in the same location/orientation when the measurement was taken? <well two questions in one there > In short what was your set up. Display A does not appear to be complete for the lower ampltude signal. Compaired to display B the first part of the waveform looks truncated otherwise the signals are identical except in the last part of the rise. This can slew your delta t measure. The rise time to the level state for B is about 2.8us but appears to be interupted about 1us into its upturn. This suggests to me some minor feed back but does not look like its caused by the coils ringing. Thats why I ask about the coils sizes and orientations.... it looks induced. I say that because the output pulse <larger waveform> shows a ring at the base of its pulse and that seems to be a response to something other than the coil. Ringing does not normally follow a pattern of this type on the front end in my experience with one shot oscillators. A display shows a sharp clean rise of about .8us to steady state and even the output pulse <the larger waveform> shows sharp features. Nice but I do want to know if the test set up is identical for both coils. Before I go any further can you orient me on these items of interest? My ignorance comes from not having done these measurements myself so I dont know what a good targets waveform would do to the steady state ie would it show as a dip and rise or an oscillation? I can try it with my scope but its a small digital modle and not fit for pictures~ field stuff. As to one of your questions, would a time delayed machien of 7us be affected by the signal time of greater than 7us.... Certianly. Depending on the sample window if the overshoot is excessive enough and the reciever is sensitive to the signal at that level it would probly result in a warble or constant tone as seen by the reciever. Its looking for a variance on the steady state that a target will make when its signal cuts the coil. Thoughts or am I on a bad track?. Talking soon, pz

Thank you Req (Thumbs Up) I'll ruminate on that a while for sure The pics are pretty good and informative for the samples taken. I'd like to get ahold of a malfunctioning machien.... Names Pat. Talking again soon, pz

Thank you Eric, Req, Parts of your explaination dovetail with my thoughts on why larger, or even poorly built, coils could cause the problems I've been reading on. No doubt your variable time delay is a good fix to assist in matching coils to a given machien. ML not having that adjustment it should be subject to funky coil problems. The Fet problem <suspect> is an output from, just a guess, the machien and has more to do with loading <or overloading> but thats just my opinion and would cause on off on again problems. I dont know that this type of problem would be intermitant~ FETS tend to work or not. Please feel free to pipe in.... As ML machiens dont "know" what coil they have on them a bit could be said about programing~ the machien does a form of self calibration in the GP series. I suspect the 3500 has some form of self adjustment due to the increased automation to compensate for the coil used. As to older SD's I think they are just more "open" as far as tolerances go, and that also affects sensitivity, and probly why they dont seem to suffer as much with coils~ but thats based on lack of problems from what amounts to third party knowledge. Only ML knows for sure if they even track such things for product development. And it goes for 'punching deeper'.... that is a sensitivity issue as much as it is a power one. Agian, opinion. I would love to see your notes. It might be helpfull in your explaination on overshoot and ringing if you could get up a couple of O'scope pictures and a little more basic explaination of why it occurs. An idea. Thanks again, talking soon, pz

hammerednick, The standard answer from the reading is a mono gives about a 20 percent increase in depth over a DD of same size. Robs estimate is about spot on. Minelab owners forums may be of some help as there are alot of British finds and users on that site. Nice stuff to. As to the tones of non ferrous signals at depth.... It'll probly be a low to high signal simular to ferrous returns or ground noise. Thats a guess based on the detectors operation~ deep signals use a seperate circuit in the detector and its response is the Woo Whee. A suggestion if you can afford it is the new NF XP coil series. They feature a switch that changes the deep signal response to the Wee Whoo via changing the phase relation of the coil. Technical but basically it would give you what your asking after if you desire the high tone response on deep stuff. Those coils are DD's as well and have other advantages when it comes to interference. My two cents. Welcome and good luck. Rob gives good advice pz