Posted 12 January 2006 - 11:36 AM
Information, comments and potential retail outlets (including on the web) would be appreciated.
And a question: I understand these things work on magnetism. But since benchmark disks are generally brass or bronze and occasionally aluminum, how can a metal dectector find them since these metals are all non-magnetic?
Posted 12 January 2006 - 12:31 PM
Here's the rub. Most detectors can't detect much further than the diameter of their sensor, and that's reduced further for buried targets. My Harbor Freight bottom feeder model can see a benchmark buried 6-8", and no more. If you want to go deeper, you need a larger head. Right now I'm looking for a mark I know is a foot or so down, if I'm lucky. I just today dug a 1' trench and used the detector on the bottom, figuring that would give me a better chance. Man, what a lot of work. Didn't find it- still need to go another 10' to the north if it doesn't kill me.
There are also interference sources like "hot" rocks. Some rocks have a lot of iron, and look like a target. The density of the dirt is a factor. The moisture content is a factor. I'm not trying to scare you, just pointing out that you have to learn to interpret what these things tell you. It's not black and white. IMO, the model Harbor Freight sells for $26-39 seems like a super deal, and works well for benchmarking. It's light and collapsible. They have a higher priced model ($60?) that I have no experience with. After that, look mostly for ferrous/non-ferrous discrimination, and the size of the head. Be sure to get headphones or adapt the ones you may already have. You need 'em to keep out traffic noise and wind, lest you not hear the changes that suggest a benchmark is nearby!
Posted 12 January 2006 - 12:43 PM
Posted 12 January 2006 - 02:43 PM
A few months ago I sprang for a larger, but will $25 model no-name detector. So far it hasn't done me any good. For one thing, I think it is broken, as sometimes it "tunes" in reverse, so that it makes noise all the time except when it detects something. I have been unable to find the setting that makes it the most sensitive to anything in order to make it the most useful to me. There seems to be some secret, but the manual just tells you how to tune it to what you are looking for, not what the tuning settings do at all. There are Discrimination and Tune knobs and you have to fiddle with them while pressing the secret red button that does who knows what.
I think I will head to Harbor Freight and see what they have.
Posted 12 January 2006 - 03:16 PM
Most metal detectors fall in two general classes. One detects changes in the earth magnetic field due to magnetic materials, like iron rods and pipes, and perhaps current-carrying wires. This is what the professional surveyor uses to find land corner pins. They can find iron at great depths, but aren't so useful for benchmarks. They sell on ebay (used) for $300-600. The major brand is Schonstedt and there are others.
The treasure finders almost all work on some principle that detects conductivity (low electrical resistance). There are variations on how the field is excited and received. Pulse versus CW excitation, and frequency of operation make some difference in discrimination capability. Some of the discrimination can come from using both phase and amplitude information from the return signal. Expensive units may use combinations of effects (conductivity and permeability) to get better discrimination.
This post has been edited by Bill93: 12 January 2006 - 03:16 PM
Posted 12 January 2006 - 03:27 PM
Place a bunch of copper pennies on the ground to simulate a non-ferrous benchmark and sweep over those to get a feel for what to expect. Do the same thing with some iron nails. "Better" detectors will have circuits to do what the zero button does automatically, but I actually prefer this arrangement.
Bill93- I was trying to keep my explanation as simple as possible, and in fact the DOT guy I met today had one of the units you described first. Great for re-bar, but appeared completely useless for benchmarks. Built like a tank though. As for the detectors we're interested in, you can pump in more power with pulsed operation, and do other tricks like phase and frequency, but IMO the basic detection always reduces down to some kind of resonant circuit and/or transformer coupling problem. The sellers make a lot of interesting claims, but I see a lot more marketing hype than science. What I'd love to know is how to do deep detection without a large head, and without resorting to that ground penetrating radar mentioned a while back, but I think natural laws just make it difficult to impossible. I'm actually considering winding a much larger replacement head for my cheapie unit- I think it's actually an unobtainable option from the factory, because they refer to it in the manual.
This post has been edited by Photobuff: 12 January 2006 - 05:53 PM
Posted 12 January 2006 - 06:29 PM
Also, I am lucky enough to have an old benchmark in my backpack to use as a test object. Any suggestions for what to use for a brass bolt (PRR) or a monel metal rivet (Reading RR), which is most likely ferrous.
Posted 12 January 2006 - 08:35 PM
Posted 12 January 2006 - 09:08 PM
Posted 13 January 2006 - 07:00 AM
As for the monel metal rivets, they are definitely steel--some sort of stainless steel. Most I have found are still shiny. The Reading Railroad loved these things more than the Pennsy loved their copper bolts! They are very small though--about the size of a dime: KW0607 Monel Metal Rivet
Usually either of these are pretty easy to find. The locations are pretty standard so I can walk up and see them, or shift some dirt and leaves and find them quickly. Some have stymied me though and I have dug and metal detected in vain, so far. KW0932 has been the worst so far. It appears to be an easy find, as nothing seems to have changed enough for the bolt to be gone. But the combination of three bridges at one location, all numbered the same, or nearly the same, plus some overgrowth, plus some communications pipes, make this one a tough find. Sooner or later I will either find it or convince myself it isn't there.
Posted 13 January 2006 - 07:21 AM
I've seen a lot of copper bolts in my territory, most of them set from 1909-1914 by the City of New York when a systematic leveling of the 5 boroughs was done. They are small, less than an inch in diameter, and have no heads, so you could call them plugs instead of bolts if you like.
Here's an illustration from the report which was published in 1914. It shows several type. They are intended to be set vertically or horizontally in massive structures, typically granite.
Here's one of the first type, set on a step of the N. Y. Historical Society building.
Here's one of the vertically set types, from the Custom House (which is now KV0579). For those interested, there is a note in my GC log for this mark giving some background.
And lastly, here is one of the smaller ones, set in the Anchoragre of the Williamsburg Bridge on the Brooklyn side.
I would guess this type of mark is less likeley to be the target of a metal detector because the typical setting doen's tend to get covered over, but one never knows.
This post has been edited by Papa-Bear-NYC: 13 January 2006 - 07:45 AM
Posted 13 January 2006 - 11:39 AM
Posted 13 January 2006 - 04:08 PM
Posted 13 January 2006 - 07:55 PM
This post has been edited by Rotareneg: 13 January 2006 - 07:55 PM
Posted 15 January 2006 - 08:56 PM
So, there are a few marks that we could probably recover if we had a metal detector. Photobuff mentioned a couple inexpensive models from Harbor Freight - $40 with a 6.5” coil, and $60 with an 8” coil. How much deeper could you detect a mark with an 8” vs. 6.5” coil? Would a model with a 10” coil for $80-100 be about right?
Posted 15 January 2006 - 09:45 PM
Posted 16 January 2006 - 12:57 PM
Posted 17 January 2006 - 04:51 PM
I dug a hole 12" deep and laid the buckle flat (like a BM disk) and covered it over about 6" and swept the detector over it. No problem at that depth, so I added 2 more inches of sand (we're here in the desert, remember.) and swept again, no problem. So I added another inch and was about to try again when the low battery light came on so I will need to try again later with fresh batteries.
It has found a 2oz tack hammer at about 10" deep.
I found it helpful with this model to sweep an exposed benchmark to get a feel for the tone it makes with brass, before heading out for a buried benchmark.
Posted 15 February 2006 - 04:02 PM
Of course all people for some reason are unable to do this. its kinda like witching for water.......
Posted 16 February 2006 - 01:21 PM
Best of all, it does not give a hoot about Poison Ivy.