I'm fairly new to surveying and in the past few days I've come across several irons that my metal detector can barely detect. Even when the metal detector is directly touching them they only give a weak whistle. I've been told that they are called non-polar irons but nobody has been able to explain to me why some irons give off such a weak signal while others have a very strong signal. They all look identical. It can become such a time waster when it is so hard to find the irons with a metal detector that are so close to the surface yet almost undetectable so I'm wondering why this is the case and is there something that can be done about it?
Thanks!
This is true. Many bars can exert very weak magnetic fields. I do think can result from beating them in the ground. Or it's possible that the bar was demagnetized before it was driven in the ground.
Always have a shovel and be prepared to use it.?ÿ
There are articles on how to magnetize and demagnetize iron.?ÿ
@mightymoe Sure enough, I just looked up demagnetizing iron and the first thing google said was to hit it with a hammer which is exactly how they are put in the ground. Now that I think of it, the weaker irons are typically in areas that freeze solid so they must have been pounded pretty hard to get them into place. I would have never guessed that was the problem. Thanks!
Many bars can exert very weak magnetic fields. I do think can result from beating them in the ground.
Over?ÿ the years I've also been puzzled by a few rebar that don't sing under a Schonstedt detector.?ÿ It's true pounding on iron can demagnetize it but only when hitting a biscuit sized piece lying on a hard surface with a sledge.?ÿ So I think the pathetic efforts of a surveyor pounding in an 18" rebar does not demagnetize it except possibly at the mushroomed top; therefore an urban lore.
So why are some rebars nonmagnetic??ÿ My anecdotal opinion is it's a lot of magnetic junk near the buried rebar which confuses the detector (fences, junk cars, magnetic minerals, et. al.) and apparently as rebar ages it loses its magnetism.?ÿ Also rebar that was heated above its Curie temperature during manufacture post rolling is non-magnetic but that's unlikely.
I'm interested in what others think about this matter.
I've run into the same thing.?ÿ And I'm sure the alloys used in all the different kinds of rebar can make big differences in what we hear.?ÿ I've seen 24" #5 rebar that hardly sings at all.?ÿ I've seen a conc. nail that sounds like the iron bumper off a semi.
Most Schonstedts utilize two coils.?ÿ One is near the tip, the other is close to halfway up the shaft.?ÿ The audible pitch is registering the differences of the magnetic field between the two detector coils.?ÿ "Fooling" the detector is probably easier than we would like to think.?ÿ Everything made with a ferrous material has some sort of magnetic field, and they all affect magnetic locators.
There's a stretch of US412 that runs through the OK panhandle.?ÿ Every bit of rock up there has some sort of magnetic field.?ÿ It renders Schonstedts useless.?ÿ We would set rebar and could hardly find it a month later.?ÿ But we set rebar cut from the same batch elsewhere and it gave good strong signals.?ÿ To me this indicated the environment in which the rebar was placed has far more to do with the signal than I originally thought.
When working up there we got in the habit of leaving the rebar sticking up about 0.2' just so we could find them later. 😉
A magnetic locator should theoretically detect a difference in magnetic field near the rebar because of one or both of two effects.?ÿ
The bar could have a "hard iron" permanent magnetism induced during its manufacture, or by pounding on in the presence of the earth's magnetic field. Not all bars will have much permanent field, and it could happen that if the field is opposite the earth's field the pounding cancels out some of what it started with.
There is also the soft iron effect whereby the bar sucks in the earth's field, concentrating it near the iron. I didn't think rebar was magnetically hard enough to avoid that effect.?ÿ Maybe someone can comment on that.
It is possible, but not likely to happen often, that the permanent magnetism and the soft iron effect mostly cancel, leaving a weak indication.
All this is theory, and it doesn't fully explain experience.?ÿ
One of the monuments at our church sits under a very large power line.?ÿ The Schonstedt has a buzz on its tone in that area due to the AC field, and the rebar doesn't seem to give much indication. It would make sense that any permanent magnetism had been erased by years of that AC magnetic field (like in a tape recorder). But I don't understand why the soft iron effect doesn't still work.
If you somehow find a rebar that gives a poor indication, it could be a good idea to rub a magnet on it to strengthen the permanent field. That helped the one at church, at least temporarily.?ÿ I should try it again.
@bill93?ÿ I think underground power and telco lines act as masks or background noise that make the pin harder to dectect.?ÿ Similar to listening to someone in a quite room vs. listening to the same person in a crowed cafateria.?ÿ?ÿ
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@paden-cash?ÿ I've always believed the "Pound out the magnetism" theory because is see low signal most often in rocky or hard soils.?ÿ I've also observed that pins in saturated soil give a much weaker signal than I'll get on a return trip when the ground is dry.?ÿ I have zero science to back up either observation.
Just touch the end of the rebar/rod with a magnet and check it again. It realigns the magnetic field, and it will sing!
There's a magnetic experiment I'd like to try, but the guy with the equipment isn't interested.?ÿ He build a "quarter crusher" (youtube demo and explanation, or demo here.?ÿ His has better craftsmanship than in the video) which charges up a capacitor to kilo-joules and dumps it into a small coil of wire that he puts a US quarter dollar in.?ÿ When it is fired, it sounds like a big firecracker and the wire vaporizes.?ÿ The brief magnetic field induces currents in the quarter that want to be as small in diameter as possible and the quarter comes out half-diameter and thicker.
A coil around a soda pop can will crush the can in the middle and cut it in two.
So what I would like to do is use a hula hoop size coil, laid on the ground where I suspect there is a deeply buried rebar that I can't detect, and see if his machine would magnetize that bar enough to find it with the magnetic locator.?ÿ He is pessimistic about the depth it would cover, and not interested in hauling his machine across town to try.
Excellent question.?ÿ
touch a magnet to the top. Magnetism restored.?ÿ
pounding them hard, flips their polarity. Instead of a big sprout of magnetism out the end of the rebar, it comes out the side, by the middle.?ÿ
N
I've run into the same thing.?ÿ And I'm sure the alloys used in all the different kinds of rebar can make big differences in what we hear.?ÿ
Yes but common rebar is made of unfinished?ÿtempered?ÿsteel. ?ÿUncoated, corrosion-resistant low?ÿcarbon/chromium?ÿ(microcomposite),?ÿsilicon bronze,?ÿepoxy-coated,?ÿgalvanized, or?ÿstainless steel rebars exist?ÿ for reasons of corrosion resistance and exceptional strength, like?ÿbridge decks, nuclear reactors, etc., but they are not encountered in property monuments so if you're fumbling around in on the back corners of a lot its most assuredly basic rebar.
There's another technique that sometimes works. Turning up the discrimination to a high pitch, and then listening for a dead tone. If there's a bunch of trash around, you'll find every beer tab from fifty years ago, but it does work sometimes.
@rj-schneider The same trick works around electric and telco lines.?ÿ Turn it up to 11 and you'll get a slight signal through the screaming of the locator or sometimes a dead spot like you mentioned.
