I've noticed in the last few months that our metal detectors are missing more and more irons. We've used three schonstedts, one of them brand new and a CST. Different jobs, different operators and sometimes even turned up all the way there is NO indication of iron. I used to have one or two times a year where the detector wouldn't sing over an iron. That was usually when the rod was driven in hard ground. Now, there seems to be no explanation. Anyone else?
Me. "What's the difference?"
T.C. Carroll "It's the difference between right and wrong!"
>sometimes even turned up all the way
Not sure what you mean by this. My old Schonstedt has two controls. One is volume and has nothing to do with sensitivity.
The other picks the "neutral" frequency for the tone, and isn't exactly a "sensitivity" control. If you turn it all the way to the low frequency end, then it's going to take more change in magnetic field to give you an audible rise in tone than if you set it to some moderate tone frequency and listen for a change.
In my experience, you can turn the sensitivity knob all the way up so its screaming and then there will be a short pause in the noise when it passes over the monument. Sort of the opposite way of using it the right way. I use this method around chain link fences.
I had that happen to me once, too. Went out one day, and looked for a corner that I was sure was in. Another day it found it quite easily. It was very cold the first time if that means anything.
> In my experience, you can turn the sensitivity knob all the way up so its screaming and then there will be a short pause in the noise when it passes over the monument. Sort of the opposite way of using it the right way. I use this method around chain link fences.
I think Brother Carroll knows well and good how to use the thing.
But, I think what you are getting at needs to also include perfect pitch hearing. On a job one day with my last boss he sends me to locate a front pin. I'm usually pretty good with the pin finder so that wasn't a problem. The problem came when I "thought" I found it but it was right next to a concrete pad with a 4x4x2' electric box sitting atop and the thing was screaming all over the place. He comes to investigate how I've done after he's done with his part and I tell what I think I may have located but with big-a$$ electric box right there, who knows. I couldn't tell. If anything I was claiming the pin must be under the pad or it's gone. He looks at how I had the pin find (Schoenstadt pistol style) settings and says that's ok and takes over. He focuses in on the same spot I did asking "don't you hear that?". I couldn't hear a difference to save my soul and he sends me to the truck after the shovel. Dam if he didn't find it and couldn't believe I couldn't hear the tells even after we recovered it. (two other surveyors before didn't find it either)
He did the same thing to me on another job with the pin buried next to a chain-link fence post with concrete all over it and metal junk everywhere.
He must have an incredible sense of tonal acuity. Apparently I don't have that kind of resolution. Tinnitus dam sure doesn't help in my case. Strike the right-most key on a piano and that's what I hear constantly without some other background sound.
My experience is when that happens, replace the batteries. My guys have a tendency to think "if they make noise, the batteries are ok". That is not necessarily true.
Another thing along those lines: If they were indeed iron, the pin-finder would find them easily. Rebar is more of a mild steel than iron. Over the last thirty years I'm convinced the manufacturing of both rebar and nails has altered the content. We use 40d nails a lot for temporary points. Finding one of those with a pin-finder use to be a snap. Nowadays you have to be right on top of it to get a signal.
my $0.02
I seem to get better results if I'm sweeping while walking towards the north. Anyone else notice that?
I've posted this before. Somebody has pounded the polarity to the middle of the pin, so it barely comes out the end.
If you touch a magnet to the end, it will sing loud and clear.
This situation is from hard pounding. I have found 3 monuments by the same surveyor, within a few tenths of each other, due to this condition. I went and measured from the witness trees, to locate the MISSING corner. I dug, and found all 3. They were in hard ground. Barely gave any signal. Your old shonstedt is fine. It's a problem with the monument.
Nate
I've been using the same model schonstedt for thirty years. And one of them we are using is brand new. We tried new batteries with no results. II've had the problem with rebar being pounded really hard. These monuments were in fairly soft ground. We've tried everything we can think of.
Me. "What's the difference?"
T.C. Carroll "It's the difference between right and wrong!"
I seem to remember that the ability of the metal detector to "detect" a target has more to do with the ability of the target to conduct electricity than it does it's existing magnetic field. I wonder if the soil surrounding the target has more to do with it than anything else. I know for a fact that the ability of a detector to energize the target improves when the soil surrounding it, is moist.
I think a detector actually works initially how Jerry described it, with the second magnetic field of the target canceling the signal that the receiving coil is receiving constantly from the transmitting coil? The audio is reversed so the lack of signal is perceived as a signal?
Hmmpppf! Tripped over the dang monumnent the other day, Schondstadt didn't even chirp!!
-JD-
I think Nate is onto it. I saw a demo once from I think a Schonstedt rep where they would put a rebar in a jig with a fixed distance to a metal locator, turn it around and one end was always stronger than the other, I think they were advocating at least checking this and marking the hot end and maybe using a big magnet to make one end hot, you then mark the hot ends and put them up when you drive them.
Also, the hot end will change over time, when it is in the transition phase there is virtually no signal, maybe the irons you are looking for have all been in the ground long enough that they don't have a hot end any longer?
SHG
Who hasn't tripped over a monument at some point in their career.
>ability of the metal detector to "detect" a target has more to do with the ability of the target to conduct electricity than it does it's existing magnetic field
There are two main classes of metal detector, with variations within each class. The coin finder type of metal detector has a response approximately proportional to the conductivity of the metal times the surface area presented to the search coil.
The other kind is the magnetic locator, which knows nothing of conductivity, and wouldn't find a silver dollar as well as it finds a nail. It responds to the difference in magnetic fields between the ends of the detector shaft. A soft iron pulls in the earth's magnetic field and the detector indicates the stronger concentration near the iron than at the other end of the detector.
You will notice some change in tone when swinging a magnetic locator, which depends on the orientation of the two sensors in its shaft, relative to the vertical and horizontal orientation of the earth's field.
Permanent magnetism, that may or may not be found in a piece of steel, can also change the magnetic field in the area in a variety of ways depending on its strength and orientation. It is possible for the permanent magnetism to have a strength and orientation to approximately cancel the earth's field in a specific place, but unlikely to do so, and would probably still show some sort of indication when either closer or more distant than those magic locations.
Stick locators locate the magnetic flux produced when a ferrous item is subject to the earth's magnetic field. Coil locators produce a magnetic field, and when a metallic item is introduced into and breaking that magnetic field, an alarm is sound.
How can I be a Metal Locator Jedi Master too?
I wish I was able to use a metal locator like that. When I get next to a fence or junction box, I find the metal locator to be just about useless and just start digging (carefully around the junction boxes though).
How can I be a Metal Locator Jedi Master too?
No kidding. I thought I was doing dam good just to get close.
I was mighty close when I thought that one pin was under the pad.
When we/he recovered it, it was just under the edge of it.
Even after being fully exposed I still couldn't tell the difference with all the racket the thing was making with that box sitting right there.
That's ok. I gave him a good lesson shooting pool later that day. 😉
Shelby, that sounds like a strong possibility. The rods were set 25-30 years ago. I don't know if it is the time period or the area though. The ones I've had problems with have been in a geographic area that I'm working in now that I only worked in occassionally until recently. My crew says it must be Chinese steel that doesn't have magnetism. I'm not sure, but it is bothersome.
Me. "What's the difference?"
T.C. Carroll "It's the difference between right and wrong!"
I have noticed a slight difference depending on which way I was facing. My old dip needle had to be pointed north.... wish it still worked.
Me. "What's the difference?"
T.C. Carroll "It's the difference between right and wrong!"
> Rebar is more of a mild steel than iron.
Steel is basically iron and carbon. The carbon content runs from a very small fraction of a percent to about 1.5 percent. Over 2 percent or so and you've got cast iron, not steel.
More exotic alloying elements like chromium, vanadium, nickel, molybdenum can also be used, but again are present in only small amounts. So calling mild steel, which is well over 90% iron, "not iron" doesn't really make sense.
Those small percentages of alloying elements can have a big effect on magnetic properties, though. Most stainless, which is mostly iron, carbon and nickel, is non-magnetic. Since rebar is often made from scrap steel, that may come into play with regard to its magnetic signature.