Why no mag on metal detectors?

[Midden-marauder]

Oh, and in case any of you are curious to try messing with this the apps I've been using with are: phyphox (best analytical app that I've found), physics toolbox, keuwlsoft AC magnetic meter, keuwsoft gauss meter, crowdmag, Lord Kelvin sensor logger, magnetometer 3d, and an assortment of compass apps (the steel compass app is the most useful) to help confirm calibration and avoid major heading errors while interpreting data. Avoid basically all of the so called metal detecting apps, they all appear to have been programmed by folks who don't actually really understand what a magnetic sensor does. The so called EMF reader apps are all mag apps but it's not an emf meter in the classic sense, these apps are so so at best.
I'd seriously love to see mags as standard metal detector features, it's a fairly powerful tool

 

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[Tesorodeoro]

Oh, and in case any of you are curious to try messing with this the apps I've been using with are: phyphox (best analytical app that I've found), physics toolbox, keuwlsoft AC magnetic meter, keuwsoft gauss meter, crowdmag, Lord Kelvin sensor logger, magnetometer 3d, and an assortment of compass apps (the steel compass app is the most useful) to help confirm calibration and avoid major heading errors while interpreting data. Avoid basically all of the so called metal detecting apps, they all appear to have been programmed by folks who don't actually really understand what a magnetic sensor does. The so called EMF reader apps are all mag apps but it's not an emf meter in the classic sense, these apps are so so at best.
I'd seriously love to see mags as standard metal detector features, it's a fairly powerful tool

care to upload a video of this phone on a stick finding a buried water pipe? just curious to see it in action.

 

[FinderTravis]

Thanks, it's all in good fun, I'm prone to nerding out on science topics lol.

Nerds for life! lol

 

[invent4hir]

Not understanding polar plot.
I had a Whites in the late 80's early 90s. Get a Signal, keeping the coil pressed to the ground, turn 90° if cut out anytime in the 1/4 rotation it was iron, or trash.

pepperj, the polar plot may only be a V3i feature as my DFX doesn't have it either. I can't find a good pic of it when there is a definite ferrous target under the coil. I'm going out tomorrow - so will try to remember to take one and post it here.

Similar to you, sometimes when I get what appears to be a good signal, I turn 90 degrees to see if it changes.

 

[pepperj]

Oh, and in case any of you are curious to try messing with this the apps I've been using with are: phyphox (best analytical app that I've found), physics toolbox, keuwlsoft AC magnetic meter, keuwsoft gauss meter, crowdmag, Lord Kelvin sensor logger, magnetometer 3d, and an assortment of compass apps (the steel compass app is the most useful) to help confirm calibration and avoid major heading errors while interpreting data. Avoid basically all of the so called metal detecting apps, they all appear to have been programmed by folks who don't actually really understand what a magnetic sensor does. The so called EMF reader apps are all mag apps but it's not an emf meter in the classic sense, these apps are so so at best.
I'd seriously love to see mags as standard metal detector features, it's a fairly powerful tool

What phone do you use?

 

[Midden-marauder]

care to upload a video of this phone on a stick finding a buried water pipe? just curious to see it in action.

I think I can manage it though you'll have to forgive me if excavating a foot down in the summer while the mosquitos launch their invasion isn't something you'll see me doing. I really, REALLY try to avoid digging up the utilities. Knowing my luck, I'd be chiseling into the packed dirt with a screwdriver and I'd accidentally punch through the pipe. We do have some other iron features around the yard I could dig up though, a few angle irons, a well casing, what have you. I could just demonstrate the principle using a railroad spike if you like

 

[Tesorodeoro]

I think I can manage it though you'll have to forgive me if excavating a foot down in the summer while the mosquitos launch their invasion isn't something you'll see me doing. I really, REALLY try to avoid digging up the utilities. Knowing my luck, I'd be chiseling into the packed dirt with a screwdriver and I'd accidentally punch through the pipe. We do have some other iron features around the yard I could dig up though, a few angle irons, a well casing, what have you. I could just demonstrate the principle using a railroad spike if you like

I was using a pipe as an example. Just curious to see it in action on any target.

 

[Midden-marauder]

pepperj, the polar plot may only be a V3i feature as my DFX doesn't have it either. I can't find a good pic of it when there is a definite ferrous target under the coil. I'm going out tomorrow - so will try to remember to take one and post it here.

Similar to you, sometimes when I get what appears to be a good signal, I turn 90 degrees to see if it changes.

I actually looked that one up, looks like the V3i has a pretty detailed display, I like the look of it, I'm a fan of graphs, charts and dry technical data so it looks like fun to operate

 

[Midden-marauder]

I was using a pipe as an example. Just curious to see it in action on any target.

Yeah, I can do a demo video for this so you can see what to expect with it. Could also demonstrate the whole scenario in which I am able to get reliable readings even in the presence of large structural distortions, the phone mag is 360 degree, it's honestly better to use it away from major urban structures if it can be arranged, better to use it away from busy streets, better to be magnetically clean (not wearing any ferrous materials or carrying magnetized objects) and if you really wanna get it as quiet as you can it's better to do it late at night when the world is magnetically quiet. I can get it to work just fine even with all of those issues but giving demonstration of how much that stuff matters would be useful, I think. If I'm wearing my usual utility belt and moving around it will produce additional noise in the data which doesn't make it impossible to get clean readings but it doesn't help. It may take a few days but I can post such a video

 

[Midden-marauder]

What phone do you use?

At the moment I'm using a moto g "stylus". When I first started messing with it however I was using a different phone, another moto g model but it wasn't appreciably different. My current phone is about half as noisy as my last phone based on the information the phyphox app provided which is great but I don't think it changes much in terms of finding a ferrous target. If your smartphone can support any given compass app it definitely has a magnetometer in it and basically all up to date smart phones have this feature. It's finding some surprising uses in a number of departments, not just magnetic surveying. It's a neat feature for sure, all the phone sensors are pretty cool honestly but that mag is what holds my interest more than the rest. I have been able to find shallow underground voids using the phone accelerometer and a big mallet but it's not terribly useful for geophysical exploration in most regards that I've found

 

[Midden-marauder]

It's not a video but it's a timed recording for a quick demo:
The large peaks and troughs are responses from the mag to me waving a lightly magnetized screwdriver back and forth about 3.5 feet away. You can see the noise in the data set, the noise floor averages at about a half micro Tesla. The absolute flux density reading is skewed because I had the phone sitting on a surface with a steel support frame inside of it. In the least distorted environments flux density, assuming things are correctly calibrated, is at about 46-50 micros depending on your heading, the absolute number changes based on the orientation of the sensor with respect to the earths magnetic field and the presence of any local distortions. When I use it I'm not looking at the absolute numbers, those simply form the conditions of the local background field, the actual useful readings are relative in nature.
But yeah, here's a recorded magnetogram from ol phyphox, just one example of what this looks like using a phone, the changes here are at about 1 1/2 micro Tesla at the described range. I could have done it further away and still gotten a useful reading but you get the point

 

[Midden-marauder]

I'll see about getting a recorded magnetogram of me walking up to a chain link fence from 10 or 15 yards away so you can see what that looks like as well

 

[Chilli]

Don't know if this helps.

https://www.minelab.com/__files/f/11043/METAL%20DETECTOR%20BASICS%20AND%20THEORY.pdf

 

[Midden-marauder]

To some degree I may have just answered the question but not entirely. I was running depth tests on various pieces of ferrous materials I have and I observed something I already realized but got a clearer picture of. Metal composition is a major deciding factor on the ability of the mag to detect the object as is geometry to a lesser degree. I was able to meaningfully detect a 4" screw at almost 18", I was seeing a response at a further range but not one that could easily be interpreted on the fly especially with the noisy phone mag. A quieter set up of hardware would resolve the issue permitting even greater detection depth but....
After testing the 4" I pulled out a rusty old bolt, 2", but I discovered that the sensor could only just begin to respond at about 2". I took an 1 1/2" stainless steel fender washer and found that the sensor could barely register it held right up to the phone. Now I knew that not all "ferrous" materials are not created equally, the degree of magnetism varies considerably depending on the alloy. Many stainless steels have only weak ferromagnetism, in some cases none at all. If it's a certain type of alloy the magnetometer can barely pick it up or simply not. Object orientation and geometry affected things in ways that might seem surprising. It's less a case of surface area or even size and more of a situation where the magnetometer strongly detects the poles of the object but rather less so the areas between said poles. This can actually be exploited to help tease out object orientation and geometry, if you only get a monopole response passing over the object there's a good chance it's oriented vertically in the ground or just one pole is facing upward, magnetic fields are strongest at the extreme ends of an object so if it's longer than it is wide the ends will present as positive/negative. If it's a really long object, a utility line for example, you actually end up with magnetic domains so as you walk along you get +/-/+/- and so on, it's oriented horizontally and you can definitely tell.
The 4" screw caused a pretty strong jump in the graph when I oriented the ENDS of it toward the sensor but lengthwise orientation it lost depth. That part is not going to be terribly troublesome in application because you're likely to run across one or both poles going over the object but it's a thing. Object composition is a far bigger factor in terms of detectability and depth meaning that only magnetic metals/materials can be detected. That could be problematic for identifying ALL ferrous objects, some simply have no magnetism. Still, if the mag gives a response, it's DEFINITELY ferrous (possibly hematite or such). It's ability to detect other magnetic features is still of value but by itself there are limits which is why we have metal detectors to nicely fill in gaps lol. It's definitely been a reliable go to if there's ambiguity regarding possible deep iron, it finds those pipes bloody well and it does a fabulous job of finding nuts, bolts and various pieces of building hardware at surprising depths in some cases. Likewise it'll pick up even weak magnets at astonishing depths, we have an assortment of random magnets on our fridge now that I located puttering around with the phone lol. Just depends on the metal/material composition, bottom line. More iron content=stronger response. I still stand by the idea that metal detectors should feature a hall sensor option, hall sensors are dollars cheap and can be used to find things a metal detector might actually miss but that's it's own topic. The ability to do even low resolution magnetic surveying can help to find subsurface structural features a metal detector just can't spot.
My 50 pound anvil, just for the record can be picked up from at least 5 feet away lol. I'm very much experimenting with this thing to see what it can do, doing a ton of research, still figuring it out but yeah, as subsurface sensors go it's kinda cool and it has it's own value in a number of applications.

 

[invent4hir]

I actually looked that one up, looks like the V3i has a pretty detailed display, I like the look of it, I'm a fan of graphs, charts and dry technical data so it looks like fun to operate

Yes, lots of bells and whistles. Darn near need a PhD to use it! I did get a pic of the polar plot over an iron target, but dont have time to post tonight. Will try tomorrow.

 

[Midden-marauder]

Yes, lots of bells and whistles. Darn near need a PhD to use it! I did get a pic of the polar plot over an iron target, but dont have time to post tonight. Will try tomorrow.

I liked the look of it but then I'm over here interpreting magnetograms and various other technical charts so that would be right up my alley lol

 

[Midden-marauder]

Oh, and if any of you are thinking of experimenting with a cell mag avoid bringing it too close to magnets, they can fry the mag with extended exposure, seriously, I killed the sensor on my previous phone doing exactly that.
A side note: it can reliably detect fire brick.

 

[Chilli]

Oh, and if any of you are thinking of experimenting with a cell mag avoid bringing it too close to magnets, they can fry the mag with extended exposure, seriously, I killed the sensor on my previous phone doing exactly that.
A side note: it can reliably detect fire brick.

Cool, thanks for that bit. I downloaded a mag app which Mr DuckDuck Go was all thumbs up on. No warnings, no "we want access to everything on your phone and blood type" 😆
Some the Duck said "no, no, no".
I did try it near a magnet on a hand grab thing so I wont do that again.
I probably got a lame one but it works on things ferrous. Wont be replacing my Xterra any time soon though.

Interesting subject though.
Here's my Case Stockman Pocket knife sitting this way and there's nothing happening.
Sideways to the phone held upright.

Now this direction with the phone held the same.

Certainly shows the difference between orientation and the angle of approach to an object.

 

[Midden-marauder]

Cool, thanks for that bit. I downloaded a mag app which Mr DuckDuck Go was all thumbs up on. No warnings, no "we want access to everything on your phone and blood type" 😆
Some the Duck said "no, no, no".
I did try it near a magnet on a hand grab thing so I wont do that again.
I probably got a lame one but it works on things ferrous. Wont be replacing my Xterra any time soon though.
View attachment 2165027
Interesting subject though.
Here's my Case Stockman Pocket knife sitting this way and there's nothing happening.
Sideways to the phone held upright.
View attachment 2165028
View attachment 2165029
Now this direction with the phone held the same.
View attachment 2165030
View attachment 2165031
Certainly shows the difference between orientation and the angle of approach to an object.

Oh, no replacement but literally it's own tool. The challenge is when you realize that detection ranges actually occur, in most cases, in the micro to nano range lol. Typical graphs I run we're talking shifts as faint as a half mike, they look bloody huge on the graphs often because I'll zoom in nice and tight on the little wiggly line and (zoom functions exist on some of these physics apps) so changes in the nano Tesla range start showing up right away and I can pick up magnetite in the soil which will be basically damn nigh invisible on an app that uses the absolute number which only changes visibly in the most powerful of fields as you were kind enough to demonstrate. 41 mT to 116 mT? Dude, "...a mountain walked, or stumbled..." 116 mT is about the time I jerk my phone away and be like, ok, where's the magnet lol. Literally off the charts reading. That's higher by far than the graph I presented and that graph is crystal clear so yeah, the output format matters, it seriously does in terms of reading the data and detecting an object 5"+ down or picking up the response of a brick or any of that.
Maybe I have answered the question why:
If you don't have a magnetogram output you aren't gonna be able to use it in any meaningful way but what percentage of folks will take the time to learn that kooky skill? I mean some definitely will which is why hunting for shipwrecks but....
Also, it's important to check calibration so there's that, like, critically important and who knows if your phone is calibrated at the moment? It effects a lot and if the phone has been repeatedly dropped or unwittingly placed next to powerful magnetic fields (it's very common actually)then all bets are off. I use a couple apps to check my calibration and it can make such a difference in output it's incredible. Another thing about the phone mag is that you can reduce noise levels some by putting it in airplane mode and turning off location services. Cellphones are not the ideal host for a mag, there's a lot of beep boop going on behind the scenes which increases noise levels, it does matter in terms of trying to catch a shift of less than a micro tesla. Cellphone mags require babying, they aren't as robust as they could be and over time can wear out if the phone gets knocked around.
I'm looking at this from an experimental point of view, I'd rather have the mag mounted in a designated gadget, not a phone lol. Still, the phone can do it, you just gotta know how it all works to get it to do the thing. When I say I can detect my anvil from 5 feet we're talking about shifts at sub micro ranges, not ten micros or 50 or more. I can likewise pick up the movement of an oscillating area fan from 6', again, a micro Tesla or two tops, not more than that.

 

[Midden-marauder]

Cool, thanks for that bit. I downloaded a mag app which Mr DuckDuck Go was all thumbs up on. No warnings, no "we want access to everything on your phone and blood type" 😆
Some the Duck said "no, no, no".
I did try it near a magnet on a hand grab thing so I wont do that again.
I probably got a lame one but it works on things ferrous. Wont be replacing my Xterra any time soon though.
View attachment 2165027
Interesting subject though.
Here's my Case Stockman Pocket knife sitting this way and there's nothing happening.
Sideways to the phone held upright.
View attachment 2165028
View attachment 2165029
Now this direction with the phone held the same.
View attachment 2165030
View attachment 2165031
Certainly shows the difference between orientation and the angle of approach to an object.

I mean you could do a static survey using absolute numbers which would be a slow, pain in the patooty method. Basically you'd lay out a physical grid and then take a single reading in each cell of the grid, maybe plot the numbers on graph paper or something, ugh. Also, using that method you can end up running into major problems with "diurnal variation" which in the end would make trying to work up such a grid basically impossible or you'd be having to use two phones and then MANUALLY correct the data😂, oh dear.