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Discrimination of pulltabs by magnetic susceptibility
Posted by: boldforgold
Date: October 25, 2018 04:09PM
I was wondering if anyone out there has any experience with magnetic susceptibility detectors. I was reading an article about different types of detectors and found that these detectors are widely used in the food, pharma, and geological industries. I also found that magnetic susceptibility (MS) is defined as the ability of a material to be magnetized when an external magnetic field is applied. A material like aluminum is paramagnetic, so it is slightly attracted to a strong magnet and has a + positive MS. Gold is diamagnetic, so it is slightly repelled by a strong magnet and has a - negative MS. (This phenomenon is demonstrated on youtube, notably at youtube.com/watch?v=0-ex-CAjQhY). Also, I found two manufacturers of geological instruments who make MS detectors, and these look a lot like conventional detectors that measure conductivity. These manufacturers are Bartington Instruments in England and Terraplus in Canada. In their brochure,Bartington says that their MS2D search loop "can locate buried rusty cans and other items of metallic trash easily, so it is well to check that high values are from natural minerals. Coins, rings, jewelry, metal buttons, etc., can all affect measurements". Handheld units are also available, I believe, from Terraplus. My question is, could a conventional metal detector and a MS detector be used together to discriminate pulltabs? The conventional detector would locate a target in the pulltab range, and then the target would be scanned by a MS detector. If the reading is positive, it might be a paramagnetic pulltab. If the reading is, let's say, negative or neutral it might be a diamagnetic gold ring or some other metal, but probably worth digging up. Could anyone tell me if this is possibly a valid idea, or is there some physical or practical reason why it would NOT work? Thanks for reading.

Re: Discrimination of pulltabs by magnetic susceptibility
Posted by: Carl-NC
Date: October 26, 2018 02:12PM
Para and diamagnetic effects in most materials are so small you would never be able to detect them. These effects are measured using incredibly sensitive scales and looking at the weight change with and without an applied field.

Re: Discrimination of pulltabs by magnetic susceptibility
Posted by: boldforgold
Date: October 27, 2018 09:28AM
New subject: Clarification of definition of magnetic susceptibility. I was under the impression that magnetic susceptibility(MS) was a measure of both para and dia magnetism. In any case, the method you described with the springs and scales sounds like a mechanical (old school) method of determining para and dia magnetism. Surely there must be an electronic (millennial) way of measuring para and dia magnetism in metals, and I thought that these MS meters I mentioned earlier were the way to do it. Guess I'll do some more research. Thanks for replying to my post.

Re: Discrimination of pulltabs by magnetic susceptibility
Posted by: Carl-NC
Date: October 27, 2018 11:09PM
Yes, para is positive, dia is negative susceptibility. Problem is, gold is about +0.0000034 and aluminum is about -0.0000022. Ordinary iron oxides in the soil are around 1,000,000 times larger.

Re: Discrimination of pulltabs by magnetic susceptibility
Posted by: boldforgold
Date: October 28, 2018 11:57AM
I knew there had to be a catch. Would still like to get my hands on one of those MS meters designed for geological core samples, because they specifically warn against taking measurements close to a metal object ( do not put the core samples in an aluminum tray while testing, etc.). So I reasoned that if it can detect aluminum that close to a core sample then it can produce some sort of a TID# for aluminum in the ground. And if it can do that then I also reasoned that this TID# might be different from a gold ring or some other metal. I further reasoned that this method might work even better in the nickel range since nickel is ferromagnetic. So much for my reasoning process. There must be a flaw in there somewhere. Guess I'll get one of those MS meters to test since the company brochure does say it will detect metal in the ground. Then I'll find out for myself why engineers do not use magnetic susceptibility as a complement to conventional metal detectors that measure conductivity. (Too much time on my hands). Thanks again for your reply.

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