How Does Frequency Affect Iron Masking?

[vferrari]

It will take a few readings to absorb all of that, but that is exactly the sort of answer I was looking for. Thanks for taking the time to explain all that.

PS: Mind if I ask how you know all this? Do you or have you worked in the industry? You sound like an engineer that has worked on detector design.

I am a trained engineer but have not worked as a metal detector designer and mainly do management stuff nowadays. I use my engineering background to research how metal detectors fundamentally work and my engineering background helps me when reading publically available articles regarding the fundamental theory and design of detectors. I don't think it is absolutely necessary to understand these complex underlying physical principles and design methods to enjoy and be successful at metal detecting, but it does help me understand whys behind certain metal detector settings and some of their quirky behavior and to a certain extent it satisfies my geeky intellectual curiosity. I actually had to do some research and reasoning to answer your question because I had never thought about your question before you asked it. I leaned heavily on a Minelab theory and design paper that is linked here: https://www.minelab.com/__files/f/11043/KBA_METAL_DETECTOR_BASICS_&_THEORY.pdf which is the closest thing I could find that sort of has the answer in it, but even I don't fully understand what the paper is saying as it appears to have some contradictions, arcane terminology, and missing pieces that make it difficult for me to put the whole picture together on how inductance, reactance, and resistance and phase shifts occur and are detected. Because of this, some details and terminology in my explanation may be suspect or subject to criticism by those who may have a better level of detector design knowledge than I, but I think the bottom line answer is correct and the basic information and theory I have presented is sound, to the best of my knowledge. HTH

Anyway, enough with this geeky stuff, lets go play in the dirt!

 

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[Iron Buzz]

I am a trained engineer but have not worked as a metal detector designer and mainly do management stuff nowadays. I use my engineering background to research how metal detectors fundamentally work and my engineering background helps me when reading publically available articles regarding the fundamental theory and design of detectors. I don't think it is absolutely necessary to understand these complex underlying physical principles and design methods to enjoy and be successful at metal detecting, but it does help me understand whys behind certain metal detector settings and some of their quirky behavior and to a certain extent it satisfies my geeky intellectual curiosity. I actually had to do some research and reasoning to answer your question because I had never thought about your question before you asked it. I leaned heavily on a Minelab theory and design paper that is linked here: https://www.minelab.com/__files/f/11043/KBA_METAL_DETECTOR_BASICS_&_THEORY.pdf which is the closest thing I could find that sort of has the answer in it, but even I don't fully understand what the paper is saying as it appears to have some contradictions, arcane terminology, and missing pieces that make it difficult for me to put the whole picture together on how inductance, reactance, and resistance and phase shifts occur and are detected. Because of this, some details and terminology in my explanation may be suspect or subject to criticism by those who may have a better level of detector design knowledge than I, but I think the bottom line answer is correct and the basic information and theory I have presented is sound, to the best of my knowledge. HTH

Anyway, enough with this geeky stuff, lets go play in the dirt!

I think you've fielded enough of my questions by now to know that I also like to know why things work the way they do, and not just take common knowledge as fact. That is why I appreciate your answers and explanations so much. So... thanks again.

 

[shanegalang]

OK, I think I better understand what you are driving at. Be careful what you ask for...

Alright, let's get ready to explode some heads...

A common misconception regarding how metal detectors work is that they rely solely measuring the specific conductivity of the target to determine the nature of the target. In other words, many folks think that the inherent high specific conductivity of metals like silver versus other less conductive metals like aluminum is the primary differentiator that a metal detector uses to determine target ID.

In fact, metal detectors look at two electromagnetic properties - inductance and conductance (or more accurately, the inverse of conductance - resistance). The inductance of a target is influenced primarily by its inherent magnetic properties while the resistance/conductance is influenced primarily by the mass/shape of the target (that is why a large aluminum can can often give a visual and tone target ID similar to a silver quarter, even though aluminum is a lot less conductive than silver. Both of these properties result in a phase change or shift in the detected magnetic field waveform that is emitted by the target and detected by the receive coil versus the transmitted field waveform that is sent into the ground by the transmit coil. The characteristics of the phase change are what detector signal processing systems use to infer the target ID of the target but more sophisticated detectors are also looking for uniformity/symmetry in the magnetic field signal as the coil passes over the target and tend to factor-in the symmetric field given off by round targets along with their phase shift because they are likely coins or round jewelry. The magnitude of the inductive component of the phase shift varies with transmission frequency for non-ferromagnetic materials such as silver, gold, aluminum, lead, brass, copper and so on. In general, the magnitude of the inductive component signal of mid-conductive targets (e.g., brass, lead, aluminum, gold alloys) tend to peak at higher transmission frequencies and the the magnitude of the inductive component signal of high-conductors (e.g., copper, silver) tend to peak at lower frequencies.

On the other hand, ferromagnetic targets such as iron nails and horseshoes tend to have a large, constant inductance signal component that is both largely independent of frequency but also results in a large phase shift that is opposite that non-ferromagnetic materials. This component term dominates the phase shift signal even though iron has much higher resistance than most non-ferromagnetic metals. Discrimination circuits/algorithms use this unique response of ferromagnetic metals to identify and filter out ferrous targets. The upshot is that frequency has a much greater effect on the signal response of non-ferrous targets than ferrous targets although it is possible to induce "false" non-ferrous responses from ferrous targets at high frequencies, especially if they have a high mass and/or are round in shape.

The total picture of target response with frequency is actually even more complex than I have described above, and there are exceptions to the simplifications I used above that even I do not fully understand.

Bottom line, set your frequency to optimize the target of interest rather than worrying about the small effect that frequency has on nearby ferrous target signal strength. Hope that helps.

Having studied Industrial Electronics I actually have a better understanding of the workings of a metal detector now. Thanks. Obviously you have some type of electronics engineering degree?

 

[smokeythecat]

V made my head explode again. I frankly don't understand the frequency stuff and now I don't especially care. Here's why I don't care: First, for me, it's hopeless to try to understand it and second, my Deus has picked up small musket balls at 11", it picked up a small 1.5" knee buckle at 11" with an iron "flat iron" 3 inches away from it, the Confederate buckles were shallow so a pie pan on a stick could have found them. I have detected stuff 18" with it, the last being a silver quarter. Iron separation is excellent at all these spots. I guess I'll keep doing what I'm doing.

 

[vferrari]

I didn't say you (especially you, Smokey) need to know this stuff to detect effectively, I just answer questions people ask (and enjoy my superpower of being able to make people's heads explode with mere words. ).

 

[cudamark]

View attachment 1706551

....

I can actually answer two of those questions!

 

[cudamark]

OK, I think I better understand what you are driving at. Be careful what you ask for...

Alright, let's get ready to explode some heads...

A common misconception regarding how metal detectors work is that they rely solely measuring the specific conductivity of the target to determine the nature of the target. In other words, many folks think that the inherent high specific conductivity of metals like silver versus other less conductive metals like aluminum is the primary differentiator that a metal detector uses to determine target ID.

In fact, metal detectors look at two electromagnetic properties - inductance and conductance (or more accurately, the inverse of conductance - resistance). The inductance of a target is influenced primarily by its inherent magnetic properties while the resistance/conductance is influenced primarily by the mass/shape of the target (that is why a large aluminum can can often give a visual and tone target ID similar to a silver quarter, even though aluminum is a lot less conductive than silver. Both of these properties result in a phase change or shift in the detected magnetic field waveform that is emitted by the target and detected by the receive coil versus the transmitted field waveform that is sent into the ground by the transmit coil. The characteristics of the phase change are what detector signal processing systems use to infer the target ID of the target but more sophisticated detectors are also looking for uniformity/symmetry in the magnetic field signal as the coil passes over the target and tend to factor-in the symmetric field given off by round targets along with their phase shift because they are likely coins or round jewelry. The magnitude of the inductive component of the phase shift varies with transmission frequency for non-ferromagnetic materials such as silver, gold, aluminum, lead, brass, copper and so on. In general, the magnitude of the inductive component signal of mid-conductive targets (e.g., brass, lead, aluminum, gold alloys) tend to peak at higher transmission frequencies and the the magnitude of the inductive component signal of high-conductors (e.g., copper, silver) tend to peak at lower frequencies.

On the other hand, ferromagnetic targets such as iron nails and horseshoes tend to have a large, constant inductance signal component that is both largely independent of frequency but also results in a large phase shift that is opposite that non-ferromagnetic materials. This component term dominates the phase shift signal even though iron has much higher resistance than most non-ferromagnetic metals. Discrimination circuits/algorithms use this unique response of ferromagnetic metals to identify and filter out ferrous targets. The upshot is that frequency has a much greater effect on the signal response of non-ferrous targets than ferrous targets although it is possible to induce "false" non-ferrous responses from ferrous targets at high frequencies, especially if they have a high mass and/or are round in shape.

The total picture of target response with frequency is actually even more complex than I have described above, and there are exceptions to the simplifications I used above that even I do not fully understand.

Bottom line, set your frequency to optimize the target of interest rather than worrying about the small effect that frequency has on nearby ferrous target signal strength. Hope that helps.

I always wondered why ferrous targets (regardless of which frequency I was using) always seemed to ring out loud and clear, and at depth, while nice shallow gold and platinum targets would bare make a squeak. Now (I think) I know. Thanks.

 

[smokeythecat]

No, I don't need to "know" the math, but you're going to have to come to my house and get my exploded head cleaned up!

 

[Iron Buzz]

V made my head explode again. I frankly don't understand the frequency stuff and now I don't especially care. Here's why I don't care: First, for me, it's hopeless to try to understand it and second, my Deus has picked up small musket balls at 11", it picked up a small 1.5" knee buckle at 11" with an iron "flat iron" 3 inches away from it, the Confederate buckles were shallow so a pie pan on a stick could have found them. I have detected stuff 18" with it, the last being a silver quarter. Iron separation is excellent at all these spots. I guess I'll keep doing what I'm doing.

I'm not saying that the answer will, or will not, change how I detect. For me, that's not the point. I am curious. I like to know how things work, and knowledge leads to more questions. Don't you ever want to just know "why"?

 

[randywa]

For some reason "pie pan on a stick" is all I remember.