PI / Magnetometers

[John Kent]

The last tiime I touched metal detectors was aabout 25 years ago, so I am trying to catch up !. Back in those days we also used to make magnetometers, which basically were 1000 turns of wire around a milk bottle full of distilled water. In use one electronically 'flashed' the coil across a 12 volt battery and this caused a strong magnetic field in the coil that realigned all the spinning protons in the water in line with the magnetic flux of the coil.

One then electronically disconnected the coil from the battery and used it as a proton detector, via a high gain amp.

All the spinning protons then 'wobble' back inline with earths magnetic field. If we measure this 'wobble' frequency it might typically be about 2000 Hz and directly proportional to the strength of earths magnetic flux at that particular spot. The presence of metal produces a positive or negative anomally(I think). I m wondering the following ....

1. Using the usual do-nut search coil under water, the hole would be full of salt water. Would this work instead of the distilled water ?

2. Using a PIC one could measure the precession frequency and have a continuous readout of this frequency, or simply look for a change in frequency during each sweep. Is there any mileage in this and has anyone tried it ?. It ought to work with non magnetic land mines as well .... since paramagnetic refill will show up as a negative anomally.

Any comments please ?. I think I am trying to work out how a discriminating PI could work.

 

[rgecy]

Simple answer: No

Why: The principle of the magnetometer you mentioned above is correct, but just having the salt water moving in and around the hole in the coil would not do. For one, salt water is not a good medium for detecting the proton shift. The second would be that the fluid also needs to be contained in a confined area so it does not flow out of the energizing and detection area of the coil.

Now, with that being said, what about the possibility of incorporating a fluxgate magnetometer into a coil? With their small size and constant reading and digital output, this seems more probable. Although I am not an electrical engineer by any means, it seems a triaxial fluxgate could be used in this situation. How? I will have to leave that up to the experts!

 

[bakergeol]

Hi

Actually PI/magnetometers are nothing new such as the SSP-3000. There are also others out there.

As an owner of a PI/mag (GS5/Ferrous Hound) I am well aware of the issues with combining the two. The operating issues with a combo(when run together) are interference to the PI and interference to the mag. However these and other issues can be overcome.

However, the real issue is the effectiveness of a PI/mag combo and not if a system can be built. The main problem with a mag is simply being able to detect all iron regardless of size and shape to the same depth as a PI. Large iron OK but how about small, thin and flat iron? Mag depths are highly dependent on shape. On my mag a 1943 steel penny will only be detected at 2 inches whereas a ball bearing of the same size would probably be detected at 8" to 10". So you can see the difficulty toward flat iron.

I can see a usage for a PI/mag combo for searching for large objects, however I never regarded it as the sole solution for a discriminating PI. At best using it with a PI that already has an iron ID capability( e.g.GS5)to check for iron/steel that the PI has problems IDing.

George

 

[Smiechu]

Hi

I know that this topic is really old, but I'd like to know more about magnetometers and where I can buy them.

 

[UK Brian]

Information on the Reeves Jones (ferrous hound) can be found here

www.reevejones.com.au

(Except its not working at this moment). Price delivered to the U.S. is under $500

Brian