field test unit no 001 "model T"

[gadgetmaker]

Eric,

Many thanks again. I will but the book and study all the other material. Have looked at earth resistivity meters and the ground is so hard here that it would make things very difficult. I am buying a much nicer scope and it will have USB capability and will take some data with a 2' by 2' coil I have. Will keep you posted and follow everyones else posts on new PI concepts.

Karl

[Ferric Toes]

Hi Karl,

If you can get hold of the book "Seeing Beneath the Soil" it will be a great help in your application. It covers all methods, magnetometers, electromagnetic including PI, resistivity and others. It was published in 1990, but the author Tony Clarke has since passed away. He pioneered the use of resistivity measurement as an archaeological tool and for a time I used to manufacture his novel resistivity meter. This may be a good instrument for you to investigate for privvy location. Keep your eye on ebay for a copy of the book. The photo in the .jpeg shows the gridding method we used (photos are pre 1970).

Eric.

Seeing Beneath the Soil003.pdf

[gadgetmaker]

Paul,

Yes they are very old; usually 120 years plus. The bottles are very beautiful as they were all hand made, have also found many coins, tokens, jewelry as anything can fall out of ones pocket. Have found several pistols; once something falls in poop no ones going after it.

The soil in the pit is usually so different from the surrounding soil. It contains ash and lime that kept the smell down; usually not to much metal, if any it would be very deep do to weight (8-10feet). I would want to look for a large change and then confirm by using the spring steel probe. The claiche soil here is a real show stopper with the probe and if the probe goes more than 3 feet you know that you are in a pit of some kind.

I have tryed magnetometers and usually work in dense downtown city areas were there an abundance of other fields....

Have a coupe of ideas for driving the coil differently of a PI unit. Use to work on very large telescopes for the Smithsonian designining building drive and mirror aluminizing systems. Hope to be a value to the forum in the future.

Al the best, Karl

[gadgetmaker]

Eric,

Thanks for the help; I will study the article and also how to construct a non-motion / non-auto zeroing unit. My goal was to take samples of privies that I have dug a refilled and the surrounding ground ; storing these plots for reference. It would be nice to have a real time unit were I could then probe to confirm the pit is there. I have programmed many high speed DSP's and would to build my own specific unit for privy detecting. Have also programmed usinf VHDL for Xilinx FPGA's. Have a lot to learn about PI units; will hopefully be able to contribute to the forum soon.

All the best, Karl

[Ferric Toes]

Look at "History of Pulse Induction" thread for paper on the detection of buried pits, etc. using PI.

Eric.

[Ferric Toes]

Hi Karl,
You also need a PI that does not have auto-zeroing, or motion mode as it is sometimes called. Ideally you would grid out the ground into 1m squares, put the coil on the ground and record the ground reading with a data logger. At the end of the survey, download the data and use a program that gives you a colour map of the highs and lows in the ground signal. Metal objects will still be found and will show up as an intense and more localised anomaly. I would use a square coaxial coplanar coil TX 1m and RX 0.75m or 0.5m. Mono coils are no good for this application.

Eric.

[moodz]

Paul,

Thanks for the reply and the explanation of the nano-tesla estimates. My main hobby is digging old privies for bottles and mainly use a spring steel rod to find them. Had a Mala GPR at one point but 250 mhz did not give the resolution needed. Have tri[ATTACH]22591[/ATTACH]ed other devices as well and always go back to the rod. Here in AZ need one with a slide hammer for the top layer. I spoke with Eric Foster years ago about one of his first PI units being built to find roman burn pits. I am taking some plots of pulse responses from dug privies and untouched privies and the surrounding undestrurbed ground. My goal is to build a PI for the explicit purposes of finding these Outhouse pits. Sorry to hijack your thread.....attached a picture of one of my finds, it is an unknown whiskey from 1883. A.T. is of course Arizona Territory. Will keep my education going by reading as much of the old posts as possible. Happy New Year, Karl

I guess if they are old ( hopefully very old ) privies thats OK. I am guessing then that you need a PI detector that does not cancel the ground .... quite the opposite in fact. Unfortunately on my current project PI the ground cancellation cant be turned off its part of the basic function though I might able to do something on the fully digital version. Best regards for the new year also. Paul.

[gadgetmaker]

Paul,

Thanks for the reply and the explanation of the nano-tesla estimates. My main hobby is digging old privies for bottles and mainly use a spring steel rod to find them. Had a Mala GPR at one point but 250 mhz did not give the resolution needed. Have tried other devices as well and always go back to the rod. Here in AZ need one with a slide hammer for the top layer. I spoke with Eric Foster years ago about one of his first PI units being built to find roman burn pits. I am taking some plots of pulse responses from dug privies and untouched privies and the surrounding undestrurbed ground. My goal is to build a PI for the explicit purposes of finding these Outhouse pits. Sorry to hijack your thread.....attached a picture of one of my finds, it is an unknown whiskey from 1883. A.T. is of course Arizona Territory. Will keep my education going by reading as much of the old posts as possible. Happy New Year, Karl

[moodz]

Moodz,

Maybe you have a useful magnetometer? What is the field strength in nT at 12 feet from that magnet? Have been following your posts and want to thank you for the interesting reading material.

Karl

Hi Karl ... the earth field being something like 30 to 80 microtesla and the 6 inch ferrite ring shaped speaker magnet I test with being around 1 Tesla ( being generous here ) ... the detector with the Earth field filter code disabled is very sensitive to the normal earth field. A very quick calculation off the top of my head says that the magnetic deviation from waving the magnet around at 12 feet is around 10 nanotesla ( using the formula for magnetic dipole at a distance d from a magnet loop of radius r ). So that would mean that the detector is sensitive to magnetic field variations of better than 10 nanotesla in change. However this is not a static measurement ... the magnetic field must be changing .. the detector response is proportional to not only the magnitude of the change but also the rate .... not sure that would be useful in a magnetometer measurement unless you have precise measurement of position and velocity of the coil.

I would be making the proton magnetometer ( see attached ) it just needs updating to modern design ... should be a winner.

protonmagnetometer.pdf

regards. Paul.

[gadgetmaker]

Moodz,

Maybe you have a useful magnetometer? What is the field strength in nT at 12 feet from that magnet? Have been following your posts and want to thank you for the interesting reading material.

Karl

[Aziz]

...
Thanks to Urban Fox for stimulating a line of investigation to retry an idea I had previously dismissed.

UFox seems to be good guy. PJ, Rafferty, Robby_H too. Who else?
These guys are stimulating good ideas.

Aziz

[moodz]

...been slaving over a hot soldering iron all day here LOL ....the first version of the detector is finished and I am conducting field tests ....so far so good .... I have one small problem that is on the verge of being solved .. the current detector auto ground balances however it is very sensitive to magnetic fields ( eg earth field and magnets of course ).. With an 18 inch mono coil the unit picks up a handheld magnet from 12 feet away which is not good unless you want to detect magnets ...however the plus side is that this means my signal processing channels are working well.

update .... earth field sensitivity is now solved without impact to the the signal processing chains or auto ground balance. The detector only shows a slight response to the ferrite of the magnet not the magnetic field itself now ... even when held right to the coil. Thanks to Urban Fox for stimulating a line of investigation to retry an idea I had previously dismissed. Field testing commences again.

[Tinkerer]

switching noise

The TEM pix above indicate severe switching noise.

Any time you hard switch 10A current, you will get considerable switching noise. The question is, how to see it and how to mitigate it.

On post 748 you see the RX signal after the preamp. Looks smooth enough, but when you look closely, you can see the switching noise of the sample switch pulses. However, there is no trace of the Mosfet switching noise.

[Mechanic]

Nope, I would not think so, 2.2R = very low impedance input path = low noise source, much less than the preamplifier.
Its a bit of a juggle trying to find a mosfet with low capacitance and on resistance. As the resistance goes down the capacitance goes up. Also the maximum current causes the cap spec to go up. The irf5801 will handle 500mA, so all round it is a very good front end fet. Higher capacitance = slower switching speeds and slower settling so it is important to find one with the lowest capacitance possible, whilst also satisfying all of the other requirements. I have been using these for at least 2 years now and never blown one with the exception of doing some silly things with them!

Cheers Mick

[dougAEGPF]

My gift to you all for Christmas...... IRF5801 I have pushed these up to I think 220v, possibly 240v before avalanche and it survived Very low capacitance and on resistance.

Cheers Mick

Is the RDSon of 2.2 Ohms of any concern?
Merry Christmas to all .
dougAEGPF