Ask A Question--Get An Answer

You can learn just about as much in air testing if you do it right. Get yourself a pine dowel, a small piece of pine square and some BluTack, that sticky putty used to put posters on the wall.

Drill a hole near the very edge of the pine square for the dowel. Glue it in before you mark the dowel. After gluing mark measurements down to 1/4 inch or 5mm with a fine tipped marker. You should now have an "L" shaped device with measurements on a dowel and a flat square on the end. Easy. Now you have your measuring device.

Lay your detector on a wooden table or dangle the coil over the edge a fair distance from large objects. A good sized cardboard box works well for this. Mount your test object on the square bit of your test jig at the approximate center point for your coil. For non motion machines, simply move the object into range to see how far you can detect it. That will give you your detection depth plus or minus about 5% from the real world which contains other variables like soil condition and et cetera.

For motion machines, do the same but swing the dowel up and down as you bring it closer and away from the coil. The marks on the dowel will again tell you the story.

This is where you really do benefit from this test jig. Adjust your dials one by one to see the effect the settings have on detection, depth range and material type. You can learn a lot from this with no need for a test garden. To be really useful a garden would need a coin of each type planted at a range of depths. Such gardens are not really very accurate but simply give a go/no-go test.

Record your data and carry it along on your next field trip to see how it compares with your actual results in the field with your test settings. It may surprise you to find out that your air tests and your ground tests are almost always nearly identical.

Funny story here. A friend of mine was new to detecting and was using one of my spare machines which I ultimately gave to him. He kept calling me over frustrated that he'd hear a signal and drop to his knee to dig only to find nothing at all. Standing again and sweeping, he'd get a good signal and not find a thing as he dug. I ran my coil over his diggings and over the hole and found nothing. He did it without headphones and there it was - a solid signal! Again, I tried with no results at all. How odd.

After a few minutes of head scratching, I noticed that he had on hiking boots and I ran my detector near his boot. Wham! Huge signal. His steel toe was being detected by the edge of his coil when he was standing up. All he had to do for that day was lean a bit forward and not detect his own boot toe. I've also seen folks detect their own eyelets in shoes so I make it a point to be sure there is no metal at all in my shoes. Except for that 5c piece I glue into my left shoe near the toe. That happens to be my test piece that I use when I lack confidence in my machine or its batteries. I run the coil over my toe and can tell by the detection if my batteries are good or not. The Australian 5c piece is the same size as the US dime. We don't use pennies here any more.

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Every detector varies so your answer will have to come from experimentation or as the result of a direct question in the sub-boards that deal with your detector model.

Testing as I described is indeed used for determining the overall characteristics of your machine and noting how the settings make a difference. Say you use a penny and run your tests at home as I described. Record your settings in your notebook using a variety of settings to get the best result. When you find your next penny in the field, see if there is any correlation to your readings in the book. If not, try and duplicate them from the book or record them for later comparison at home. Take that penny and dig to the same depth as in your book. Bury it again and see if any of your findings compare with those recorded in your book. Getting to know your detector is the object and doing all this will tell you all you need to know.

After you have done this for all of the common things you may find, try it with some borrowed jewelry. An earring, a pendant, a large ring, small ring, bracelet, Medic Alert bracelet and so on. Change the settings as you do the air test to see where you gain the most depth for each type of item, coin, jewelry or whatever. Try it with common and not so common junk. This is a perfect rainy day activity and classifies as research.

As to where to set your machine to eliminate trash, I am the last guy you want to check with. I always use zero discrimination and dig every target no matter how it is reported by my detector. Always. You might be surprised to learn that the majority of gold rings that I have found were reported as trash. Not all of them but nearly so. It is the shape and the fact that gold is non-ferrous that causes that.

The technical reason for the confusion can be blamed on eddy currents set up by your detector. In ferrous objects, the magnetic field generated by your coil is organized in a line right through the object where some of it is absorbed. A weakening of the signal received as you pass over a ferrous object happens and that is how your detector can sense it is ferrous. You noted that you also want to find relics so it is important that you not reject iron or ferrous objects. When a magnetic field from your detector passes through gold and any other non-ferrous metals there are eddy currents set up on the surface of the object rather than through it like in iron. A ring made of silver or gold and a ring top pull tab are all shaped the same so the eddy currents that are set up look the same to your detector. They are random and can be seen using

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to be swirls. Those eddy currents make the object look larger than it is because they radiate a bit from the surface where they are formed. If you eliminate the ring pull tabs, you will probably eliminate gold and silver rings as well. It happens to be a law of physics and no amount of settings change or coil experimentation will make it go away.

There has been promise shown in spectral analysis of the signal received and Fast Fourier Transform of the signal to determine what is really there but there are limits to what can be determined by one sample. In the case of the pull tab, there is no need for multiple sampling as the mass and shape are very close to another of the same item. The rings though vary in size, density and thickness with some of them matching exactly the characteristics of the pull tab.

I realize that his seems to be highly technical so I kept it simple. It really is far more complex than my explanation here. The engineers and technicians out there may disagree with my descriptions but they ultimately should relax and realize that a discussion among engineers would look markedly different than what I presented here.

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Multiple detectors are an offshoot of being in the hobby a long, long time. I have got a few more that are not listed. I have two that are of my own design that I experiment with. One day I may release the designs but for now they are unique to me. The Groundhog was my first top of the line detector purchased back in 1973. The Minelab was purchased two years ago and the Bounty Hunter just last year for my wife. It is lighter than any other I have and she wanted it to be that way. She used it once and decided it was boring so it became part of my inventory. The Sea Hunter was used for wreck diving. It has paid for itself in so many ways. In fact, aside from the Bounty Hunter, all of my machines have paid for themselves many times over.

Rivers are fresh water so ground balancing is easy. Nothing in a rive stands still because of the roaring waters at one moment and the smooth flow at another. Something lost in a riverboat wreck could wind up miles away from the site. Hunt the fresh water swimming areas to start out and the picnic areas along side the river. Both are good areas. Dig everything for a while and see what I mean about finding more treasure that way. Don't believe anyone who tells you that you gain anything with settings until you have verified it using what I related in those other posts.

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To set ground balance on most machines you use the technique of first clearing the area of all metal. You only need a small area but if there is metal there it will be a problem. So, clear it and then lower your coil close to the ground but not touching it.

Adjust your ground balance from full off to full on and note the point at which it goes silent. Then, bring it back to just exactly where you first get a bit of sound. Raise the coil quickly and if it goes away, move it back to the "just silent" point and you are there. If it gets louder, you have to go again until it goes silent when you raise it. Confusing? You bet but the types of soil drive the reaction not human logic. Ground balance has to be set for every soil and only works with no discrimination. If you have discriminate on, it can't be set. It is an all metal function. Actually, I have read where some manufacturers are claiming theirs works in discriminate but I have no idea if this claim is true.

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Your machine may have automatic ground balance. A lot of detectors do that now.

Any book is good. Dan is a member here. You could ask him. I like the late Karl Von Meuller as an author as well. Roy Lagle and Charles Garrett teamed up for some great books. Classics all.

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