Hi Frank,
One should be able to do some basic testing to determine the best settings that will allow the detection of coins and ignore most nails. Unfortunately, there are some flaws in the setup as noted on the video.
If you watch the video I made, you will see it is easy to ignore most nails while detecting coins. Again, the key is in the setup. No, you will not ignore all nails, but you should be able to ignore most.
The video mentioned earlier indicates you should set the delay at a setting of 17 usec or later and turn the ground balance (GB) back to ignore nails. What will be indicated if this is done is the nail rejection isn't as good as it is at a lower delay. At least, on my TDI it isn't.
Now, if you watch this video,
http://www.youtube.com/watch?v=7ibDh4k-DuY
you will see a poorly made video because it was a last of the minute idea, no planning, and I didn't have anyone else to help me, all of which would make a big difference. What the video does display is the fact one can ignore nails and other ferrous junk such as bottle caps.
In the video you will see it is possible to ignore nails and still detect copper pennies and silver dimes with decent depth capabilities. The key settings are the delay and the GB setting. The other controls such as the gain or sensitivity, the Frequency are adjusted for maximum depth. You also need to select HI conductor so you can ignore nails as displayed in the video.
In the video, I set the delay at 10, the GB at 5 and at this setting, most nails are ignored but silver and copper coins are detected with a reliable and repeatable response. Reduce the GB to as low as 2 and the rejection of nails is increased but the depth capability is reduced. Also, at the setting of 2, you will probably ignore Indian Head pennies.
On my TDI, if I set the delay at 17 or more and I try to ignore nails I will lose Indian Heads before I lose nails consistently. I don't do that at a delay of 10.
Keep in mind the TDI isn't perfect and you will dig some nails, but that usually happens with any detector. Also, a PI isn't really designed as a discriminator. It just turns out that when selected to the single tone mode with the right settings, most junk can be ignored and still detect the deeper stuff.
Next it becomes important to listen to the signals themselves. Deeper coins will be a nice smooth signal that normally repeats from all directions. This could be influenced if a large piece of trash is nearby, much like what happens with a VLF coin machine.
In my video, you will note, I sweep much slower than what is done in the other video. Why? Well, a PI requires a lot of filtering to try to reduce the excessive noise that is so common on a PI. The filtering is designed such to try to reduce the noise while not affecting the main signal that much. In doing so, the filtering sets the ideal sweep speed. Sweep too slow and depth is lost. Fortunately, most people do not do that, but if one tries hard enough it can happen. Normally, this sweep speed would be called a granny gear because it would be very slow.
Also, there is an upper limit on sweep speed where depth is lost. This may not be as noticeable when working with hunting coins, but it is there. Bury a coin at about maximum depth of detection and try different speed and this becomes more important. This upper limit becomes even more important when nugget hunting.
Ok, the final thing to remember is use the single tone mode when trying to ignore nails. This technique works best for hunting silver and copper coins and makes ignoring bottle caps, pull tabs, most nails and tin foil quite easy to do. In the case of silver and copper coin hunting, use the high conductor mode.
Keep a couple things in mind. First, use a nice smooth sweep keeping the coil level above the ground. The reason for this is because the GB will be off a little so by keeping the coil level, there will be little interference from any ground signal. Next, remember that in most cases a buried coin will give a nice clear smooth repeatable signal from all directions. If it occurs from only one direction, then move around 90 degrees and try again.
If the signal stutters, double blips, has a sharp edge to the signal or is not repeatable, it is most likely ferrous trash of some type. With practice, you will ultimately know what I mean.
Finally, if you are too close to a nail, you will get a signal regardless. This holds true for other large ferrous junk. If you suspect this is the case, simply raise the coil and test the target again. A coin will have signal that fades smoothly, while trash can fade or better stated, disappear abruptly.
So, there are a lot of little things to learn to get the best out of the TDI. Sweep speed is one, GB settings how they are affected by the delay is another, coil height above a target and how the target signal can alter things if too loud, requiring the coil be raised, and finally, to use the single tone mode for the best results.
One last bit of wisdom and that is, too much of a reduction in the GB can result in ignoring Indian Head pennies also. So, it does take practice to find the best combination for the location.
Reg
I think that's half the fun