Hi "Dude",
Sorry, didn't have any other name to use.
Now, since Eric is away, I will try to answer your questions as best as I can.
First, any metal object such as gold, iron, silver, etc, can be simulated by using substituting an inductor and resistor when trying to find a mathimatical answer to what will happen. Basically, this means each object acts much like a combination of two electronic parts. Changing the inductance and/or resistanced values is much like changing the conductivity and size of a metallic object.
On a VLF coin detector, the size, shape, and composition of a coin is a constant so it is much easier to develop a means of determining a target. However, even on the VLF's, it is impossible to catagorize something as basic as gold rings simply because too many variables are changing. It isn't as much the gold composition as it is the size, shape, etc of the object.
PI's are a different beast all together so trying to determine objects even coins is much more difficult.
Now, on a PI, the decay of the spike is what is analyzed to determine if a target is present. More complex analysis can be done by taking multiple analysis (samples) of the decay signal.
What is done is to sample this decay signal and watch for changes in the sample voltage to determine if a target alters the decay signal.
In a nutshell, when the pulse current is shut off quickly, a large spike occurs that quickly decays back to zero volts. At this point a sample is taken. When an object enters the field of the coil, this decay signal is altered slightly and the sample voltage changes. This sample voltage change is amplified and ultimately becomes the signal we hear.
Just how the decay is altered is a function of the metal involved as well as the size and shape of the object. Gold will alter the decay for a much shorter period of time than a piece of iron so it becomes important to have the spike decay very quickly otherwise the signal from the object may be gone before the sample occurs.
On an Eric Foster PI, where the sample delay can be adjusted, one can increase the delay time to see if an object disappears. If it does, then the object is suspected to be made of a lower conductive material such as gold, lead, aluminum foil, etc.
When looking for iron objects, the pulse on time is generally much longer and the sample is taken much later in time.
The pulse repetition rate (PRR) simply determines the number of pulses (and samples) per second. Increasing this number simply increases the number of samples that are taken which can help somewhat in averaging the signal. This can produce a little increase in sensitivity and generally helps the signal to noise ratio.
One of the big mistakes people who are unfamiliar with PI's make is to try to associate PPR to something like a VLF frequency where the VLF frequency is a major factor in how well the detector detects small gold. On a PI, the PPR isn't the major factor. It is the sample delay that is critical in determining just what small gold objects can be detected.
Beachcomber asked about different types of gold and their conductivity. The purity of gold is in important factor but is not the only one. The size, shape, and even the physical characteristics are extremely important. On a PI, a very small piece of pure gold will decay much faster than a larger piece of similar composition.
A large gold ring or a 1/4 oz nugget will generate a signal that is easily detectable when the delay is out to 20 usec to 30 usec, while a 2 grain nugget signal may be almost gone in 10 usec or less. Pure gold will alter this delay curve but not as much as the size, shape, etc.
Iron objects, even small ones may produce a good signal when the delay is out 50 to 100 usec or more. Very large iron objects may have signals extending out in the millisecond range.
Copper, silver and other highly conductive objects will generate signals that extend out for much longer periods of time, much like small iron objects.
One can take into account other factors such as coil current, number of coil turns, etc as means of altering just how the PI will react to different objects. Increasing the coil current generally increases the return signal from an object in the field. However, increasing the coil current also increases just how fast the initial decay can occur. Increasing the coil inductance also generally increases the time it takes for the signal to decay. So, it is a balancing act to find the right combination when designing a PI.
A PI designed to find small gold will be quite different than one designed to find large iron objects.
I hope I have answered your questions.
Reg
