Hi Allan,
Boy, you have asked the key questions about PI's. Unfortunately, the answers could fill a book so I will try to just touch on a few things that I have observed. As for the details, if we could just get Eric to write the book, I know I would buy it in a hearbeat since most of my knowledge has come from him.
Ground balance is required when certain ground conditions create signals that sound much like one large continuous target deep target beneath the coil. The closer the coil gets to the ground, the louder the response. Under such conditions, one has to keep the coil very level above the ground to reduce the audio variations caused by the ground if no ground balance is present.
Now, the objective of ground balance is to eliminate the signals caused by the ground and to do this, about the only way I know of, is to perform some form of subtraction process such that the signal being subtracted is equal to the offending ground signal. If this is done, then the ground response is minimized.
I say minimized because the ground response is not linear, so any form of simple subtraction will not be equal over the range of signal changes that occur as the coil is lowered. The shorter the delay, the more likely will be the degree of error. Now, to compound matters, certain targets will have responses such that they are very similar to the ground signals, so they will be eliminated also.
If the pulse is lengthened, then the ground signal characteristics change. Also, if the primary sample is taken a little later, a subtract process will require a different subtraction ratio which will then cause different objects to be ignored. Finally, if the target signals from the two different pulse lengths are combined, a full range of targets can be detected.
Discrimination is another can of worms even if just iron objects are to be rejected. The reason lies in the fact that iron objects span a wide range of target responses. Large or thick iron object decay signals look nothing like signals from something like a piece of an old tin can.
Simple delay techniques may minimize the digging of only certain iron items (i.e. large or thick iron) while other pieces of ferrous metal (pieces of tin cans) will respond more like a good target. Now, flip things around and we will see a large non-ferrous target such as a very large gold nugget may respond much like a iron object by having a long decay time. So, the design of a good discriminating PI is much more complex than most people realize.
As for the cost factor, this is in Eric's field. I can say that the only way advancements will be made is through research. This takes time, a lot of time. To be done right, almost requires full time research and since people have to make a living, then some means have to be made to pay for such research. Normally, the recovery of such costs is incorporated into the cost of a detector. Finally, one should also realize that it is not uncommon in the industry for it to take 1 to 2 years to develop a new detector and this is true if the person is working on it full time.
With this in mind, we should really admire Eric. Designing detectors is a full time job, building detectors is a full time job, and answering questions on this forum as well as emails, almost becomes a full time job. So, he is doing the job of 3 people. I know, I couldn't do it.
It is because of the latter that I have a tendency to jump in and try to answer some of the questions. Like I said before, most of what I know about PI's has come from the guidance of Eric, so, I try to provide answers and reduce Eric's load somewhat. I just hope he doesn't mind and I also hope he will jump in and correct any errors I may state. To error is human and I have found that I can be extremely human at times.
Reg
