Quote
Reg
Here is some info posted by Eric Foster many years ago that might help.
"Here are the curves I have used for many years. As Robert said, the range reaches a maximum when it is equal to the radius of the coil. Coils larger or smaller than this optimum will result in less range. To show how this works, along the bottom axis you see coil diameter, which is obviously 2 x the radius. So for an 11in coil, if we go up the vertical scale to A, we have 5.5in. Also note the diagonal line and the series of ever increasing semicircles. Everything to the left of this line shows increasing detection range up to the maximum where it intersects the line, then decreasing range to the right, where the semicircles are shown dashed.
If a certain metal object is just detected at 5.5in with the 11in coil, then going larger in coil size will cause a reduction (going down the dashed side), and going smaller in coil size will have a similar effect. Initially, it wont be much, i.e. going from 11in to 8in will only make 0.5in difference but below 4in diameter, the range will drop rapidly.
Now, suppose with the 11in coil, you can detect an object at about 12.5in (B on the vertical scale. This indicates that the coil is not an optimum size for that particular object. If we carry on up the curve (direction of arrow) we can see that by using a 20in coil, we could gain another 2.5in (C). The curve peaks at 15in with a 30in coil. But the extra inch gained hardly makes such an unwieldy coil worth while.
Other factors come into play of course. The curves assume that the number of turns and the coil current is the same in all cases; which it isnt necessarily. For the same inductance value, a smaller coil has more turns, which counteracts to some degree the loss in range. Also a smaller coil will pick up less electromagnetic noise, earths field noise and ground effect, which make for a smoother threshold.
The end result is, that with a small nugget that can be detected at between 5 and 7in with the 11in coil, so that it is on the top part of the curve, an 8in coil may well give a similar range. That is not to say that smaller coils do not have other advantages. Small coils and probes are very useful in rocky areas or searching in undergrowth. They have less drag too for water hunting, and less pickup from mineralised soil or conductive sea water plus better signal separation on close or multiple objects.
One other point regarding PI, is that the small object sensitivity is largely determined by the sample pulse delay. If an object is so small, or thin, or made of high grade stainless steel, such that all the signal has decayed before sampling takes place, it would not matter how small a coil you made, it would never be picked up.
Big time problem for the depth junkies If the Aquastar with a 10in coil can pick up a ring at 20in, what is the optimum size coil for maximum range, and what range could theoretically be achieved? Thats the depth at which a mere pulltab could trigger suicidal inclinations
Eric.
************************************************************Hello Reg.
Browsing the PI forum I noticed this thread on coil size and depth.
Without going into detail, I would like to offer my thoughts on the highlighted section.
It is an erroneous suggestion that max depth is:- ...... 'when the object is at a range equal to the coil's radius'.
That is simply wrong...for many reasons, so don't use it as a 'standard' in of your knowledge base.
The chart is meaningless without reference to the originator's list of parameters upon which he bases his assertions.
They're not provided, so rendering the curves pretty much meaningless.
YOU cannot relate a coil's flux field patterns effectiveness without you state its driving parameters.
i.e. DC or frequency driving source, including pulse drive. The peak current. Number of turns. Details of wire used. Coil shape (Circle, square, elliptical...or whatever!)
Any such derived flux diagrams must then be defined with reference to the 'medium' in which they're portrayed....air....or alternatively, other medias.
To expand on this subject, requires a larger volume of space generally afforded by a forum's posting facilities.
That means answers must be relatively brief, and by that restriction can lack important detail.
Without reasonable detail, their implied messages may be open to mis-interpretation.
I offer the above comments without reference or offence to those who have contributed to the subject matter.....Matt.
Edited 1 time(s). Last edit at 01/18/2012 06:47AM by TheMarshall.
