pi response

[Anonymous]

i am working on a design similar to carl morland's pi project. so far i can get a response at 10" on a nickel using a 8.5" coil, but only get about 8" on a similar size copper item. is it normal for a pi to get less depth on higher conductive items of similar size and shape? also without ground balancing ability, will it null/signal over a hole in the sand?
thank you and hh.
waiting for spring in the frozen north.
john.

 

[Anonymous]

John,
Copper item is more conductive than nickel. That means you need more energy to get same distanse from it. You must use longer TX pulse, because the eddy currents need more time to decrease before pulse ends. When you make sample, the still flowing into the copper object, but in "wrong" ( opposte ) direction and are substracted from the signal.
Every object can be conserned like inductor and resistor in series. Because the copper have a little resistance, it's time constant is bigger. The time constant is L/R. Greater time constant, more time is needed for the current to decrease ( usualy 5 to 10 time constants, but 5 is enough ).When the fly back is changing the magnetic field, the eddy currents into the target also try to change their direction.
But that is not so easy. Every inductor ( remember
L/R of the object) does not like changes. It needs a time. And, because R is small, L is dominant in this case. And, you give it a time. But time is an energy.
No way to make an universal detector. There are many trade off's between power, weith, coils etc.
Best regards
Tony