P.I. recieve coil question

[Anonymous]

Reg, Eric and all
I am trying to absorb all the P.I info as quickly as possible, but I feel like a bear seeing a feast of salmon swim by. This may have been discussed before and very simple I'm sure but... since the recieve coil is made of a conductive metal(copper) is there not an eddy current in the rx coil after the tx turns off. Is this looked on as noise? Is the thin gage in the rx coil wiring keeping it from being a problem?
The reason for the question is a post that said the tx coil is strong enough(cross talk problems) but the rx coil could to be designed for better reception.

 

[Anonymous]

Hi Wyndham,
Eddy currents are generated in the receive coil in two ways. One is the very fact that it is a loop of wire in itself, and the other is that the wire has a conductive cross section which will support eddy currents. Dealing with the second point first, the eddy current path in the cross section is interrupted by either using stranded wire or Litz wire. Eddy currents in the overall loop must have a decay time which is very much faster than the shortest object decay time that you want to measure. This is generally achieved by the damping resistor which gets rid of these "spurious" eddy currents as quickly as possible. Use too low a value of damping resistor and you will certainly see the coil loop signal. Since the TX and RX coils are coupled as in a transformer, what happens in one will be induced in the other. For example, if you used copper plumbing pipe for the transmitter winding and Litz wire for the receiver, you would still see the eddy currents induced in the copper pipe.
Some PI detectors use solid wire instead of stranded, but whatever you do, any eddy currents generated in either coil must have a fast decay compared to the fastest target decay you want to detect.
Eric.

 

[Anonymous]

Thanks Eric, Are there other materials that could be used but not wire, say carbon fiber or silicon carbide, in the rx coil to enhance the signal.
In pottery, silicon carbide shelves are only used in non electric kilns because of the conductivity of the material. in another case, rods of this material are use by applying electricity to them and the resistance causes them to produce heat(glow bars).
I have the kilns and material to make hi fired ceramic material that might not be available to others, that might be interesting as a rx material.
I'm not trying to get way out there, just trying to get up to speed and eliminate some dead ends.
Thanks again for the time and patience of all. wyndham