Negative Conductivity?

[Anonymous]

I have encountered a response, seen with both a CZ5 and a XL500 (PI) detector on the beaches that confuses me. From time to time I come across a spot that reduces the threshold of both detectors. Of course this shows up as signal in motion mode on the CZ5 but when pinpointing the threshold goes DOWN. Can anybody explain?

 

[Anonymous]

Hi Charles,
I think that what you are seeing is a patch of less positive conductivity, rather than negative. Wet beaches do give a signal due to the salt water content and this signal is normally fairly constant over quite large areas. The threshhold can be adjusted, either manually or automatically, depending on the detector, to cancel out this background signal. If the beach conductivity should lessen for any reason then the threshold will go negative. Things that can cause a localised area of less conductivity are:- a buried rock. Fresh water from a spring, stream or river, percolating through the sand. Water from an outfall pipe. A non porous rock is virtually non conducting and will interrupt the currents set up in the beach. Fresh water is also a poor conductor and will dilute the conductivity of the sea water.
The XL500 has a manual threshold control so one would expect that this would need adjustment from time to time to compensate for beach conductivity changes. Detectors like the Surfmaster PI, Barracuda or Deepstar have self adjusting threshold so that the electronics compensates automatically for normal changes. The other point about the XL500 is that the search coil is unshielded so the capacitance between the coil and the beach will cause a negative signal. This is especially so if the beach is touched with the coil or you brush against wet seaweed. Once the coil is immersed in the water you are OK because the capacitance then stays constant.
Eric.

 

[Anonymous]

Thanks very much. The spots that I see that have this problem are small being about coke can sized ( except less positive rather than more positive) The CZ5 reacts to these targets in much the same way, with a decrease in threshold when pinpointing ( a non-motion mode).

 

[Anonymous]

Hi Charles,
Have you dug any of these "negative" spots to investigate? It would be worth doing to see if there is any visible cause and whether it ties up with my explanation.
Eric.

 

[Anonymous]

I have a CZ 20 which is the same electronics, but water proof. If you take a fine gold chain and air test it with the same autotune threshold. The tone will go silent and come back. I have found two ankle chains (gold) by digging this signal. When the weather is bad I go into the wifes ring box and the rings that I have found and air test them for the different tones. This how I noticed this neg. tone. Another thing someone told me and maybe Eric can help is a chain is only seen as an individual link and each link cancels the other out and if it has an odd number of links it can be detected?

 

[Anonymous]

Yes, I have dug sone and find only a black /brown layer of sand that looks like rust. This is generally found at the 1ft level so I stopped digging them.

 

[Anonymous]

Hi Kurt,
I have found that with some induction balance detectors, there is a point where a thin low conductivity item gives the same response as piece of ferrous metal. i.e. negative. This effect is predictable from the phase response but the point at which it occurs depends on the frequency of operation. This should not happen with a PI except for the reasons given in my earlier post.
A chain is seen as individual links, unless it is tightly balled up, however there is no cancellation effect from alternate links. The signal from individual links add up but only in amplitude. The decay rate of the signal is the same for 100 links as for one, so if an individual link is totally undetectable at the pulse delay used then it doensn't matter how many you have, they will still be undetectable.
I once made a large boat towed coil which used several pounds of lead shot mixed in with the epoxy to weight it down. Same principle.
Eric.