I didn't need another project -

[Ferric Toes]

In wet salt sand I believe the decay is reasonably exponential. I assume the tau depends on salinity but probably on the order of a couple of microseconds. In full submersion it may behave differently, I've never investigated. I would also assume that moving the coil away from wet salt sand will require more distance than a coin to cut the signal in half because the salt "target" is so much larger.

Yes, a Figure-8 will cancel salt effect.

In geophysical prospecting a salt wet sandy beach would be regarded as a 'conductive half space', the background decay of which obeys a t^-5/2 law. Tests done back in 1968, by lowering a coil into the sea from a boat, indicated the same decay law.

Eric.

[Carl-NC]

In geophysical prospecting a salt wet sandy beach would be regarded as a 'conductive half space', the background decay of which obeys a t^-5/2 law. Tests done back in 1968, by lowering a coil into the sea from a boat, indicated the same decay law.

Eric.

Good to know, thanks.

[green]

Sometimes a picture helps. Charted saltwater slope -5/2, ground slope -1 and a nickel TC=10. The decay slope for the nickel is the same when delay equals nickel TC times power slope. Nickel decays faster at longer delay time.

[Ferric Toes]

Sometimes a picture helps. Charted saltwater slope -5/2, ground slope -1 and a nickel TC=10. The decay slope for the nickel is the same when delay equals nickel TC times power slope. Nickel decays faster at longer delay time.

Thanks Green. Yes, a picture does make things clearer. I have a sea water graph plotted at the time and compared to that of an underwater gas pipeline, which is not very relevant to hobby use. My problem would be finding the plot amongst all the papers going back the the 60's and 70's.

Eric.

[Pipeliner]

Hello Ferric Toes, I registered to this site and your post came up first with the search. I'm recently retired and hoping to spend time detecting. I have the same story as yours with the xl500. Picked up off Ebay about 2 weeks, same price as you out of Jacksonville Fl. Had the identical experience as you. Factory batteries have been replaced with 8 sub C installed at batteries plus. 9volts at the batteries. Coil attached and no activity on the gauge or headphones. Any direction/guidance/opinions would be appreciated! Hope your rebuild has gone well and you're already enjoying it!

[Ferric Toes]

Hi Pipeliner,
Yes, I am enjoying the XL500 and have done a few modifications (upgrades) to the electronics which, IMHO, greatly improves the performance. It is now a 'motion' type detector with a smooth, sharp response. I have yet to take it to the beach, but hope to in the near future.
First thing is to get yours working. On first switching on, the meter should read battery volts for a few seconds and then revert to the signal reading. The meter is scaled 0-10 and this relates to battery voltage when switching on. If there is no reading,the next thing is to see if the battery voltage is reaching the circuit board. On the component side under the meter is a large cylindrical capacitor and the battery volts should appear across this. All you need is a d.c. voltage test meter to do this. To make things easier, unscrew the four red cap retainers on the solder side and swing the circuit board sideways on the leads so as to have the component side up. This makes it easier to connect to this capacitor.

Do this first and report back.

Eric.

[Pipeliner]

Hi Eric, Battery check is good just below 9v. Only reading below scale at white capacitor, with red positive lead on metal frame and black ones wire then white terminal, got 2v reading. Thanks again for taking time with suggestions...Rob