salt water interference in PI

[Anonymous]

A few years ago I had the opportunity to test the Fisher Impulse, White's Surfmaster PI, and CZ-6 on the beach. I discovered that both PI machines ignored the salt water if the water was still, but if you held the coil in or near the water while it was running in fast or going out fast, the machines went screwy. The sound on the Impulse seemed to be proportional to the water velocity. With the Surfmaster it just bounced all over because the Surfmaster has autotune. In my judgement the Impulse was the more usable of the two in the moving water, but that isn't saying much.
Inasmuch as proximity to salt water in and of itself seemed to have no effect, this was not a problem with defective shielding.
The CZ-6 just ignored the salt even with water rushing in over the searchcoil.
Since two very different pulse machines exhibited a strong response to moving water, whereas the CZ didn't care, I drew the conclusion that this odd behavior is probably characteristic of pulse machines.
So-- three questions.
1. Do all pulse machines do this?
2. If some do and some don't, what's the difference in the machines?
3. Does anyone know the underlying physical mechanism?
Looking forward to learning a thing or two,
Dave J.

 

[Anonymous]

You were not seeing salt water causing a problem, it is the air and salt water. When you lift out a PI detector from the water it ground balances so to speak into the air and at that instance it makes a noise. When you try to detect right where the waves are washing up either you have to be under the water or over it, not mixed in and out, or not in frothy water. Hope this staightens out your problem. Steve

 

[Anonymous]

Thanks, Steve.
I was observing this with the searchcoil completely submerged, with the Impulse which is a fully static (non-motion) machine. When the water was rushing onshore, the threshold went up, as I recall (but maybe it was down?). When the motion of the water stopped, the threshold returned to normal. When the water was rushing out again, the threshold went up again, all in proportion to water velocity.
As I recall, with the loop just above the water surface but not touching the water, the same effect could be observed. However this was a long time ago and my memory may be faulty on that score.
As I recall, moving the searchcoil rapidly over the surface of still water didn't produce the same effect, but I attributed that to not being able to swing the searchcoil at anything like the velocity that water runs in and out on a moderately steep beach.
Some machines (not limited to PI) ping when you touch something wet with the searchcoil, but I was able to go in and out of the (still) water with the Impulse without appreciable change in threshold.
With the Surfmaster, its autotune system prevented me from establishing a definite proportionality, but what I observed with it with the searchcoil immersed in the water seemed to be the same phenomenon. Still water, stable threshold. Moving water, noise severe enough to make searching for anything impossible when the water was really rippin' along.
--Dave J.

 

[Anonymous]

Hi Dave,
There is still a lot to be investigated regarding sea water and its effect on PI detectors. One of the problems is the physical difficulty of doing meaningful tests and measurements in the surf zone. In the past, I was able to do considerable testing in deeper water, as there was a research grant in place that covered hire of boats, divers etc. This laid the groundwork for designing successful diver held and boat towed detectors. However, since that time, shallow water detectors have become popular which operate in a border zone between wet sand and two or three feet of water. Also, these detectors operate at higher sensitivities than deep water units and are more susceptible to water effect.
Sea water is a conductor and, although much less conductive than even poorly conducting metals, makes up for it in the vast volume that the field from the coil excites. When searching on a wet beach, the coil will generate eddy currents in the saturated sand. The fact that sand itself is generally made up of an insulating material limits the amplitude of these signals so that they are not too much of a problem. Also, the first derivative filtering in motion type PI

 

[Anonymous]

Hi Dave,
Welcome to the PI forum.
Gee I feel I should be asking you the questions, not the other way around<grin>. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
I can understand why the CZ6 didn

 

[Anonymous]

Thank you, Steve M., Mr. Bill, and Mr. Foster, for your replies to my question about salt water interference. Here are several additional pieces of information.
1. When the water was about a foot deep, and relatively still because it was at its maximum advance toward the land, I was able to plunge the Impulse searchcoil into the water and back out with little change in threshold.
2. The beach was on the central California coast and had moderate mineralization-- didn't measure it, but based on appearance as well as the behavior of the metal detectors I mentioned as well as a 1280-X discriminator, I'd guess magnetic susceptibility was in the mid-hundred micro-cgs and loss angle probably a couple tenths of a degree. Of course the mineralization was highly variable, the sand grains being sorted by wave action.
3. The Fisher Impulse, unlike any other pulse unit I know of, operates at low voltage and uses a triangular current waveform. This would seem to eliminate any possibility of detecting the electrical conductivity of the salt water (not to mention the flimsy gold chains that everyone wants to detect on the beach). Also, the pulses as well as the demodulation are bipolar and symmetrical, which should eliminate effects from low frequency modulation of earth field by wave motion, etc.
Here's a summary of what I think I learned from our exchange.
1. In the shallow wading surf, it is common
to observe salt water effects with PI's.
2. The problem is not limited to one or two
specific models, but is probably exhibited
by most or all PI's.
3. There is probably more than one mechanism
involved. It is probable that some designs
are immune to at least one of these
mechanisms.
4. Those who beep, not infrequently observe
things which have no known satisfactory
explanation. What I saw with the Impulse
on that occasion (only time I had it on
a salt water beach) seems to be in that
category.
I spent about 10 years fooling around with PI. To build one that works moderately well is deceptively easy. About 1985 I built a prototype that would discriminate out iron and aluminum trash (and cancel maghemite) while finding the high conductivity coins. It was a real sweetheart on a mineralized freshwater beach (never had it in the salt)-- didn't have to dig any trash, whereas VLF discriminators were almost useless. Whole machine weighed less than 3 pounds and ran darn near forever on a 9 volt "transistor" battery. Unfortunately it didn't have enough "air sensitivity" to be a marketable product.
What I constantly ran into with PI, were funny things happening that couldn't be observed on a 'scope, or couldn't be duplicated, or which had no satisfactory explanation when they could be seen on a 'scope and/or duplicated. On one occasion I ran into a problem that was eventually identified as a transmission line effect-- something you'd never be concerned with in a VLF type unit. I attribute the manifold mysteries of PI to the extremely wide bandwidth that PI machines have, what with diodes and/or fast switches right there in the receiver coil circuit. I suppose you could detect a cellphone call with some models.
So, after the Impulse came to market, I threw in the towel on PI and decided to stick with multifrequency VLF, which holds far fewer rude surprises. My hat goes off to those folks who have hung in there with PI and turned them into competitive machines, by finding application niches which play to the strengths of PI, and by actually solving some of the problems that crop up with PI.
Have a happy holiday season.
--Dave J.

 

[Anonymous]

Hi Dave,
Good to see you on the forums. I met you at the Mesa Gold Show earlier this year, I appreciate the time you took to chat about detector design.
I have a SandShark that I've taken to Florida a few times. This past summer I had it in some moderate surf action, I remember that I wished I had a coil angle lock. Don't recall getting any falsing from the surf, other than when the coil flopped around. The SandShark has SAT and the spiral PC coil, I doubt the spiral coil would prevent falsing but maybe the SAT does.
- Carl

 

[Anonymous]

Thanks, Carl. I wonder if the lack of salt interference in Florida with the Sand Shark had something to do with the lack of iron minerals, the difference in design, or some other unknown variable?
Is there anybody reading this who uses the Sand Shark on mineralized beaches who can comment from experience on the effect (if any) of the combo of iron minerals and salt?
Is there anybody reading this who uses the Impulse on mineralized or nonmineralized beaches who would care to make a post on this subject?
--Dave J.