A Note On Saltwater Detector Performance

[Steve Herschbach]

The original 1985 Fisher Impulse metal detector

A note on saltwater. On dry land and in freshwater pulse induction (PI) detectors can be run to max limits. Not so in saltwater. There is an inherent limiting factor in saltwater that tends to flatten top end performance on all PI detectors, or even VLF detectors for that matter. The problem is simple - saltwater is conductive and so is visible to electromagnetic devices like metal detectors.

Pulse induction detectors saw their earliest use as beach detectors because of the pulse delay control. Lower pulse delays expose items with a shorter time constant, and this usually means low conductive and/or small items. As the pulse delay increases a PI loses overall sensitivity. Early beach PI detectors all came with a preset, relatively high pulse delay that made saltwater invisible to the detector. 

In general somewhere around a pulse delay of 10 uS saltwater becomes visible to a PI detector. The number varies for several reasons, First, salinity varies greatly around the world, everything from salt free fresh water, all the way to the Great Salt Lake, which is water supersaturated with salt. Oceans and bays vary, and especially bays that have large river inflows, and therefore lower salinity levels. Large, shallow, enclosed bays with no rivers may act as evaporation dishes, and have abnormally high salinity levels. Further, this effect is accentuated the deeper you take a detector, and so detectors used for true SCUBA diving must be run at higher delay levels at depth. You can't just pick a number like 10 uS and say that is the magic number for dealing with saltwater. 9 uS may work well, or it may take 12 uS to eliminate feedback from the saltwater in your location.

The coil detects like a globe in all directions, and so it is not just the water under the coil, but all around the coil that is affecting it. This large ball of saltwater is like a giant target. Many hunters riding the edge of sensitivity can tell you a detector can pick up waves as they pass over, making the water deeper.

This long winded explanation is to make people realize that you can't just magically make the detector itself more powerful and get "more depth" or "more sensitivity." The salt range overlaps the tiny gold range, and so if you make a detector able to detect fine gold chains and tiny gold ear rings, it will detect the saltwater. If your set the pulse delay to eliminate the salt signal, you lose the tiny gold items. This is an inherent wall on both PI and VLF small gold performance in saltwater. We have had detectors for decades that can detect tiny gold that people say they want to detect in saltwater, the Fisher Gold Bug 2 for instance. The problem is the Gold Bug 2 will not work in a saltwater environment. The water is just a huge signal to a Gold Bug 2. I have gone round and round with people for the last twenty years trying to explain why you can't detect certain fine gold chains, small ear rings, small platinum, etc. in saltwater. The problem is not the detectors - it is the saltwater.

The same problem exists to some degree on trying to detect larger items. You can make a very powerful detector, but you have to inhibit the detection of saltwater, and this tends to put a ceiling on the maximum attainable performance in saltwater. No matter the machine you use, once you hit the saltwater you can only advance the pulse delay and sensitivity controls to a certain point before the detector starts to protest. The exact settings where this occurs will vary by location. This all assumes "no mineral" sand. Add magnetic soil content to the beach or bottom being hunted, and you have yet another limiting factor to contend with.

Add this all up and do not expect to run the Impulse AQ Limited at a pulse delay of 7 Us and maximum sensitivity in a typical saltwater environment. You will likely have to lower one or the other or both settings to get stable performance, and this requirement tends to be a limiting factor on all PI performance in saltwater. It is this knowledge that keeps me from ever expecting miracles to occur when I try new detectors in saltwater environment. The problem is not the detectors - it is the saltwater.

The original 1985 Fisher Impulse metal detector

[Tony]

Great read Steve......all PI and potential PI users must understand this concept.

Also, moving saltwater in shallow depths can be much more of a challenge than calm water in deeper depths. Eric Foster has mentioned somewhere in an old post that generally the deeper the saltwater then the bigger the signal will be generated and that is why diving PI detectors such as the Aquapulse AQ1B have a 40uS starting delay but after about 20 to 30 feet of salt water the salt signal levels off...that is to say, it is no worse deeper than that.

[Skull diver]

50 minutes ago, phrunt said:

It is interesting and different parts of the world have different salinity levels too, cooler water tends to be less salty.  The polar regions are unable to be salt mapped as they can't see the salt through the ice so they're just white.  So detectors should in theory work slightly better in different parts of the world. 

The numbers to the right are the salinity, 32 grams per kilogram being the lowest, and 38 the highest.  The deep ocean has a lot less salt than the surface, warmer climates have more partly due to evaporation.

Thanks for the post Steve, I now have a much better understanding of the issue with salt and detectors.

Thanks to this map I confirmed my doubts!

Regardless of a reduced gain, of a turned off GB and really slow coil movements, there's no way for me to keep my Tdi usable without 17uS delay minimum.

I'm in the more higher salt concentration area!

In this way, (underwater), forget tiny thin stuff, forget depth and forget to be effective, spending time in over digging activity.

I'm also convinced that a smaller coil will change almost nothing to this machine reactions, unless a lower magnetic field impact.

This is where I hope the AQ can make a difference. If I only will be able with it to keep 60 to 70% sensitivity and 11.5uS delay should be fine enough.

[Joe D.]

Thanks Steve, 

That has to be one of the clearest layman explanations I've seen on PI's!  

   So what do you think is going to be the "Special Sauce" for the AQ to be deeper than your previous PI experiences?? If any?? Other than what you've stated previously!👍👍

[Compass]

I read this article a couple of times and it really helped my understanding of the effects of salt water on detector performance- thanks! It also made me think about all the targets that I've missed over the years when the Dual Field with its 15us pulse delay was my primary beach detector. 😞

[Steve Herschbach]

1 hour ago, Joe D. said:

Thanks Steve, 

That has to be one of the clearest layman explanations I've seen on PI's!  

   So what do you think is going to be the "Special Sauce" for the AQ to be deeper than your previous PI experiences?? If any?? Other than what you've stated previously!👍👍

“Deeper“ is a one dimensional way of looking at metal detecting. My goal is to be more efficient and to locate more “good” targets per hour of metal detecting. The Fisher Impulse AQ offers great ergonomics, superb audio, an advanced discrimination system (for a PI), allowing focus on ring type targets, and as much power as the hunting environment will allow. It’s not just “more depth”, though the AQ has as much as is possible, but the total package that matters to me. When it comes to the total package, including depth, the Impulse AQ delivers. 👍🏼

[Joe D.]

Sorry Steve,

I ment "depth" in a more generic term than i may have stated! I get that the AQ is a more "focused  detector" than other PI's before it! I understand that there is more to detecting than "raw depth" onto itself!  Hence the intent to split into several specific models! You've done the "dig everything" routine, and are acutely aware of where you would like to focus!

I, myself have never used a PI, and am considering weather my beach detecting would warrant the expense to purchase the AQ, or another more generic "dig it all" PI!  I still love finding coins, as well as jewelry, and other metal targets; especially on the Treasure Coast! The AQ is shaping up to be an awesome addition to a stable of PI's, but I'll probably only be able to make that "one" purchase into that realm, for a long time forward! So i am just a student, for the time being!👍👍

 

[Skull diver]

2 hours ago, Compass said:

I read this article a couple of times and it really helped my understanding of the effects of salt water on detector performance- thanks! It also made me think about all the targets that I've missed over the years when the Dual Field with its 15us pulse delay was my primary beach detector. 😞

Actually, if You're able to keep 15uS and to hear a stable noisefree treshold with the 12'' dual field, You're doing pretty good.

I mean, I (air) tested with higher delay rings of various shape and weight, whithout big signal difference till 25uS!..

In saltwater however You'll find Yourself to do a detuning, to use in the right way the machine unless You want to fight with boring noise.

So the trade is between stability and sensitivity / depth on small stuff...I've seen gold chains captured underwater with Your machine, but the clasp was the reason!

There's a point few people think about...Depth means also blind machines to environment's created noise and this thing almost doesn't exist unluckily...

I'm a vlf user prevalently, and when I need depth, I go back on the same place with the PI to double check for less relevant things in terms of urgency...

When a time window is open and I need to be fast, there's no way my Tdi can be the first weapon of choice...I ended up to quit with the idea to use the GB turned on, to obtain sound discrimination..The only way to use it efficiently is like a straight PI with GB off, otherwise loosing half of depth I can do the same with a CTX and leaving (not so much) deep stuff in a lot less time too. 

[ALEXANDRE TARTAR]

Basically I wanted to call it AQUAMANTA and not IMPULSE, but I had no choice.
There is as much commonalities between a potato and a ford raptor V8, as between a 1985 IMPULSE and an AQUAMANTA ... 

I wanted us to forget all notion of PI, because metal masking is very powerful on AQ.
It is so advanced that in the years to come masking will seem to be confused with the discrimination of a multi-frequency detector....

[Compass]

Thanks Goldskulldiver,  I used to think that the Dual Field had a somewhat stable, noise-free threshold. It is pretty stable but compared with the threshold of the Beachhunter it is a bit noisy which probably prevented me from hearing some weaker signals.