Minelab detectors: Same settings, different results, one possible answer

[Mayo South Elgin]

I contacted one of MinelabUSA's tech people regarding the question of why would two E-tracs with the same settings produce different results regarding target identification.
For all those interested, here is the answer I received...

"On the subject of two identical detectors on the same targets, I do not have an answer for that. If the coils were the same and the users were at the same height to the ground and same speed of swing then there is a possibility that one of the detectors needs to be checked out to make sure it is operating properly."

"I have run into this more than once when more than one person is testing a machine. When I test detectors side by side I make sure to time swings and heights by using a template to the ground and a count for total swing time. Just the difference in two users is generally enough to make results null."

Following are my thoughts on it (not the tech's response)
So if anyone is able to test this in the field, one person should do the detecting on both machines, making sure to keep the height to ground the same, the swing speed the same, all the settings the same including ground balance channel etc and see if that changes anything.

Another thing I have noticed is the field surrounding the coil is much more sensitive than one would expect and I believe that what a person wears, what's in their pockets - change, keys, knife, metal eyelets on shoes, etc. - may have an over all effect on detection and target ID. I plan on testing this concept with as little metal on me as possible, and then my typical mode of detecting with a pocket full of change and car/house keys, shoes with 10 metal eyelets in each.

 

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[kimsdad]

A while ago, Extractor sent me a link to a good article written on an MD'ing site about the effects of carried metal on MD signals. Darned if I can find it now... The author pretty much stripped himself of all unneccessary metal after the testing he did with his machine.

You might be on to something, Mayo. Darned if the idea to compare our two machines didn't come to me when we were out last time.

 

[Montauk3]

I think along these lines is when I sent my Excalibur in for repair. Before being sent in, I could pretty much tell the difference between a bottle cap and something else.
I was getting pretty good at distinguishing objects from one another.

When it was returned, It is no longer possible for me to guess as to the object.

Thats another reason that I dig everything.

 

[bugs]

This is just a thought, but are the raps of the coil cord around the shaft exactly the same on both machines? If one of the machines has more raps then the other resistance is created. Does that matter?

 

[Montauk3]

Good question.

 

[Lowbatts]

It might be a safe assumption that the units are bench-tested and calibrated in the factory w/o the coils on. Very simple reason for this is having a battery of tests performed w/o direct connection would result in a very noisy environment, making all tests essentially innacurate.

At any given moment, there may be 2, 5 or even 10 units simultaneously being tested/calibrated depending on the production capacity and build dynamics in the process for a given mfr.

Lets stay simple and say two benches are involved. Bench 1 and bench 2 are hooked direct connect to the unit through a serial buss and the coil connector. Tests run, parametrics measured and evaluated and units passed on for final closure and packaging, where the coils are added. Coils go through a similar process. Maybe again two benches for simplicity' sake.

Now take into account that the rx pin on the cable connector on unit bench one is several-thousandths of an inch off from the one on bench two. This can cause the phasing values to be slightly higher on that unit in either the tx side or the rx side depending on how the system manipulates test results.

Now add a weak or slightly different pin combination on the coil benches as well and you have how many possibilities for differently phased sets of coils/control units available at the packaging end of the process?
Bench 1 control- Bench 1 Coil
Bench 1 control- Bench 2 Coil
Bench 2 control- Bench 2 Coil
Bench 2 control- Bench 1 Coil

Yes, one of these four combos may run hotter than the others, especially if most of the phase values are set according to values measured through the serial buss instead of the connector. Now do that math again with four or six benches.

While for the most part you'll have equal machines, it's a small thing that may make dramatic differences in the field.

Not sure about wrapping the coil cable around the pole as a big enough factor, the shielding in the cable should be sufficient to overcome those stray capactive or inductive values that might be affected by that issue.

 

[bugs]

OK I see the machines values fall in parameters, (close enough, good enough) to have a machine with a specific dead on value would cost more then we would want to pay.

 

[RW]

Were they on the same channel? Have read of people getting different results with same machine on different channel. The results led one to believe that certain channels favor different frequencies or cause machine to GB a little different. No proof of course...

 

[Digger]

If you've ever owned multiple detectors of the same type you know each has its own character.

I've owned 4 DFX's and each acted a little different, but one was totally different. I actually sent in in to be fixed not once, but twice because I was sure something was wrong. It acted so much different. Got it back the last time and had to realize it was just going to be different.

I owned 2 E-Tracs and on the first one FE was virtually worthless below 4". I'd see people talking about the FE 12 and I'd say that just doesn't work unless the target is on top the ground. I got a second E-Trac and its FE was far more accurate and usable.

 

[p2c]

So,
Getting back to the original point, coil height, swing speed, etc... perhaps Etrac007 should go into business coaching metal detecting form. I'd like to think that's his advantage and not that we all have lousy machines in comparison to his.
I do believe that every machine will have its individual variance due to the fact that every component has a tolerance range considered acceptable and those components go together to make the system. My Etrac is spot on with Nickels. Most the time I know when its a nickel. The higher tones are good, but I don't think quite as spot on. Several silver dimes I've gotten recently I'd swear would have been quarters with a consistent 12-47, in fact I've recently had several 12-47 memorial cents.
Not sure about the FE on my Etrac. I refuse to dig anything with a consistent FE over 27, but will dig a jumpy FE. Maybe some of those 32-47's are goodies deep, but I just call 27 my line.