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Fisher F75 Coil Comparison


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Ran out of likes 😞

I always questioned how accurate the pvc tube at an angle really is because it leaves an air pocket around the target.

Direct comparision is find an low EMI area (nearly impossible) and do a straight up air test. For all metal modes I go to where the target fades by audio but not necessarily by TID as 2 of my machines all metal modes will fade and only my Gold Racer will produce a precise TID at the point where the audio just fades out (without boost mode). So for me it is just a raw detection. Discrimination modes tend to respond with a TID on the other 2 machines better.

For ground test if you have a pi or access to one then dig all targets. I think for testing purposes it would be important not to clean the soil any more than that other than just remove targets. Then make your test garden. US Nickel, Silver Dime, Copper Penny and Silver Quarter if you like. More important would be the nickel, (low conductor), copper penny (mid conductor) and silver dime the small high conductor.

Purpose of your test garden is to help you in your conditions so cleansing the earth won't give you an accurate sample of your conditions.

Not to go too far off topic, I had taken a plastic container, flipped it upside down, put a silver dime below and some rust on top. I was farting around with different ways to hear the dime below and realized that 14khz, 19khz on MK, 17.6khz Tejon, and 15khz AT Pro had great difficulty in hearing the target. It would be very sputtery and often produced no TID though you could hear a spark of something. At 5kzh mk I would get a 99 on display but no wrap or overload. Basically the dime had up averaged, what many have described in their high iron land.

So I was thinking if that happens then maybe the response of the conductors are not linear in relation to conductivity and frequency of the machine and maybe they work in a sine wave? So if the response is in a sine then the response of a dime + iron at 5khz, where would that response be replicated at another frequency? Would it be in the hz or ghz? This also got me wondering how do the manufacturers map all those signal combinations to the VID and if they miss a mapping does the machine become blind to that combo?

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1 hour ago, EL NINO77 said:

Could you repeat this test with Fisher F75 on another program?

 

3 minutes ago, kac said:

Direct comparision is find an low EMI area (nearly impossible) and do a straight up air test.

Well, OK, guys.  I'll put these on my list.  Good news is that it's cold enough this week (highs around 35F = 2C) that I don't really want to go detecting.  Bad news is that it's cold enough this week that I don't really want to do a lot of testing either!  😁

5 minutes ago, kac said:

For ground test if you have a pi or access to one then dig all targets. I think for testing purposes it would be important not to clean the soil any more than that other than just remove targets.

I actually used my White's TDI when I built the setup 2 (or is it 3?) years ago, but either something got added later or I just missed this target.  My intent is not to clean the soil of its natural ferromagnetic content but simply to find that pesky piece of iron (and any others).

 

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I agree with Mike H's opinion that this test has the major flaw that you're increasing air-gap to mimic a deeper coin. Problem number one is that it's not realistic, as you don't normally sweep your coil 6 inches off the ground. Problem number two is you're not being consistent - you've got one coil 6 inches above the ground, one close to it, so you've made multiple differences between tests, making a comparison almost impossible. And problem number 3 is the real flaw: adding air-depth plus coin depth together, and assuming this is equivalent to a deeper-buried coin, when it's not. When you hold a coil 6 inches above the ground, not only are you reducing the target signal, you're also reducing the ground signal. Your target signal may be reduced 20-fold, but your ground signal is reduced 5-fold as well. So you are proportionally-speaking making it easier for your detector.
The ground signal a detector picks up is significant, and finding a deep coin can involve finding a target that's one-fiftieth the strength of the ground it's buried in. Seperating these two signals is what differentiates a 'deep' machine from a poor one.

Tests that involve the same piece of undisturbed ground, and targets that are inserted through PVC tubes etc to varying depths, are fairly realistic. Obviously a properly-buried target is best, but that then means multiple coins at multiple depths in a test-garden.

The subject of search-coil size vs performance on the F75 does interest me, as I have an F75. I saw no shortage of forum chat about how some larger coils worked well on the Tek T2, but those for the F75 always seemed disappointing. Eventually, I bought a Mars Tiger at a sensible price secondhand off eBay, so could evaluate one for myself. I was deeply unimpressed. No noticeable depth improvement over stock, and no tolerance to a small air-gap, either. So on any rough grass areas it was worse than stock. It very soon went back on eBay. ( it seemed well made and solid, with good strain-relief on the coil connector, and a little rubber boot to protect it )

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On 12/20/2020 at 1:38 AM, Cal_Cobra said:

I wasn't a big fan of the Detech Ultimate on my F75 LTD2.

Same here. The Ultimate 13 is no better than the stock coil. I felt it was a waste of money.

 

 

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Yes I agree with PimentoUK , the air gap must be small( 1 or 2cms max )when testing a detector depth , . This means that if you test different targets , they must be buried at different depths so that they are at almost their max detection depth , with almost no air gap between the coil and the ground surface.

I use 2 test targets when I test a new detector/coil , a small 2g bronze coin and a bigger 10g copper coin. I use 2 plastic boxes filled with moderated mineralized ground with no iron , one for each target . The 2g coin is buried at 16cm and the 10g coin is buried at 28cm . It took me a little time to adjust the soil thicknesses to find the optimum target depth but now this works quite well and it is realistic for VLF detectors with medium/big sized coils.

 

Picture below /  the first test box ( 10g copper coin ) + the 2 targets in my hand :

test_box.PNG

 

Testing the garrett apex on the 10g coin test :

box_test1.PNG

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Sometimes larger coils get smothered so the gain is minimal. In the case of Nel Big vs stock on the AT Pro it will be about 50% deeper at max but often in the field depending on emi and ground conditions the gain may be as little as an inch or so. Other factors is maybe the F75 edition GB is using is already close to being maxed out in performance so it just might be at the cusp of the extents of the machines ability with the stock.

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My test garden has a row of four silver dimes and a row of four silver quarters. The depths in each row get progressively deeper in one inch increments. The two deepest coins are buried at a depth just beyond the detection of most detectors so that the coil must scrub the ground to get a signal if you can get a signal at all. This makes it easy to see what settings get the best detection depth.

 

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16 hours ago, Badger-NH said:

My test garden has a row of four silver dimes and a row of four silver quarters. The depths in each row get progressively deeper in one inch increments.

Now all you need is an F75 and you can add data to this thread.  😁

Regarding the discussion of air gap, I just remembered I have some (four, I think) ziplock bags with soil in them.  Each one is about 1" thick, so short of having more targets buried at increasing depths as you have, I can utilize those to get a more realistic ground effect.

I'm not ready to throw in the towel on the Detech Ultimate.  Simon would never forgive me if I did.  But today is another one with good enough detecting weather that I probably won't get any testing done until later.

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2 hours ago, GB_Amateur said:

Now all you need is an F75 and you can add data to this thread.  😁

Regarding the discussion of air gap, I just remembered I have some (four, I think) ziplock bags with soil in them.  Each one is about 1" thick, so short of having more targets buried at increasing depths as you have, I can utilize those to get a more realistic ground effect.

I own both the F-75 LTD and T2 SE. I haven't tested them in the garden for a couple of years. As soon as we get some warmer weather I will try to do that. Maybe Feb/Mar.

I don't know if the ziplocs would be that realistic. I think having the target metal in contact with the soil and having no interruptions within the soil matrix surrounding the target make a big difference. You can't get any more realistic ground conditions than that.

 

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