Don't be afraid to turn down the sensitivity!

[Mark in NC]

I was hunting (mainly experimenting) in my parents yard Sunday afternoon. Their yard has an electric cattle fence running along side it, and is notorious for driving my detectors bonkers. I thought I would see how the F-75 hunted up close and personal to that much broad band rfi. Well, I turned down the sensitivity to 30 and experienced wonderfully stable operation. I thought to myself "Well, there's no way I'll get any depth with her cranked down this low"... As usual, I was incorrect. I hit a nice high tone (4h mode) that sounded deep. Heads up depth indicator verified it. So, pinpoint indicated 8 to 9 inches. I cut my plug, and began digging. At a MEASURED 8 inches comes...a memorial penny 1961 to be exact.

The moral of the story is: You still get outstanding depth with the F-75, even when it's cooled down! I could have run it even hotter, but I wanted to see what it could do when set this low. This thing continues to surprise me!

hh'n

Mark in NC

 

[MIG73]

n/t

 

[Bill Ladd]

I've said this before, & don't know about anybody else, but I could never get my Fisher CZ's over 5 or 6...never near 8 or 9. But, many want to run the F75 on 90 and expect it to be silent. Nasa Tom said once that CZ's got no deeper over 4.5 on the dial, so I always kept 'em on 4 or 5 for stability & great depth. The same is quite similar with the F75. This is why I tell most beginners, or those who complain it's "noisy", to simply turn down the sensitivity to 40-50 & still go plenty deep especially for coinshooting. Climb a little as you get more hours under your belt. I often hunt around 75 or so. In JE 70 seems "hot".....
Now, I know when folks first get a new detector they want to crank it up to "see how deep it will go". But often that can be counterproductive, at least until you know it's sight & sounds alot better. Yes, I have posted a few times I ran my F75 sensitivity in the 80's & even 90 once. But, that was a low mineralization site & I was enduring "chatter" to try to get to some very very deep musketballs.....
A C$ is another Fisher that is very similar. "Noisy" with sensitivity on 9, & I always preached sensitivity 5 (525) on that one too.
HH,
Bill

 

[Mike Hillis]

Not only does the Sensitivity setting determine how sensitive the unit is to electrical interference and small signal sources in the ground, but it also controls the shape of the coil's detection footprint at both the surface and at depth.

The F-75 makes you listen to the site conditions and forces you to adjust it properly. That is a good thing.

Mark, were you using JE or DE mode? JE runs real well at low sens settings. I believe if site conditions require you to lower the sens, that switching to JE mode is a must.

They thought of everything

HH

Mike

 

[Mark in NC]

I'll try the same site with the je mode and see what happens at low sensitivity... should be interesting how it handles the electric fence..
..and I agree..I do think they thought of everything!

 

[Streak!]

you wrote: but it also controls the shape of the coil's detection footprint at both the surface and at depth.
Thats a common misconception. the "footprint", or size/shape of the field is totally unaffected by the sensitivity settings, on ANY machine. The "sens" adjustment controls the gain on the recieve circut side, NOT the size/ strength of the transmit field. On most machines out there, the transmit power (front end power) is fixed. You can only modulate the recieve strength/gain. the only thing that will affect the size/shape of the field, is introducing a conductor into the field. Try this: hold your detector rod just above the coil with a watch on your wrist, and wave a good target under the coil. Cant get hardly any depth can ya?? the watch distorts the coil so much, its loses a ton of depth. thats one of the major reasons iron masking if such a big factor with detectors. it simply distorts the field so much it greatly affects the performance of the machine. AND, the effect multiplys with mass. Put a little iron washer next to a coin, and the detector will still get the coin. Put the coin next the a bigger chunk of iron (like a cannon ball frag), and you cant get the coin from any direction, even if its 2 or three inches away from the frag on the same plane! (on top of the ground).

 

[MattR]

[quote Streak!]you wrote: but it also controls the shape of the coil's detection footprint at both the surface and at depth.
Thats a common misconception. the "footprint", or size/shape of the field is totally unaffected by the sensitivity settings, on ANY machine. The "sens" adjustment controls the gain on the receive circuit side, NOT the size/ strength of the transmit field. On most machines out there, the transmit power (front end power) is fixed. You can only modulate the receive strength/gain.................

the only thing that will affect the size/shape of the field, is introducing a conductor into the field.[/quote]

[size=medium]You are absolutely correct Streak......[/size]

All the funny shaped illustrations produced in magazines etc., are nonsense unless accompanied by the specific target to which the 'response- curve' relates.

Such diagrams do not represent the radiated TX flux-field......MattR.UK.

 

[Mike Hillis]

Air tests are only good for air. You need to get out of the air and into the ground.

The coil detection footprint in the ground is directly determined by three things. One is the ground mineralization, and second is the sensitivity setting and the third influence is the target itself.

The transmit field is only a potential, not the footprint, and only really comes into play with target size. The receiving field creates the true coil footprint. The first affect on this is the ground minerals. Mineralization condenses this field shape, and in some cases will even warp it. The Sensitivity setting will change the shape of the field within the crunch that the ground minerals place on it. This is coil dependant, but once understood, you can directly know, and control, what type of ground coverage at any depth point you are getting in the ground.

Target size plays a role in depth, but the receive field footprint stays basically the same, just appears larger. So in-ground testing with a dime will give a particular shape. In-ground testing with a quarter will show the same shape but on a larger scale.

This is something that is easily seen in a test bed or a test box and will probably be quite shocking to you if you have relied on air tests to tell you what to expect in the ground.

You can test in the air and be blind in the ground. Or you can test in the ground and start to see.

HH

Mike

 

[Streak!]

I was simply explaining the effect mass has on masking whe I made reference to the targets being beside each other on the ground (the big iron VS the small iron on the same plane). that has nothing to do with airtesting, and frankly, I dont even know where you came up with that(airtesting), as it wasnt even mentioned in my post. BUT, I stand by my origonal premise, which is, sensitivity settings dont affect the fields at all. I wasnt taking about depth either. the depth or size of a field, VS the "detectible" depth of a given target are two very different things. As I said in my first post, the only thing that will affect the size/shape of a given field is introducing a conductor (and that INCLUDES ground minerals). We can debate this all week, but it wont change the facts. your actually taking about "percieved/useable" depth.(which is affected by many many things) I'm talking about the field generated by a static coil winding with a fixed driving current. Perceived depth is greatly affected by strength of the gain, Disc levels, amount and type of mineralization, target compostition AND mass. Not to even mention RF interference and a myriad of other factors. BUT transmit power and the subsuquent generated field are affected only by things that actually interact with said field. We are taking apples and oranges here. And by the way, I have a very large and extensive test bed, thats been my best friend for probably 20 years now.(nice and seasoned targets) My only regret was that I didnt start using one till I had already detected for about 15 years. They can be quite telling cant they?? Still a good test bed is only good for the area the test bed is in. I've detected from California to the UK, and detecting conditions vary as much as the people that detect! As for your contention that the transmit coil is only a potential, you absolutely correct, but that potential translates to a measurable constant, when you factor in all the local variables. After all, what is it that generates the eddy currents in the target? The recieve coil? nope, the transmit coil. Without the energy from the transmit coil, there would be nothing for the recieve coil to do! Anyway, I appreciate your opinions, but I think we are talking about two differnt things. Thanks for the input. Respectfully, Streak!

 

[Jackpine Savage]

On several coins and coin sized targets in the 6-7" range detected at "normal" settings I found that the T2 would still respond when the sensitivity was turned to minimum. The only difference was the audio volume level. I should have tried it on the weaker foil range signals to see what happened with those. Perhaps theres a tip there for coin hunters in trashy areas.

Tom