Coil output and mineralization

Please correct me if I'm wrong, but I recall reading that the constant transmitting by VLF machines builds up an electrical charge in iron mineralized soils and reduces the VLF machines effectiveness (compared to a P.I machine). I can also remember reading that by reducing the coil output this issue can be reduced, and even with reduced power the machine may be more effective, because there is less excited iron mineralization.

It's my understanding also, that the F75LTD has a constant non- adjustable output, and buy reducing the sensitivity only reduces what you hear (the receive circuit), not the output power itself. If this is the case, then wouldn't it be advantageous for Fisher to make their output power adjustable rather than fixed? I know other manufactures do.

Now if the output is fixed on the F75 it's my further understanding that there is Default Process (dP) then Boost Process (bP) goes a little deeper and Cashe Locating Process (CL) allows deeper again.

So if the F75 is a regulated output then, say for this example would CL =100% power output... bP =80% and say dP =60% all controlled by the internal programming in the machine.

Thanks

Ross

As a signal voltage is induced into a target, It will charge to a percentage of the original source. But being an alternating source, when the source goes negative, the induced voltage will discharge. Thus using Direct Current the source stays constant and never allows the induced voltage to discharge. Which causes stored energy. (electro Magnet). By reducing the source voltage or power, you are still inducing a stored voltage into the object that is still the same percentage to the source as it would be if at a higher level. In theory, it should have the same effect at any level of the source and only have a direct impact if the target size or quantity increases.
Higher minerals the greater the impact.

The power put into the coil is rather low. It doesn't need to be high, you're only trying to find things 10 inches away, not 100 yards away. Government regulations (FCC, EC, etc) limit the maximum power too. Reducing transmit power doesn't severely affect depth, but it does start to worsen existing issues like circuit noise, electrical interference pickup etc. It's generally considered simplest to leave transmit power fixed, and vary the sensitivity of the receive circuit. One noteable exception is the XP Deus, which has a transmit coil powered by a small battery, and reducing the (variable) transmit power gives longer battery life.

On the F75-LTD, I believe the BP(boost) mode increases the receiver sensitivity, but to prevent circuit overload, the coil needs to be raised to lower the ground-pickup. Additional (software) filtering helps compensate for the noise issues. CL (cache) mode also uses digital filtering to extract weak signals from the ground noise. Neither of these modes are simply equivalent to 'Upping the transmit power'. Hence asking 'if Cache mode =100%, is .... etc etc' is not the simple question it appears.

It would be interesting if you could tell us where to find the article about this 'magnetisation phenomena'.



Edited 1 time(s). Last edit at 01/15/2012 01:12AM by Pimento.

Look for the definition of 'Hysteresis' (magnetic 'memory').

True......... the F75 SE/LTD............ and............. for that matter............ nearly all other metal detectors on the open market............. transmit a 'fixed' amount of power. . . . . and at that............ a very low power........ in the very low end of the mW range. The different 'boost process' modes in the T2/F75 are merely a different timing & sampling rate; not power output.

It is doubtful minute' (if any) hysteresis is imposed on ferromagnetic elements with a VLF IB metal detector. PI's are a different story.

After quite awhile of internet searching, I cannot seem to find specific requirements by the FCC in regard to IB or PI consumer metal detector regulations.

I was at one time under the assumption (or thought I read it somewhere) that there was a 100 mW limit. Unless someone can produce otherwise, the only thing
that seems pertinent is the FCC's regulation: .summarized.... part 15 of the FCC rules that:
Operation is subject to the following two conditions:
(1) this device may not cause harmful interference, and
(2) this device must accept any interference received

I was under the impression that increasing the power output of IB detectors, was more impractical due to extra battery weight and shortened corresponding battery life.

However, there must be something else......that makes increasing coil tx pwr. output more impractical.

The nature of the beast being what it is, as more power is transmitted......stronger eddy currents are induced in ALL metals, including the natural ground mineralization, as well.....as the trash....and the desired targets........unless of course....one is fortunate enough to have the luxury of inert dirt.

So, has anyone actually tried....increasing near-field documented testing (that is generically non-proprietary enough) to prove to us that it is or is not worth the effort.

I did find some interesting info IRT detector power output levels: ( [www.findmall.com] )


Tri

I knew I'd seen 100mW mentioned before, I just didn't pay much attention to the fine-print, as I knew detectors typically ran nearer 10mW. Up to 250mW would be a viable coil power whilst still giving adequate battery life, but it would be of limited use - high EMI locations being one possible. Lowering TX power, as per the original posters question would have minimal benefits re. battery consumption, and would start to cause noise-related issues.
So, Ross, have you found the 'electrical charging' article, so we can assess if more or less power makes any difference?



Edited 1 time(s). Last edit at 01/17/2012 01:54PM by Pimento.