Referring to sensitivity as "transmit power" on Tesoro machines is a bit of a misnomer, because there is no way to increase or decrease the actual power transmitted at the coil on these machines. In fact, there are very few machines on the market that allow such (some upper-end White's and Nautilus machines being an exception).
As Monte mentions, "sensitivity" is a receiver circuit based function rather than a transmit mode function, something that occurs or is controlled after the signal has already been received back from the coil. Some compare it to the "squelch" control of a CB radio in which higher levels on the control allow you to hear fainter and fainter return signals, or increase the response from those already being heard. That unfortunately includes good targets and bad, including (in all metal mode) the ground mineralization when there is no discrimination function used to eliminate such ground effect.
"Full Power" needs to be qualified when discussing this kind of topic, and most coin machines have far lower gain levels than gold machines, the ElDorado vs. the Diablo uMax being a good comparative example. The gold machine uses (and requires) far greater gain levels than the coin machine, simply because gold machines are generally designed to find much smaller targets than coin machines. But due to the areas where gold is usually found in its natural state (nuggets), that same level gain requires it to be adjustable to compensate for some pretty harsh ground mineralization.
So, with this in mind, understand that most coin machines at "full power" are still operating at far lower gain levels than those available on the gold machines like the Diablo uMax and LST. And that is the reason that "full power" on some machines creates few problems, while "full power" on others would be a nightmare to operate in the same ground conditions. It's just a matter of "gain availability" or audio sensitivity as to what the machine is capable of "hearing". While "transmit power" can also vary from one machine to another, this function is usually "fixed" internally within the circuit designs rather than adjustable by the user. This perhaps is also why Tesoro and some other manufacturers prefer to refer to receiver gain as "sensitivity" rather than "power", with POWER denoting the transmitter side of the circuits and SENSITIVITY denoting the receiver side.
Something to also remember in depending on VDI readings to determine ground mineralization is that the VDI value only gives you an indication of the relative conductivity of the ground rather than the "saturation level" of the actual minerals themselves. You might get a VDI reading of 75 in two different areas, where one is nearly impossible to detect due the the "quantity" or saturation of mineralization in the ground, while in the other area the ground is much more detectable due to a lower saturation of the same minerals reading in the same 75 range. A higher VDI is indicating a more conductive ground mineralization than a lower one, but neither high or low numbers will give any indication of the actual "quantitative value" of the ground minerals. Such numbers are really more useful as an indication of what targets you will have difficulty in detecting in that particular ground. Supposing you have a ground reading of 75, and a particular coin that reads in the same range. It will be all but impossible to detect a "75 coin" in "75 ground" because the detector is unable to distinguish one from the other. The coin just blends in with the ground. Of course, you will rarely find a particular coin and ground combination that are identical, but then again, how would you know ? <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="
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"Consistency" of the ground you are hunting is another factor in how much gain can be used with a particular machine. If the ground mineralization is changing every few feet (or inches), you obviously have more difficulty with ground noise or the need to re-ground balance more often. Ground balance, by the way, is nothing much more than a low-level discrimination function in which the audio (receiver) circuits are balanced against a known (transmitted) signal (induction balance), and although the detector itself still "knows" the mineralization is there, the audio is silenced for the benefit of the operator. Ground balance then is like a "specific point" discrimination function, where progressive discrimination" involves a wider range of targets as the discrimination control is advanced. You might even consider it a "notching out" of the relative conductivity of the specific ground mineralization since ground balance operates in much the same way as notch discrimination, only usually within a 5 to 7 degree phase range that covers most ground minerals rather than the wider range used for true notch discrimination of more conductive metallic targets.
Hope this provides some help with your questions and concerns.
Ralph
