TNSS over on “forum D” was bugging me over the depth of the AQ in discriminate vs in all metal. I have been collecting and reviewing all my stuff on PI detectors for a couple of weeks now to put together a PI intro for AQ customers in the “great bye-and-bye” when it goes on sale.
so I sat down and wasted a few hours trying to explain what I THINK I know about the function of the AQ’s iron ID function. In spite of having had my hands on three different ones since October ‘18, I never got far enough along with any of them to really document actual controlled results in the ground. Anyway, if you want to ruin your afternoon or get a headache - press on....
As far as Tone ID at depth, first remember that in all metal, all targets return the same high tone. I will now try and explain the discrimination function and its results.
Now if the question is do the ID modes of multitone and mute provide the same depth of detection as the all metal mode, the answer is no. The depth difference has been progressively reduced as the development of the AQ went on.
The the depth penalty is due to the action of the discrimination system. the discrimination is achieved by analyzing the decay of targets return signals over time.
WARNING - I am not a physicist or an electronics engineer. The following is based on what I believe to be correct based on several years of studying PI detectors and the information Alexandre has shared on various forums and in private communications between him and I.
Pulse Induction detectors do their target analysis in the time domain - that’s why Whites called the TDI by that name. A strong current is created in the coil which creates a magnetic field. This is transmitted to the ground and any magnetic or metallic targets are energized by it. This current is abruptly cut off with the result that a very high voltage pulse is created. Targets which have been excited by the pulse of current continue to radiate the induced signal at a an ever decreasing signal strength which decays exponentially (drops off quickly and then persists at a lower rate of decay for some time longer. Each target has its own curve - called its time constant.
Low conductors of all sizes decay very quickly and high conductors and ferrous targets persist longer - have a longer time constant.
In all metal there is only an early sample taken of the target signal and so the decay is less - all targets give their full short pulse delay signals and all sound a high tone.
When either of the multitone or mute modes are selected, an addditional sample of the delay curve of the target signal are also taken at a later point in time and therefore further down the delay curve where the signal strength is ugh weaker than it was in the first sample. A comparison and analysis of the two samples result in longer persisting targets being either assigned a low tone (multitone) or are muted (mute mode).
The action of this analysis of the second sample is on the later and therefore relatively weaker signal. It involves amplifying the second sample so that the analysis can take place. This results in an increase of noise and the slight reduction of sensitivity to longer persisting targets.
In this process the depth of detection of all targets is somewhat less than in all metal which operates on only the earlier stronger signals. The lower conductors, with their fast decaying signals are less effected however than higher conductors or ferrous targets with their longer persisting signals.
Alexandre has posted elsewhere that the effect is that the loss in discriminate modes for all gold jewelry below 22kt (regardless of weight) is minimal perhaps 10% less depth than the same target in all metal. So a thin gold ring detected at 12” in AM might be detected at 10.8” in discriminate. As the conductivity and therefor the target’s time constant increases however the difference is depth is larger - for 22k it might be 20%, for 24k depending on size they might return a low tone or be muted.
So, in practical terms, the iron ID function works to the full depth of detection, but that depth is somewhat less than in all metal - although jewelry below 22k is not severely affected.
But what about deep iron or that large 24kt Asian ring? If the system works as I have explained, if you can get a signal from it in multitone, it will ID correctly with a low tone. If you hear a deep iron target in all metal and you switch to multitone, it should either give a low tone in multitone or disappear due to the depth loss described above. Remember, long persisting targets seem to suffer more depth loss than low conductors with their fast signal decay
What about a 14k ring at the edge of detection depth in all metal? You may or may not get a signal in multitone or mute due to the 10% or so depth loss.
If all this was clear as mud - my apologies, but I have actually worked pretty hard to BEGIN to unserstand this stuff. Once I have a machine to play with for an extended period it will be easy enough to document actual tests of various targets at various depths.