explain "normalize"

[Downdeep]

By far the vast majority of my hunting is for civil war relics, so I have been attempting to do some fine tuning on my V3 to get more depth and better target ID (duh, who doesn't)

I have been playing with using the 7.5 single frequency as it is supposed to hit harder on brass/copper/lead than the other freqs. My depth has increased noticeably but my target ID is still bouncing around quite a bit. In doing so I noticed the normalize option checked so decided to play with that. I have a copper penny buried in my yard that I use to test different settings just to see what difference it may make. When running the 7.5 freq with normalized checked, my ID bounces anywhere from 0 VDI to into the 80's. With normalize off, I run a consistent VDI range in the mid to high 70's. Now, from what I read in the manual, though I can't fully wrap my mind around it, is that the normalize option should actually give me a more consistent reading? Or am I understanding it wrong.........If I can run with normalize off when hunting relics and get better ID, I'd like to do that, but I don't want to lose anything.

I also am thinking that the normalize option adjusts the VDI for targets that may not be picked up by a certain frequency very strongly. Such as lead would read more consistently with the 7.5 freq, but gold might not read correctly with normalize off?

Help?

 

[jgedde]

The different frequencies show different phase shifts between the TX waveform and the RX waveform for an identical target. This means they will display different VDI numbers. Normalize corrects (normalizes) the VDI's for 22.5 kHz and 2.5 kHz so that they all read using the traditional VDI numbers from the 6.59 kHz units of old. I would suspect normalize could yield less jumping around of VDI in three frequency mode when using the "Best data" mode.

John

 

[TheMarshall]

Hi there DD.

Normalise.........'the meaning of ',.... hopefully without over complicating matters....

In the case of metal detecting and VDI's for specific targets, coins are a typical standard because of their regular shape, dimensions and metals / alloys.

White's, many years ago, generally adopted their ' 7 KHz' numeric evaluation of such targets, as 'The norm', or normal reading.

If you utilise a different search frequency, the detector 'sees' or evaluates those same coins as numerically different.

This is caused by the 'Skin effect', whereby the current induced into the metal alters as the frequency alters.

The VDI 'number' is a reflection of the amount of induced current that flows around the mass of any target.

With the advent of microprocessor usage in metal detectors, it became possible to 'measure these differences' at differing discrete frequencies, and so apply a correction factor.

This then allows the designer to 'multiply' the target's frequency dependent VDI, by a 'normalising' factor, so bringing the differing reading back to the 7 KHz standard value.

Now the detectorist is able to associate a particular VD! number to such standard items as coins, regardless of their varied reaction to different frequencies.

It is simply a process of artificially making the 'same' target look the same numerically, despite the change of eddy current levels flowing at different frequencies.

As an example when using the two frequencies of the DFX :- 3Khz and 15 kHz.

The VDIs quoted are 'rough' in-air values, of some degree of variability due to the manual process involved.

An English copper penny, will at 3 kHz give a VDI of 64....non normalised
........................................... at 15 kHz give a VDI of 87....non normalised

The DFX, in 'dual frequency mode' produces a VDI of about 78.........normalised. (This is typical of the 'standard' VDI on the 6.6 kHz White's XLT.)

The simplified maths using whole numbers.....87 minus 64 gives approx. 24. Divide this by 2, giving the average of the difference, as 12.

Add 12 to the lower VDI of 64, give an approximate normalised value of 76. This is near enough to the 'normalised' value of 78.

Trouble is DD, that the response curve of our metal detector to varying targets, is not a linear function, So the k factor of 'adding half the difference' etc., will also vary.

It all gets a little more complicated due to the physics involving that aforementioned 'Skin effect'.......I've tried to explain without over doing matters, but somethings cannot be ignored without loosing the plot.

Hope it has helped in some small way........................TheMarshall

p.s.

Normalisation is not responsible for any erratic VDI's produced.

That is probably caused by external factors at the different frequencies involved.

Such as EMI or ground mineralisation reactions, or even target characteristics changing at the 'other' frequencies....

 

[Downdeep]

Thank you for the responses. That is kinda what I thought it did, thuogh the explanation here did a better job than the manual. BUT it still doesn't explain why I get a more stable reading using the single 7.5 frequency with normalize off. I just went out and double checked it...I am not imagining things!