— SteemCN

My dear @henrychidiebere! I know it can only be you always with your intelligent questions. Thanks for visiting my blog and for asking such an important question. I really envisaged a question such as yours coming, and would have included the details in this article in the first place, but I didn't because the article was getting too long and I like to keep my writings within readable threshold, and then engage people with questions with further details in the comment section.

Now, in response to your question; Very Low Frequency based metal detectors employ what's known as phase shifting phenomenon which confers a sort of discrimination capacity on such detectors, enabling them to distinguish between different metals with very high detection accuracy.

Phase shift is essentally the difference in timing between the frequency of the transmitter coils and the frequency of the target object that results from variations in the inductive and resistive behaviors of different objects to the flow of electricity. | _Source

For instance, an object that is a good conductor of electricity would be slow to respond to changes in the current. In this case, we say that the object is of high inductance because it allows electricity to pass through it easily. One way to appreciate this is to think of inductance as a deep river. It would certainly take some time to notice a change in the amount of water flowing into such a water body.

On the other hand, an object that is a bad conductor of electricity would be very quick to respond to changes in the current owing to its high resistance. Applying our water analogy above in this case, we think of resistance as a shallow stream of water. Now, change the amount of water flowing into a stream and the change will be easily noticed.

On the basis of the foregoing discourse, a target object with high inductance value would equally have high phase shift value because it would certainly take some time to sufficiently change its associated magnetic flux. And the reverse is true for a target object with high resistance.

Attention @henrychidiebere

VLF metal detectors use a discrimination feature known as phase modulators. And since different metals (target objects) have different inductance and resistance values, when the vehicle comes under the scrutiny of a VLF metal detector, the detector by simply using its phase modulation discrimination technology probes the different metal components of the vehicle and compares their values with a particular type of metal being searched for. Should the reading falls within the range a target object would likely have, the vehicle will be suspected and taken into custody for further investigation. It's been exhaustive response. Thank you.

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RE: The Science Behind Metal Detectors

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