Pulse Induction Detectors

 One of the more popular metal detectors used for nugget hunting today is a type of detector commonly called the Pulse Induction or PI for short. A lot has been written on the general principles of operation but many questions are still unanswered or not answered completely about this strange machine. Also, there is a lot of misinterpretations of information that has been written about PI's and how they work.

  As an example, in some books there is a statement that a PI does not "see mineralization" so it is therefore a great detector to use in mineralized areas. Is this really a true statement? The answer is both yes and no.

  PI's basically do not respond to the typical iron mineralization such as magnetite or black sand. However, other minerals of the same family can and many times do cause a response. Iron oxides such as maghemite, clays, and other things such as salts commonly found in the ground can cause a PI to produce a rather strong signal. So, generally a very sensitive PI, normally used for gold hunting will respond to ground signals, especially if it does not have some form of ground balancing circuitry built in.

  One question that is often asked is what is the operating frequency of a PI. This question is often asked by someone who is trying to relate their knowledge of VLF's to the PI. Unfortunately, because of the nature or differences between types of detectors, comparing a PI to a VLF is sort of like comparing an apple to a potato, so trying to relate the operating frequency of a PI to a VLF or sensitivity to small gold is of little value. The differences between the two types of detectors or the affects of their operating frequencies are quite dramatic so it is best to not try to use the same standards when trying to determine certain things about a PI.

  As for a PI, the pulse rate or pulses per second (pps) refers to the number of high current pulses that occur over the time specified. Rates vary from a few hundred to several thousand per second: Generally, more pulses allow for a little better averaging and thus a little better signal to noise ratio. However, a detector will have a tendency to consume more current with a higher pulse rate. A faster pulse rate doesn't mean a detector will detect small gold better. In fact, it is quite easy to build a PI that has a very low pulse repetition rate (PPS) that is very sensitive to very small gold while designing a PI with a high PPS that is not sensitive to small nuggets.

  Now, both PI's and VLF's will detect metals, respond to different ground conditions, and even respond to salt water. Both use a coil, specialized circuitry and usually generate a similar output to indicate some object has been detected However, the techniques, circuitry and in many cases, the coils are dramatically different.

  VLF's generally produce a relatively low power continuous sinewave into the transmit coil and, analyze a signal received with a separate receive coil winding. A signal from an object will increase the amplitude of the receive signal level but will also shift the receive signal with respect to the transmit signal. Thus, an object can be analyzed by not only the intensity or amplitude increase of the signal but by just how much the signal has shifted.

  VLF's generally operate at a single frequency but can be produced to operate at different frequencies. However, each frequency has to be analyzed as if it is the primary frequency and as such, both the signal strength and the shift are used to determine the presence of an object as well as type of metal.

  PI's are a different beast all together. Instead of transmitting a low power continuous signal, the PI generates a brief high current pulse to energize the coil and this pulse is repeated at some nominal repetition rate, which can vary from a few hundred pulses per second to thousands per second.

  The technique to determine whether an object is present is to analyze the signal coming from the receive coil shortly after the high current pulse is turned off. This is done by sampling the signal coming from the coil some time after each high current pulse. This time after the pulse is often referred to as the delay time Remember, on a PI, the transmit coil may become the receive coil once the transmit signal is turned off so there is no need for a separate receive coil winding. This type of coil is often referred to as a Mono coil.

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