Detecting a Target While in Null

[Cody]

If you have followed any of the discussions on the other forum on this topic I think there is a way to still detect targets while in null. If the null does not drive the threshold any more than say a click below our threshold then we would still be able to detect while in threshold null. So far my tests have not show that to be correct but will look at this in more detail.

If we can detect a target in null then there is really no need to keep a threshold when detecting in heavy iron unless the audio bothers us by going from a threshold to a null. This kind of brings us around to why not just set the threshold at one click below the audible threshold and use the Explorer as a silent threshold detector. Anyhow, I will look at this closer to see what I can find out and will also talk to the guys at Minelab.

Have a good one,

 

[AK in KY]

Cody - I did a test with two nails and a penny. Imagine two nails 4" apart pointing north and south with the penny in the center of the two nails. When using IM -12 sweeping east to west the EX nulled but the penny was detected. Turning 90 dedgrees and sweeping north to south in the same direction the nails are pointing the EX nulls and the penny is not detected. Not very scientific but may shed some light on the subject.

 

[shawnv(or)]

I just watched my new video that came with my new EX II and in one section the host Gary Brun put a fifty cent piece next to a nail and then put the nail on top of the coin. The EX II detected the coin both times and you could tell the machine was nulling. Is it not what the machine was designed to do. And isn't the threshold level just personal preference. What a great machine and a great forum to learn by. Keep up all the great post(s). If it stops raining I may actually get to try out my new machine.
Happy Hunting
Shawnv(OR)

 

[Cody]

Try that with a piece of iron other than a nail. A nail can look like a coin due to the flow of eddy currents around the axis and down the length of the nail so will be interested in what happens with a flat piece of iron, any iron shaped that is not like a nail, so you may well be right in it depends on the iron.

Good test and thanks,

 

[Cody]

We have to be a little cautious with some of these demonstrations. The silver is so much larger than the nail that any machine will detect the silver. However, put a nail on a silver dime and see what happens. Then put the dime over the nail and see what happens.

One more point is a nail will give the same reading as a silver coin in conductive sounds. That has been a real irritant to many users. Try the same test using a flat piece of iron. Try a piece larger and smaller than the coin.

Another good test and thanks,

 

[shawnv(or)]

Because it s cold and raining I have had time to play and experiment with my EX II and while my daughter serenades me with her violin lesson I'll write what I have discovered about Ex II in the short couple days I have had it. You can put coins next to all kinds of discriminated objects and still detect them. I could actually find the null on the floor in my office and lay a dime directly over it and still detect the dime. I wish the weather would change so I try some real detecting.
HH
Shawn

 

[Cody]

That is interesting. I am looking forward to your posts and settings.

 

[Ralph Sun Ray]

Cody and all,

I have been using an Explorer since it first came out in January of 2000 and have been using a Sovereign since February of 1993. Most of the coins I and my wife have found have been in areas where the threshhold was nulled as many of the areas we have hunted have been in either heavily mineralized areas which will cause a null or heavy iron infested areas, or both. This has also been while using many sizes of coils, but with many old silver, nickel and copper coins being found in the above conditions using a 12" coil.

The rule of thumb is that you will find coins in a null, but not ALL of them. There are so many variables involved in hunting situations that different sizes and types of coils, detector settings, ferrous and non-ferrous trash situations, etc. will allow you to either detect or miss desirable targets. You can change a coil size or type, change one or more settings, change directions and sweep speeds and find desirable targets in the same areas previously hunted that you previously missed. Also, ground moisture plays a big part in the above conditions, especially in iron areas, as those "auras" are enhanced in wet conditions creating either a larger masked area or, as in the case of rusty bottle caps, can cause them to change ID and come in more like non-ferrous coins and targets.

The best way we have found to continue to eck out a few more desirable targets, is to keep going back to those old productive areas with different size/types of coils, detector settings, sweep speeds and sweep directions, etc. Also, going back after a rain as well as in dry conditions.

As to iron nails sounding like silver coins in conductive sounds, that can happen some times. But normally if you have your "Variability" setting maxed out at 10 the iron nail will have a higher squeaky pitch than a silver quarter (which is the highest coin pitch in conductivity on the Explorer). In digital it will usually read a 31 on the conductivity side. In Smartfind the little cross-hair window will only show the bottom two squares with the top two buried in the top of the screen. So there is "usually" a difference, but there are always exceptions.

We have also found that it is very helpful to have the threshhold barely audible to alert us to changing conditions, whether it be heavy ground mineral or iron trash, so we can not only find potential hot spots, but also change coils, or settings in certain areas, to go back over more carefully. In our opinion, the null is a valuable form of discrimination and you can learn from it so you know what strategy to use in hunting certain areas.

I just thought I would jump in here with some in-field observations that Elaine (12 year Sovereign veteran) and I have made in hopes of adding information to this thread. I hope the above helps.

Ralph (Sun Ray)

 

[bing! (L.A.)]

I agree that the explorer can detect a target when in NULL but not all the time.

This is quite a complex subject with many variables but I would like to share my thoughts about it.

How can the explorer detect a target when it is in NULL? Because its not in NULL. Being an automatic ground balancing machine, the detector continously takes in samples and ground balances. It may null on a target, but within in milliseconds rebalance and be ready to detect the next target. This is pretty much the same as detecting with the explorer with the sensitivity set too high. You'll still find stuff, a lot of stuff, but not as much as when you set it just right.

Why is it that you can detect a dime in the middle of two nail pointed north to south, and not the same dime when the nails are pointed east to west? I posit that the explorer DD coils take samples like a wiper and not like a mop (concentric coils). Which is probably why oval coils seperate targets better than round coils. If true then the first sample is viewed by the explorer as 3 targets, vs the second one which will be viewed as one.

/bing

 

[Cody]

Yea, I think this is one where terms, null, threshold recover, and the like does more to obscure the operation than explain it. Good insights and comments on your part.

It appears to me that the audio is immediately force to the ON state when a target is detected even if it was OFF to indicate a rejected target. When two positive hits are detected the audio will indicate both and settings will help he user to hear tones for both.

I did not do nearly enough to explain my answer that we cannot detect when in NULL. Even if we can say this in an "absolute sense" the users cannot perceive the millisecond switching times.

This is where my background in needing to be absolutely as correct as possible and thinking in "electronic time and speed" can be a problem for me. I spent so many years with engineers, programmers, technicians and teaching in the classroom where engineers would back me up if not "exact" that I continue to think in those terms.

I have made a promise to myself to stop doing that and think more from the users practical point of view.

 

[Ralph Bryant]

If the circuits in the Explorer are truly sending and receiving multiple frequencies/channels on a simultaneous basis, would they not be capable of responding separately but on a simultaneous basis also ?

Just a thought.

Ralph

 

[Cody]

A sine wave can be generated in a "natural" way by rotating a conductor in a magnetic field. As you know that is how we end up with the typical house voltage and current. Engineers could take many sine waves and shift the phases and to generate multiple frequency detector. My guess is this has been tried.

The obvious way it to use a square wave because the characteristics of a square wave contain multiple frequencies. A square wave generator is basically a wave generated by adding odd harmonics of the basic frequency. The Explorer use a wide and narrow rectangular pulse so there are two basic frequencies and then the harmonics.

They process 28 frequencies and as you suggest only they know exactly what their algorithms are do do so. I do know that the signal in the receiver is fed to 3 pairs of demodulators where the outputs are three time constants, long, medium, and short, then filtered and processed. In addition there is a Noise Cancel that shift the 28 frequencies into 11 different channels. That is if we have something like 3, 6, 9 they can shift that to 3, 5, 7 as an example. Each shift is a channel and I believe there are 11. I only mention this to suggest that they do a lot with the 28 frequencies that is not going to be shared and as we know patented.

I have often thought it would be nice to be able to upgrade based on versions since there seems to be so many options. So, I guess the answer is I think they already do that and have options for a lot more magic.

 

[Ralph Bryant]

WHEW ! I love long answers to short questions. It's the best way to learn IMO.

I remember asking my wife "why ?" one time, about 6 years ago.

She's still explaining !

Ralph