TDI discrimination

[scs]

Shovel is the deepest discriminating detector.

[Reg]

Geo,

The conductivity of the coins shouldn't have changed and still should be a high conductor at a delay of 10 usec and a GB at 9 to max. So, something else is wrong.

If you are looking for maximum depth of detection on those gold coins, then it appears you can gain depth by increasing the delay to maximum delay which should minimize or at least reduce the ground signal and then turn the GB off. You will have no form of discrimination this way, but will maximize depth.

With the GB off you should be able to turn the sensitivity to maximum which should help gain more depth also.

Then you can try a larger coil and gain a few more inches in depth.

BTW, what size coil are you using? Also, what type of coil are you using? The normal coil sent with the TDI here in the US is a dual field coil. Detectors sent to OZ normally are sent with a plain mono coil. A plain mono coil should gain a small amount of depth when compared to the dual field design. Also, there will be a little less ground response with the plain mono coil.

Have you ever tried to build a coil? It isn't that difficult if you can solder properly.

If you know someone who has a Minelab PI, you might try to borrow a DD coil and try it. With the GB off, there will be less ground signal if you use a DD coil, especially if the delay is advanced. A ML PI will give you odd signals on near surface iron objects, so there is a little discrimination, but not much.

Reg

[Geo]

Hi.
Some years ago i posted the below table.
As i see now, TDI detected one gold coin as high conductivity with delay=10 and GB between 7 and 11. But for 45 coins it reversed the conductivity!!!!!.
For delay=10 and GB= 7...9 TDI has the lower depth for gold. So I must make again the test with the 45 coins and i think that it is time to change the adjusting of the TDI and the way that i use it. Maybe it is time to keep low or high GB and the coil far from the ground ......

[Carl-NC]

I'd be a bit more concerned about the coins than the detector. Real bulk coins of 22ct purity simply must trigger high conductivity reaction in any detector. No way 22ct bulk gold would not act as a good conductor.

Sorry, not true for ANY detector. The inductive conductivity of a target has way more to do with skin depth than the DC conductivity of the metal. For the exact same size and composition, making the target thinner will result in a lower conductivity reading. As an example, make up a set of small square (I use 1"x1") targets out of aluminum foil. One target should be 1 layer thick, another 2 layers thick, then 4 layers thick, then 8, 16 and 32 layers thick. I sandwich them in clear packing tape to keep them tightly pressed together. You'll see that 1x thickness has a radically different phase response than 32x thick, even thought they are otherwise identical.

Gold, silver, copper, lead, all do the same thing.

- Carl

[Reg]

Davor,

On the TDI, more than the purity of the gold is required to trigger a high conductor response. Just like other detectors, the size of the target also makes a difference.

On a VLF, at least some of them, a $5 US Gold coin (1/4 oz 22k) which is about the same size as those buried would respond with a TID less than some pull tabs. A $10 US gold coin (1/2 oz 22K) would respond much like a screw cap. I tested different gold coins many years ago and just mentioned what I found. Now, this can vary depending upon the detector, but the earlier TID units such as the old Teknetics and Whites would react this way.

On the TDI, the adjustments of the controls will determine whether a gold coin such as those buried will respond as a high conductor or a low conductor. In other words, that same coin can respond as a low conductor or a high conductor depending upon the settings of particular controls. The two main controls that will determine this are the delay and the GB (ground balance control).

On the TDI, the GB control can be adjusted over a fairly wide range. Normally, the GB setting is somewhere between 8 and 9 on the control to minimize the signal from the ground. At this ground balance point, a nail will usually respond as a high conductor. Now, at this ground balance setting not only will it minimize the ground signal, but any object detected that has a very similar decay curve will also have a signal minimized. An Object that has a longer decay curve than the ground signal will respond with a low tone, which indicates a high conductor. Objects that decay faster than the ground signal will create a high tone, which represents a low conductor. Again, at the setting where the ground signal is minimized is the ground balance point.

Change the GB setting and if you adjust the GB to a lower number, then a new balance point is created and the ground now causes a low conductor signal. Because there are a multitude of objects that balance out at different GB settings, something else now will be balanced out. Continue to adjust the GB downward to somewhere around 4 or so and many solid nails will now create a minimal signal. Very old deteriorated nails may vary from this setting as mentioned earlier.

If you pass this nail balance point by decreasing the GB control, you will now change the signal from a nail to a low conductor which is now a high tone.

So, you can make a nail or most other thicker ferrous trash respond as a low conductor or a high conductor simply by adjusting the GB control. Well, this same thing happens to many gold objects, even those that are 22K. In the case of gold coins, size now becomes an important factor as to just where on the GB scale, they balance out.

So, as far as the TDI is concerned, an object that can change tones within the range of the GB control can create a low conductor signal or a high conductor signal, depending upon the GB setting.

This change in an object's response is often referred to a way to determine what it might be and therefore is often referred to as a form of discrimination because a person who becomes very familiar with the various adjustments can determine quite a bit about an object just by manipulating certain controls.

Please keep in mind that the TDI was developed to be a nugget hunting detector and the GB control is there to minimize the ground signal. Any form of discrimination or a means of determining more about any object is a bonus and not the main reason for the GB control. Because many people have found these various changes help in determining different objects, the TDI is now used for other reasons than what was the initial intention of use of this detector.

Reg

[Davor]

I'd be a bit more concerned about the coins than the detector. Real bulk coins of 22ct purity simply must trigger high conductivity reaction in any detector. No way 22ct bulk gold would not act as a good conductor.

[Reg]

Geo,

Make sure the delay is at 10 usec. If the delay is at a later delay, then the GB settings all change and the gold will most likely respond as a low conductor even at a higher GB setting.

In simple terms, the GB and the delay interact. Increase the delay and objects that change from a high conductor to a low conductor will do so at a higher GB number. Reduce the delay and objects will change at a lower GB number.

Reg

[Geo]

Geo,

You are right, you will have or could have a lot of ground signal noise if you are in the all mode or the low conductor mode if you turn the GB to max. However that shouldn't be the case if you use the high conductor mode.

Try to advance the GB to or near max with the delay at 10, and then select High conductor. This should minimize any ground signal if you set the threshold almost at no signal. Then check the gold coins for a signal in the high conductor mode with the GB at max.

In other words, you can use the high conductor mode to reduce any excessive ground signal caused by having the GB at a higher than perfect GB setting.

If you can detect the coins fine with the GB at max and in the high conductor mode, then reduce the GB and test until the coins seem to disappear. My guess is this will happen maybe at 7 or so. If that is true, then you can use this feature as a means of guessing the target because most iron objects won't disappear until maybe 5 or so.

If you select the high conductor mode, most thicker iron objects change tones (disappear if you are in the high conductor mode) somewhere between 4 and 5, so if a target disappears at a higher GB number, then most likely it isn't iron.

Try this and see if it works for you, if you don't mind. It works for me.

Reg

Reg,
with GB at 7 don't see the gold as high, remember that when i located the coins the GB was at 8 and saw them as low. Rust iron rejected near to 3. But i will try to remember so tommorrow to take out the TDI from my car and to check with the GB near to 10.
You understand that if has not the ability to discriminate between gold and iron then no reason for discrimination.

Regards

[Reg]

Geo,

You are right, you will have or could have a lot of ground signal noise if you are in the all mode or the low conductor mode if you turn the GB to max. However that shouldn't be the case if you use the high conductor mode.

Try to advance the GB to or near max with the delay at 10, and then select High conductor. This should minimize any ground signal if you set the threshold almost at no signal. Then check the gold coins for a signal in the high conductor mode with the GB at max.

In other words, you can use the high conductor mode to reduce any excessive ground signal caused by having the GB at a higher than perfect GB setting.

If you can detect the coins fine with the GB at max and in the high conductor mode, then reduce the GB and test until the coins seem to disappear. My guess is this will happen maybe at 7 or so. If that is true, then you can use this feature as a means of guessing the target because most iron objects won't disappear until maybe 5 or so.

If you select the high conductor mode, most thicker iron objects change tones (disappear if you are in the high conductor mode) somewhere between 4 and 5, so if a target disappears at a higher GB number, then most likely it isn't iron.

Try this and see if it works for you, if you don't mind. It works for me.

Reg

[Geo]

Geo,

I would think that if you have the delay at 10 usec and set the GB to 9 or above, you should detect the coins in the high conductive mode. You might not get that much depth so, you might turn the GB to max to see if that helps with the depth of detection. It will be noisier but should be able to be in the high conductive range and detect the coins. Then you can select high conductor mode to reduce the noise. When you do this, the ground signals, what you call ceramics should be ignored.

Try it and let me know if this works.

If you advance the delay, many of the high conductors will change to low conductors. If you reduce the GB from 0 to 1, many high conductors will now become low conductors. If you do both, you might gain a little depth. It all depends upon where the object changes tones.

As for the mode and settings for maximum depth, that depends upon the object you are trying to detect. If you can find the setting where the object changes from high to low conductor, then the farther you are from that point on the GB setting will give the greatest depth of detection.

So, if the object changes from a low conductor to a high conductor at a GB setting of 8, then going to maximum GB will increase the depth but not as much as going back to 1 on the GB scale.

Where did you have the GB and delay set when you were able to detect the coins as low conductors?

In the case of your buried coins, try different GB settings and delay settings to see what works best.

Also, see if the coins can be detected as a high conductor if the delay is at 10 usec and the GB is at max.


Reg

Hi Reg.
If delay is 10 then i can't adjust GB at max because without right GB it beep at every moving. With delay 10us i can put the GB between 8 and 9. When i made the test the GB was at 8,5 about and the delay at 10usec. I will make test with GB near to zero or to 1 and i will inform you. Next month i will go again to my friend (he is 200 km far) and i"ll make real tests with the TDI at every delay and GB position.

Regards

[Reg]

Geo,

I would think that if you have the delay at 10 usec and set the GB to 9 or above, you should detect the coins in the high conductive mode. You might not get that much depth so, you might turn the GB to max to see if that helps with the depth of detection. It will be noisier but should be able to be in the high conductive range and detect the coins. Then you can select high conductor mode to reduce the noise. When you do this, the ground signals, what you call ceramics should be ignored.

Try it and let me know if this works.

If you advance the delay, many of the high conductors will change to low conductors. If you reduce the GB from 0 to 1, many high conductors will now become low conductors. If you do both, you might gain a little depth. It all depends upon where the object changes tones.

As for the mode and settings for maximum depth, that depends upon the object you are trying to detect. If you can find the setting where the object changes from high to low conductor, then the farther you are from that point on the GB setting will give the greatest depth of detection.

So, if the object changes from a low conductor to a high conductor at a GB setting of 8, then going to maximum GB will increase the depth but not as much as going back to 1 on the GB scale.

Where did you have the GB and delay set when you were able to detect the coins as low conductors?

In the case of your buried coins, try different GB settings and delay settings to see what works best.

Also, see if the coins can be detected as a high conductor if the delay is at 10 usec and the GB is at max.


Reg

[Geo]

Hi Geo,

The conductivity of a metal as it relates to the TDI is more complex that making assumptions on all sizes of the metal.

For example, a nail may be a low conductor or a high conductor depending upon the size of the nail and the setting of the ground balance (GB) and delay control. Set the delay for 10 usec and leave it there and then the nail will vary as the result of the GB setting. If the nail is long and large enough, it will respond as a high conductor with the normal GB setting of 8 to 9. Reduce the GB to 3 or 4 and that same nail will now respond as a low conductor.

This same situation can occur with other objects including gold coins. If no gold is available, use pieces of an aluminum ingot. If the aluminum ingot isn't available, then use simple lead. Select different sizes and you will find sizes that will respond as low conductors and others that are big enough will be detected as high conductors.

This characteristic that metals can be either a low conductor or a high conductor on the TDI exists for most metals. As Carl stated, even silver if it is small and/or thin enough can respond as a low conductor.

So, rather than make an assumption as to whether something will respond as a low or a high conductor, it is a good idea to check first. It is very possible, a lot of testing and experimenting will be required before this makes sense. At least it took me a long time to adjust and even now, I get fooled at times.

Now, compound this conductivity issue with the fact that the depth of detection is also altered and the use of the GB and how it causes different things to occur further makes the TDI more complicated that people realize.

Actually, it isn't that complicated, but it can be somewhat tricky if a person is trying to obtain maximum depth of detection.

Reg

Hi Reg.
If the detector can't discriminate the iron, the gold, the silver or the ceramics then what we need the discrimination mode???.
Remember that i hunting old golden treasures that are inside or beside with ceramics....
Anyway i will play a little more with delay and GB. Maybe Carl who has the schematic can tell me where there is the best depth for high conductivity.. at GB-1..2 or at GB-8..9.

Regards

[Geo]

Hi Geo,

Are you able to check a singe gold coin to see how it responds on the TDI or do you know the size and weight of the coin? Individual gold coins can vary between low conductor and high conductor by size, weight, and GB setting. A small gold coin such as a $1 US, or a $2 1/2 US will almost always respond as a low conductor. A 1/4 oz which is about a $5 US gold coin can respond as either a low conductor or a high conductor depending upon the GB setting. The US $10 will do the same.

So, a single gold coin can respond as a low conductor or a high conductor depending upon the size, weight and shape. Now, take a bunch of them and toss them into a pile and most likely, the conductivity will not change, especially if the conductivity of a single coin is no where near the setting where it is about to switch from a low conductor to a high conductor.

This is why it is important to know more about the gold one is trying to find. This problem of low conductor or high conductor becomes worse when it comes to gold nuggets. Some places gold nuggets are very solid but more importantly are very pure, while other locations the gold has a higher silver, copper or other metal alloy content. Alloy gold with other metals radically alters the conductivity which then alters the way it will respond.

I used to refer people to a particular website that displayed that gold can change from a high conductor to a low conductor simply by the alloy content. Unfortunately, that website no longer posts this information.

Getting back to gold coins, don't be surprised if a bunch of gold coins respond as a low conductor, especially if a single coin responds as a low conductor. Whether gold responds as a low conductor or a high conductor is not just a matter of the object being gold. Again, size, weight, and shape all are factors as well as the GB setting as to whether gold will respond as a low conductor or a high conductor.

Now, as for the VLF's having a problem, I am assuming you are using the discriminate mode and not the all metal mode when trying to detect those coins. Did you try the all metal mode to see if that mode will detect them? I suggest you try. I would expect that you can still detect the coins in the all metal mode. Let me know if this is true if you don't mind.

Reg

Hi Reg.
Coins was English pounds. They are 8gr each so it was 360 gr gold coins 22 karat. When i am out for treasure hunting i have the switch at High conductivity because at low it detects the ceramics. At high it detects silver, bronze, copper and rust iron. Normal iron, gold and ceramics at low conductivity. So there is big problem. I have some gold coins at home so i will try with delay 25 and GB -2... but i am afraid that i will have problem with ceramics.
Now about VLF.... i tried them at both modes... all metal and discrimination but without results.

Regards

[Reg]

Hi Geo,

The conductivity of a metal as it relates to the TDI is more complex that making assumptions on all sizes of the metal.

For example, a nail may be a low conductor or a high conductor depending upon the size of the nail and the setting of the ground balance (GB) and delay control. Set the delay for 10 usec and leave it there and then the nail will vary as the result of the GB setting. If the nail is long and large enough, it will respond as a high conductor with the normal GB setting of 8 to 9. Reduce the GB to 3 or 4 and that same nail will now respond as a low conductor.

This same situation can occur with other objects including gold coins. If no gold is available, use pieces of an aluminum ingot. If the aluminum ingot isn't available, then use simple lead. Select different sizes and you will find sizes that will respond as low conductors and others that are big enough will be detected as high conductors.

This characteristic that metals can be either a low conductor or a high conductor on the TDI exists for most metals. As Carl stated, even silver if it is small and/or thin enough can respond as a low conductor.

So, rather than make an assumption as to whether something will respond as a low or a high conductor, it is a good idea to check first. It is very possible, a lot of testing and experimenting will be required before this makes sense. At least it took me a long time to adjust and even now, I get fooled at times.

Now, compound this conductivity issue with the fact that the depth of detection is also altered and the use of the GB and how it causes different things to occur further makes the TDI more complicated that people realize.

Actually, it isn't that complicated, but it can be somewhat tricky if a person is trying to obtain maximum depth of detection.

Reg

[Reg]

Hi Geo,

Are you able to check a singe gold coin to see how it responds on the TDI or do you know the size and weight of the coin? Individual gold coins can vary between low conductor and high conductor by size, weight, and GB setting. A small gold coin such as a $1 US, or a $2 1/2 US will almost always respond as a low conductor. A 1/4 oz which is about a $5 US gold coin can respond as either a low conductor or a high conductor depending upon the GB setting. The US $10 will do the same.

So, a single gold coin can respond as a low conductor or a high conductor depending upon the size, weight and shape. Now, take a bunch of them and toss them into a pile and most likely, the conductivity will not change, especially if the conductivity of a single coin is no where near the setting where it is about to switch from a low conductor to a high conductor.

This is why it is important to know more about the gold one is trying to find. This problem of low conductor or high conductor becomes worse when it comes to gold nuggets. Some places gold nuggets are very solid but more importantly are very pure, while other locations the gold has a higher silver, copper or other metal alloy content. Alloy gold with other metals radically alters the conductivity which then alters the way it will respond.

I used to refer people to a particular website that displayed that gold can change from a high conductor to a low conductor simply by the alloy content. Unfortunately, that website no longer posts this information.

Getting back to gold coins, don't be surprised if a bunch of gold coins respond as a low conductor, especially if a single coin responds as a low conductor. Whether gold responds as a low conductor or a high conductor is not just a matter of the object being gold. Again, size, weight, and shape all are factors as well as the GB setting as to whether gold will respond as a low conductor or a high conductor.

Now, as for the VLF's having a problem, I am assuming you are using the discriminate mode and not the all metal mode when trying to detect those coins. Did you try the all metal mode to see if that mode will detect them? I suggest you try. I would expect that you can still detect the coins in the all metal mode. Let me know if this is true if you don't mind.

Reg