Jump to content

Effects Of Ground Balance On Target Depth?


Recommended Posts

Yes this on and off GB can mess things up quite a lot, Doing side by side tests with the TDI Pro and the TDI SL really had me wondering what the devil I had done when buying the SL, Having done the GB off thing with the Pro and being impressed with the Air test results and then doing the same with the SL gave me an "Oh dear" moment,

In that video where the guy on the beach test many machines you will see the Pro with the GB off actually smooth out the ground which is great if you have no bad coil control habits, and what the pro is doing is sort of self adjusting to a happy medium as long as your sweep is controlled,  But with the SL if you turn the GB off the SL will loose depth and there is none of that smoothing effect that the Pro has running the same settings, But on the SL if you turn the GB on it actually increases the depth and does that smoothing thing that the pro does with the GB off, 

With the SL if you wave a target under the coil "IE" it will give a faint signal on a quarter at about 16" but if you wave it 3 or 4 times the signal will get fainter and now it will only signal at around 12 to 13 and a half inches, ( Don't worry ) 

Now with the SL that drop gave me concern but when you are sweeping the coil over the ground not Air Testing The SL is actually focusing on the ground so when it see's the Coin the coin signal stays and the SL smooth's out the ground,

Back to the Air test,,,, So you have swept your coin in front of the coin and got a signal at 16" and on the second sweep it is not so sharp, 3rd and 4th sweep your back down to 13" +/-  Now hold the coin about 16-18 inches in front of the coil Now bring the coin straight up to the middle of the coil so it makes a loud tone and rapidly pull it back away again, "Now" sweep the coin in front of the Coil again and it will signal again at 16", because when you put the coin right up to the coil is overloads it and makes it reset to a different type of signal and it will do this all the time that the Coil can not see the ground even though it has manual GB, When you Overload the SL with the GB On and do the same with the Pro with the GB off when I measure the distances there is less than 1/4 to 1/2 difference between the too,  The SL is murder to Air Test and because people do not know about the overload trick and smoothing effect they tend to think that the pro is a lot deeper,

So I hope this helps but remember the SL must see the ground with the GB On to maintain maximum depth and in an Air test it will smooth out about 3 or 4 inches because it is focussing on the Coin and has nothing to deduct the coins signal from.

 

hope this helps,

John.

  • Like 2
Link to comment
Share on other sites


All of the three or more TDI SL detectors I have used all gain depth in air tests by turning the ground balance off. Considering the subtractive nature of how the ground balance works I fail to see how it can be otherwise. What you are saying John also directly contradicts the owners manual, page 14:

Ground Balancing

One of the key features of TDI is its ability to balance out ground mineralization while maintaining pulse induction depths. This is achieved using a differential technique whereby two decay samples are compared; the Ground Balance control is adjusted until the two samples have the same response, thereby canceling the ground signal.

Targets below the GB point produce an overall positive response, and targets above the GB point produce an overall negative response. Broadly, the two ranges of targets depend on conductivity, which gives TDI some ability to discriminate. In rare cases a target might fall so close to the GB point that it gives either no response at all or a double-tone response.

Because ground balance is achieved using a subtraction method, there is a slight loss of depth any time ground balance is on. Some ground, notably neutral beach sand, is mild enough that no ground balance is needed. In such cases, Ground Bal- ance can be switched off and TDI will run in straight PI mode. This will slightly improve depth, but loses the ability to distinguish between low and high conductivity targets. 

Link to comment
Share on other sites

Yes that was my understanding too, but for some reason my SL is back to front, , It does/has caused me some head scratching so much so I showed it to RickUK when he came round one day, and the result of comparing the 2 TDI's I have stopped using the SL, It has the power but it goes against all of what we know about TDI's in general I still have it but I can't sell it knowing what is does and I have stopped using it for over a year now.

John

Link to comment
Share on other sites

Well I Apologize if my post my post appears misleading but I phoned whites in Scotland and they know very little about the machine and found my self clutching at straws So I just gave up with it and assumed that is how it is meant to behave, and it is not worth paying two lots of shipping to and from the US to get it resolved,

John.

Link to comment
Share on other sites

3 hours ago, Steve Herschbach said:

Well, if your machine is defective using it as an example to others of how a normal TDI SL should behave is not very helpful.

Steve, if Johns unit is defective, than so is mine. My testing results, during my  battery voltage tests, exactly mimic John's machine. There's a definite improvement in depth with the GB on, even if it's set at it's lowest level. And, with the 16.8 v battery pack, my SL can clearly see a  1 grain nugget at 3" with the 12" DF coil.

Jim

Link to comment
Share on other sites

Some data with a PI I've been playing with. The timings were adjusted to cancel ground signal. Signal strength for the quarter is higher with ground balance on. Noise increased with ground balance on so detection depth is lower with the nugget and nickel and about the same for the quarter. If noise stayed the same, detection depth the same for the nugget, less for the nickel, increase depth for the quarter. Double signal strength increases detection depth about 1inch. Different timings different results. 5975fac63940a_GEB_QNNugget.jpg.b98059b2a8d9e7755c208206e44d130d.jpg

  • Like 2
Link to comment
Share on other sites

The technique used to ground balance a TDI so that it can ignore or cancel the ground mineral signals is done by taking multiple signal samples over a period of time. The later signals are amplified such that the signal from the ground in the later sample equals the main sample and the latter sample is subtracted from the main sample. The result is the ground signal appears to go away. What also goes away is part of the signal from many targets.

If the main sample is the full signal, and the basis of results for ground balance off, and then you subtract from that by engaging the ground balance to subtract the results of a later sample - how exactly is this subtraction of part of the signal adding signal strength to targets like a quarter?

Perhaps long time constant targets like a quarter have not decayed appreciately by the time the later samples are taken, and the amplification process is boosting the signal for long time constant targets over what was achieved in the original sample?

Jim, the TDI SL comes with a 12V battery. Do you have a modded version of the detector?

I sure am not going to dispute what you guys are saying. I admit a nickel is my standard test item and so I have not observed what happens on a quarter on the TDI SL with GB both on and off. It just highlights to me that there are multiple versions of the TDI in play and their performance characteristics are not consistent across all versions.

  • Like 1
Link to comment
Share on other sites

2 hours ago, Steve Herschbach said:

Perhaps long time constant targets like a quarter have not decayed appreciately by the time the later samples are taken, and the amplification process is boosting the signal for long time constant targets over what was achieved in the original sample?

Exactly, assume the target signal didn't decay and the second sample needed a gain of four to cancel the ground signal. The subtracted target signal would be minus 3times the target sample.

  • Like 2
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...