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Battery voltage, coil size and more
Posted by: Reg
Date: March 21, 2018 09:18AM
Depth is directly related to voltage applied and coil size. There is no regulator for the voltage applied to the FET so add voltage to the detector you increase depth to a point.

Increase coil size and you increase depth to a point. Go too big and you will have to increase the delay for the coil to work right. The same goes for increasing the voltage and using a bigger coil together.

A simple rule of thumb is if you use a coil, lets say a 12 inch diameter and the maximum depth of detection is 6 inches, then going larger in size will reduce the depth of detection. In other words, if you can only detect an object to a depth of about 1/2 the diameter, that is as large as you can go size wise. Keep in mind this does not take into account possible loss due to noise.




Re: Battery voltage, coil size and more
Posted by: Jim in Idaho
Date: March 21, 2018 10:03AM
Thanks, Reg. One thing I noticed in my testing was increased voltage didn't help much with the smaller coils. But, it made a big difference with the larger coils. Along with the 12DF, I tested the MJ 5 x 9, and the factory 7.5DF.
Jim

Re: Battery voltage, coil size and more
Posted by: Reg
Date: March 21, 2018 11:27AM
Small coils are not designed to find objects deep but rather to be used in tight places and look for small objects.

Years ago, I used a modified Beachscan which I added my version of GB so I could use if for gold hunting. One trip to AZ, I went to a club I belong to where they operated a large D9 cat to move topsoil for club members so they could hunt new ground. By this time, the ML 4500 was just out, but the TDI didn't exist.

Anyway, I was using a home made coil which was made with a Bill Hays housing, about 3" by 12" in size. I got to the site about 4 days after the push so the site had been beat to death by ML's. I had a few guys kind of chuckle at my detector but that didn't bother me. After about an hour, I got a sweet signal that turned out to be a 8 grain nugget. The nugget was down in a narrow rut made by the blade of the cat.

Some time later a guy came by using a ML3500 and kind of grinned when he looked at my machine and asked if I had any luck. I told him I only had one nugget found and you should have seen the look on his face. It seems that particular push only yielded a few nuggets, so my find was quite rare. Literally dozens of ML's found nothing, but my little home made detector had been successful. Why was I successful? Simple, I was searching the furrows and managed to get down close enough to the nugget to detect it. Later the guy with the 3500 asked if he could detect the nugget so we compared depth of detection and I found I could match his unit with mine. The difference was the coil allowed me to get close enough to the gold.

Just thought you might like a true story.

Reg

Re: Battery voltage, coil size and more
Posted by: Jim in Idaho
Date: March 21, 2018 12:11PM
Great story, Reg! The deal with the extra voltage is not only increased depth, but increased sensitivity, which is I guess, sort of increased depth. With the additional voltage the 12" DF was just as sensitive to the 1 grain nugget as either the MJ, or the 7.5DF. All of them could detect it at 3" in an air test. There was a very slight advantage for the 12"DF. I need to get that test done with the Super Pulse 350, too.
Jim

Re: Battery voltage, coil size and more
Posted by: Reg
Date: March 21, 2018 05:57PM
I finally found the writeup that goes with the chart. Here it is.

For a given object size there comes a point where detection range will fall as coil size is increased. This is illustrated by the accompanying chart. The coil diameter and detection range can be in any units i.e.inches, feet, centimetres provided both scales are the same. All the solid curves to the left of the 45 degree line show increases in range, while the dotted curves to the right show decreases in range. For example take an 11in coil (marked on Coil Diameter scale) and you find that you can detect a small object at just 5.5 inches (A on the Detection Range scale), the intersection falls directly on the 45 degree line and any increase in coil diameter would be on a decending curve giving less range. If, however, you can detect an object at just over 11.5in (B) with the 11in coil, then you can follow the curve up to get increased range with larger coils. A 20in diameter coil will give 14.2in and a 30in coil 15in range. You can see that as the curve approaches horizontal, a big increase in coil size only gives a small increase in range, the difference between a 20in and a 30in coil is less than an inch in range. Continuing along the curve, a 40in coil only gives the same range as a 20in and beyond that it will fall rapidly. These curves do not take into account the fact that larger coils will pick up more noise so that the full potential may not be realised. However they can give an idea as to what improvement is possible. Say you have an 8in coil and you can detect a coin at 12in. Find that intersection on a curve and follow the curve up. You can see that a 15in coil will gain you another 4in; quite significant. The curve maximum is always when the detection range equals the coil radius e.g. 5.5in for an 11in coil.

The chart and write-up were made by Eric Foster and posted in 2000 or 2001 on the PI technology forum.

Reg



Edited 1 time(s). Last edit at 03/21/2018 06:05PM by Reg.

Re: Battery voltage, coil size and more
Posted by: Jim in Idaho
Date: March 21, 2018 07:10PM
Reg, I would really like to have that chart. It didn't show up in your post.
Jim

Re: Battery voltage, coil size and more
Posted by: Reg
Date: March 21, 2018 09:19PM
It's there but in an earlier post dated March 21, 2018 9.18AM.

Reg

Re: Battery voltage, coil size and more
Posted by: Jim in Idaho
Date: March 21, 2018 10:04PM
I didn't realize it was the same chart...thanks! One question, do you use noise as the determining factor when increasing the delay?
jim

Re: Battery voltage, coil size and more
Posted by: stephenscool
Date: March 21, 2018 10:28PM
Bit off topic.. but since depth is related to power to the coil... isn't it possible to put 24v or 36v through the coil. Using ground balance might be deeper than all metal using stock battery. I'm sure some changes to the pc board would need to be made to accommodate this much higher voltage.. . Perhaps even an external battery pack would also dissipate heat

Re: Battery voltage, coil size and more
Posted by: Reg
Date: March 22, 2018 10:18AM
Jim,

I seldom change the delay since most of my hunting is for gold. However, When I hunt for silver coins, I will advance the delay to eliminate masking by foil. VLF's suffer greatly from masking, thus tons of coins are easily within range of a good PI.

When I first tried a PI for coin hunting, it was with the GS5 after I created the first conductivity switch before the TDI existed. It was only after I found a large gold ring did Eric Foster try my conductivity switch and recommend it be added to the TDI.

As for noise, there are two kinds. Noise created externally and noise existing internally. The external noise should be dealt with the frequency control. Internal noise will take part replacement after redesigning.

Reg

Re: Battery voltage, coil size and more
Posted by: Jim in Idaho
Date: March 22, 2018 11:47AM
OK....many thanks, Reg.
Jim

Re: Battery voltage, coil size and more
Posted by: Reg
Date: March 22, 2018 01:47PM
stephenscool,

4 liion cell is about max voltage the SL can handle safely and not let the smoke out. Once the smoke is out, the fun and expensive repair work begins.

Reg

Re: Battery voltage, coil size and more
Posted by: stephenscool
Date: March 22, 2018 05:59PM
Quote
Reg
stephenscool,

4 liion cell is about max voltage the SL can handle safely and not let the smoke out. Once the smoke is out, the fun and expensive repair work begins.

Reg
Hi Reg,
I was referring to 1 of the older models. Just out curiosity/experimentation have the unit run a massive voltage. Tinker with the board to not fry it. It might be possible to run it in hi or low conductivity with impressive depths.
Not,sure how easy it would be to mod the board to accept the extreme voltage though

Re: Battery voltage, coil size and more
Posted by: Reg
Date: March 22, 2018 07:05PM
The original TDI is running on a 5 cell setup now if they are using the Reidman special battery, but it is marginal at best. Use a ML coil along with the special battery and you are really pushing it. Personally, I expect failures using the ML designed coil along with the 5 cell battery.

Running in high, low or all has nothing to do with power applied or current used except for the little difference caused by the audio and that isn't much.

Reg

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