TDI LOW DELAY

[Jim in Idaho]

There are a couple of things going on there. What's called the pulse delay is actually the delay before the "listening" phase. so to speak. That's important, but so is the transmitted pulse width. Any good PI can see a 1 grain gold nugget. The question is, how far can it detect it? The reason a short delay helps is that unlike ferrous metals, the magnetic field around anon-ferrous targets deteriorates really quickly. So, the sooner you start "listening" for those targets, the better you can detect them. But, if that delay gets too short, you start picking up interference from the transmitted pulse.
Jim

[eclipse]

I measured the TDI SL delay and it's not even 10us, more closely to 11us; The Sierra Pulse Pro (SSP) version is even worse - 12uS

[Qiaozhi]

Thanks for the reply. Was thinking 1grain of gold would have a time constant 1us or less and would be hard to detect with a 10us delay. I had looked at a couple sites that sell metal, didn't have 1grain gold bars. Like you takes about 30 seconds to find them if I search for 1grain gold bars. Some times I over look the obvious when using a computer. I keep wondering if I haven't asked you the same question in the past, apologize if I have. Found a formula that I had written down, calculates 2.5us time constant for the 1grain gold bar(3x6x.186mm). Maybe someone could calculate the time constant for a 1grain gold bar. 2.5us time constant would make it easier to detect.

Using this equation:



where:




If a 1 grain gold bar is 3 x 6 x 0.186mm, then the volume = 3.348 mm³.
The equation assumes the material is in the shape of a sphere, and a sphere with a volume of 3.348 mm³ has a radius of 0.928 mm.
Using this value for the radius gives:



I also found some different dimensions for a 1 grain gold bar at https://bsmclient.com/product/prod25/
Click on "Additional information" to get the size.
The dimensions were: 6 x 0.125 x 4 mm, which calculates out as a time constant of 4.18us.

You have to bear in mind though that this calculation is for pure gold, and would probably not be representative of a gold nugget of a similar size.

[Qiaozhi]

I've heard people say that lead is a good substitute as a gold nugget target.
This may be so, as the conductivity of lead is 4.55 x 10⁶ Sm^-1, which is nearly a factor of 10 lower than gold.
If true, then a lead sphere with a radius of 0.928mm would have a time constant of only 499 ns (~0.5 us).

According to Jim: Any good PI can see a 1 grain gold nugget.
which implies that an approximate conductivity value must lie somewhere between pure gold and lead.

[waltr]

I've heard people say that lead is a good substitute as a gold nugget target.
This may be so, as the conductivity of lead is 4.55 x 10⁶ Sm^-1, which is nearly a factor of 10 lower than gold.
If true, then a lead sphere with a radius of 0.928mm would have a time constant of only 499 ns (~0.5 us).

According to Jim: Any good PI can see a 1 grain gold nugget.
which implies that an approximate conductivity value must lie somewhere between pure gold and lead.

Yes, gold alloy has a much lower conductivity than pure gold. Even 22K is way less than 24K. Natural gold nuggets are typically is the 10 to 20K range.

[Carl-NC]

I measured the TDI SL delay and it's not even 10us, more closely to 11us; The Sierra Pulse Pro (SSP) version is even worse - 12uS

What signals are you comparing?

[eclipse]

What signals are you comparing?

Here I have a SPP (TDI SL with the only difference - fixed primary delay at 12uS). Scope is 2uS/DIV

In my book the TX is MCU PIN6, the Sample1 delay is PIN26. The test points I use are (TP1) on the board which is TX, R37 on the board (MCU PIN26) which is Sample1.

I'm working on a new chip board addon which will take care of the delay problem, probably adding auto min delay depending on coil /starting 8uS/ so the initial delay will be auto-calibrating during search.
Must be on a new board because the pin out is QFN64 and requires a bit components soldered, and the -5V_D regulator will be replaced with -3.5V regulator. I'm also thinking of giving the TX_WIDTH option some breath, this will adjust the TX width up to 120uS with the frequency ~3Khz and below to stay in the proper range of TX cycle and if the pot is turned >50%, the frequency will go lower(600-800Hz) and pulse width - 120-200uS. This could be used for hoard/relic hunting with large coils etc. In this case also a mod of the 2x470K in the SAT stage will be upgraded with a dual pot 1M+470K so the SAT can be adjusted properly /large coils = slower swing/

[daverave]

someone who was involved with the technical development of the tdi series told me the sample was set at 12/13 uS and not 10 uS which was quite a supprise

[Carl-NC]

As I recall the delay between the clock pulses is ~11us but the delay between the actual coil turn-off and the sample pulse is close to 10us.

[green]

As I recall the delay between the clock pulses is ~11us but the delay between the actual coil turn-off and the sample pulse is close to 10us.

What is actual coil turn-off? Picture I posted awhile back, turn-off command at 580us. What time do you consider actual coil turn-off?

[Carl-NC]

The start of current shut-down, which would be the 580us mark in the sim.

[Elliot]

ive been using a low delay of 8 uS on a modified TDI and to be honest ive found it a draw back on the beach on the slightly wet dry sand...the detector picks up very tiny bits of foil and other things which sometimes are hard to see with the eye...so im thinking maybe now its best to set the delay at around 10 uS or upwards as digging these tiny bits of foil are a real pain.

Using a very low sample delay is a good thing, as it means you can also find deeper objects that generate a faint signal. Those small bits of foil etc could just as easily be a deeper gold earing or nose stud. But, I do know what you mean about the amazing sensitivity of small objects at 8uS - Check out the video at around 12:40 - don't mind the dog, it wasn't a killer! https://youtu.be/1b__o20ziJw

[daverave]

are you getting much ground effect at 8 uS on very wet sand ???

[Elliot]

It depends on the beach. Whites seemed to suggest that the delay had to be at least 15uS on wet sand, but that has not been my experience with the Pro Pi Aqualand at all. I suspect a lot of that folk law has more to do with the extra effort/cost of building a machine that can operate down to 8uS. You can see from the YouTube video, there was no trouble at all with the head in salt water or on wet sand.

[bbsailor]

It depends on the beach. Whites seemed to suggest that the delay had to be at least 15uS on wet sand, but that has not been my experience with the Pro Pi Aqualand at all. I suspect a lot of that folk law has more to do with the extra effort/cost of building a machine that can operate down to 8uS. You can see from the YouTube video, there was no trouble at all with the head in salt water or on wet sand.

A simple way to tell if wet sand may be a problem is to put a coin sized magnet in a plastic bag, attach a string to the bag, and throw it into any black sand you see to tell if it is metallic based or organic based. The plastic bag just makes cleaning the metallic stuff off the magnet easier and keep it clean for the next test.

Joseph J. Rogowski