A
Anonymous
Guest
In fact, I was there for rather longer than that, in spite of the weather not being as warm as forecast. My purpose was to see if one could use even shorter delays on wet sand and even in the water
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A
Anonymous
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Eric,
I'll bet you had to do some pretty creative things to get the coils to break the 5uS barrier. I can only speculate about the coil parameters. With the best coax you have about 120pf in the 2.125m coax; about 100pf in the coil-to-shield capacitance, with a self resonance somewhere between 600 and 650 Khz on a 300 uH inductance coil. I would certainly like to know the coil details on this super fast machine? I had to be very careful with reducing all capacitance to get my coil working a 7.33 uS.
In previous posts you mentioned that about 10 uS is about the fastest practical salt water beach speed due to the response of the salt water. Now that you have worked this new PI machine at 5 uS, is it possible that we will be seeing a new breed of beach/prospecting machines?
Congrats on breaking the 5 micro-second barrier.
bbsailor
I'll bet you had to do some pretty creative things to get the coils to break the 5uS barrier. I can only speculate about the coil parameters. With the best coax you have about 120pf in the 2.125m coax; about 100pf in the coil-to-shield capacitance, with a self resonance somewhere between 600 and 650 Khz on a 300 uH inductance coil. I would certainly like to know the coil details on this super fast machine? I had to be very careful with reducing all capacitance to get my coil working a 7.33 uS.
In previous posts you mentioned that about 10 uS is about the fastest practical salt water beach speed due to the response of the salt water. Now that you have worked this new PI machine at 5 uS, is it possible that we will be seeing a new breed of beach/prospecting machines?
Congrats on breaking the 5 micro-second barrier.
bbsailor
A
Anonymous
Guest
Hi Eric,
One thing you may not have given to much thought to since the main thrust with PI's is thier operation in the slatwater environment is that there are many of us here in the States that hunt Freshwater Lakes as well as Ocean and Bay Beaches. So if you made machines with 5us or even shorter pulse delays there would be a market for them even if the shorter pulse delay went beyond what could be effectively used on an Ocean Beach. For all practical purposes whatever the shortest pulse delay that will work on land will also work in a Freshwater lake. And as you mentioned the shorter the pulse the delay the better for detecting gold chains, toe rings, bracelets, and gold earringsm gold nuggets and other small targets. Glad to hear that the new circuitry and coil worked out so well for you on it's first trip to the beach!!!!
HH
Beachcomber
One thing you may not have given to much thought to since the main thrust with PI's is thier operation in the slatwater environment is that there are many of us here in the States that hunt Freshwater Lakes as well as Ocean and Bay Beaches. So if you made machines with 5us or even shorter pulse delays there would be a market for them even if the shorter pulse delay went beyond what could be effectively used on an Ocean Beach. For all practical purposes whatever the shortest pulse delay that will work on land will also work in a Freshwater lake. And as you mentioned the shorter the pulse the delay the better for detecting gold chains, toe rings, bracelets, and gold earringsm gold nuggets and other small targets. Glad to hear that the new circuitry and coil worked out so well for you on it's first trip to the beach!!!!
HH
Beachcomber
A
Anonymous
Guest
Hi bbsailor,
The 5uS is measured from the start of the transmitter switch off, so it includes the high voltage flyback time. The scope was externally triggered by the drive pulse to the Mosfet.
The shielded coil plus cable resonant frequency is 555kHz. This is measured by inducing a pulse from another coil, so it does not include any extra capacitance contribution from the electronics.
Coil inductance is 344uH and coil resistance 3.8 ohms, inclusive of cable.
The transmitter pulse is 20uS wide and the two sample pulses 5uS wide, when running at the 5uS delay. TX p.p.s. is 20,000. At the other end of the delay control, the TX is 60uS wide, with delay and samples at 15uS, and p.p.s. is 6500.
The considerable reduction in useable sample pulse delay is the sum of attention to three areas. 1) the coil 2) transmitter output and drive circuits 3) front end amplifier. At this point in time, this detector is purely experimental, just to see if the limits can be pushed out a bit further.
Eric.
The 5uS is measured from the start of the transmitter switch off, so it includes the high voltage flyback time. The scope was externally triggered by the drive pulse to the Mosfet.
The shielded coil plus cable resonant frequency is 555kHz. This is measured by inducing a pulse from another coil, so it does not include any extra capacitance contribution from the electronics.
Coil inductance is 344uH and coil resistance 3.8 ohms, inclusive of cable.
The transmitter pulse is 20uS wide and the two sample pulses 5uS wide, when running at the 5uS delay. TX p.p.s. is 20,000. At the other end of the delay control, the TX is 60uS wide, with delay and samples at 15uS, and p.p.s. is 6500.
The considerable reduction in useable sample pulse delay is the sum of attention to three areas. 1) the coil 2) transmitter output and drive circuits 3) front end amplifier. At this point in time, this detector is purely experimental, just to see if the limits can be pushed out a bit further.
Eric.
A
Anonymous
Guest
Hi Beachcomber,
No, I hadn't really thought of freshwater use, but as you say, there may be an application there for the higher sensitivity. There is no signal at all from freshwater, so the limitations of ocean beaches do not apply. The only problem that may arise is with iron trash, as this does not rust away as fast as in salt water. From what I hear, many freshwater sites are heavily contaminated.
Eric.
No, I hadn't really thought of freshwater use, but as you say, there may be an application there for the higher sensitivity. There is no signal at all from freshwater, so the limitations of ocean beaches do not apply. The only problem that may arise is with iron trash, as this does not rust away as fast as in salt water. From what I hear, many freshwater sites are heavily contaminated.
Eric.
A
Anonymous
Guest
Here is a picture of one of the nuggets that is very difficult to detect. As can be seen. the gold is fragmented and spongey. The 0.4gm solid nugget is for comparison. Magnification is x10 and taken with a Digital Blue computer microscope, which plugs into a USB port.
<img src="http://www.findmall.com/gallery/albums/userpics/10976/Invisible nugget 1.jpg" alt="" />
Here is a x60 picture which gives more detail of the difficult nugget.
<img src="http://findmall.com/gallery/albums/userpics/10976/Invisible nugget 2.jpg" alt="" />
Eric.
<img src="http://www.findmall.com/gallery/albums/userpics/10976/Invisible nugget 1.jpg" alt="" />
Here is a x60 picture which gives more detail of the difficult nugget.
<img src="http://findmall.com/gallery/albums/userpics/10976/Invisible nugget 2.jpg" alt="" />
Eric.
A
Anonymous
Guest
Hi Eric,
Actually the freshwater lakes aren't that bad. People have been hunting them with PI's as well as vlf's for years and the PI's take care of most of the iron. Every now and then I do dig up a deep very rusty bobby pin or nail but that isn't to often.
But a high power PI with the capability of a very short pulse delay could finally give the PI's an edge over the vlf's in the freshwater environment especially in some of the mineralized lakes that severely reduce the the VLF's depth. The currently manufactured detectors leave a great deal of gold behind. Some of us have converted the higher frequency gold detectors to freshwater use. Machines like the GB2 and the Lobo ST and find an amazing amount of gold in heavily hunted lakes. Even so mineralization can become a limiting factor for the VLF's.
I wish you continued success in your research on short delay PI's and look forward to hearing more on the subject!!!
HH
Beachcomber
Actually the freshwater lakes aren't that bad. People have been hunting them with PI's as well as vlf's for years and the PI's take care of most of the iron. Every now and then I do dig up a deep very rusty bobby pin or nail but that isn't to often.
But a high power PI with the capability of a very short pulse delay could finally give the PI's an edge over the vlf's in the freshwater environment especially in some of the mineralized lakes that severely reduce the the VLF's depth. The currently manufactured detectors leave a great deal of gold behind. Some of us have converted the higher frequency gold detectors to freshwater use. Machines like the GB2 and the Lobo ST and find an amazing amount of gold in heavily hunted lakes. Even so mineralization can become a limiting factor for the VLF's.
I wish you continued success in your research on short delay PI's and look forward to hearing more on the subject!!!
HH
Beachcomber
A
Anonymous
Guest
Hi,
Short delays below 5us are possible...
Here is a scope shot of my DIY home-brew mono coil.
The measure point was taken at the damping resistor of the electronic. The TX pulse was 80us and the coil cable is 2m long.
The coil parameters are 310uH, 10 inch diameter, 20 wdg.
Chris
Short delays below 5us are possible...
Here is a scope shot of my DIY home-brew mono coil.
The measure point was taken at the damping resistor of the electronic. The TX pulse was 80us and the coil cable is 2m long.
The coil parameters are 310uH, 10 inch diameter, 20 wdg.
Chris
A
Anonymous
Guest
My most sensitive PI machine is a modified C-Scope CS6PI operating at 13.6 K PPS, about twice the normal frequency range. I added a 4-pin microphone connctor, that just fits the existing hole, to allow easy coil changes.
I found that some coils would only start to work at about the 9 o'clock dial position (with 7 o'clock or fully CCW being the highest speed). Then by doing careful measurements and reviewing all of Eric's and Reg's postings about coils I managed to better understand coil dynamics as it relates to speed.
I have wound many coils, some with plain magnet wire; AWG 22, 24, and 26 stranded hook up wire; served litz wire; and one of the best coils is made from a 50 ft spool of Radio Shack Kynar coated AWG30 wire wrap. 18 turns will just fit on a 10.5" diameter coil form, which is designed to fit inside a Hays Electronics 11" round coil housing. For shielding, first I wrap the wire bundle with dental floss, then a single layer of electrical tape the a 1/4" OD spiral wrap as a spacer to reduce coil-to-shield capacitance. The final shield of 3M 1190 copper fabric tape is wound around the spiral wrap. I have about 100 pf shield-to-coil capacitance. Here, lower is better.
I only use about 32" of coax between the coil housing and the CS6PI control box. I was lucky enough to have about 25 ft of old "CZ Connector and Cable 8/U" blue coax cable that is not detected by the coil at the fastest PPS speed. Neither is the 3M 1190 copper fabric tape shield able to be detected. I tried many types of coax and lucked out with this.
The real test of coil quality is the free resonance when measured by a pulsed square wave being induced into the coil under test by another coil connected to the pulse generator. Scope probes add capacitance to the resonant circuit, which is very sensitive to even small capacitive loading at resonance. Therefore, measuring the induced coil self resonance, as Eric mentioned, is a more accurate way to do the measurement. Measure the the coil, shield and coax together for self resonance and use this as a figure of merit when comparing coils. I have gotten some of my 320uH coils to be in the 600 to 625khz range. Higher is better! The self resonance then translates into the optimum value of the damping resistor. Again the high value is better, meaning there is less capacitance or stored energy needing to be damped.
One interesting test you can do is to compare the free resonance of the coil to the ringing resonance of the coil connected to the PI machine with the damping resistor removed. Here you can see the effect of the PI circuitry on the coil resonance. MOSFETS typically add the most circuit capacitance.
What is the self resonance of your coil operating at less then 5 uS?
bbsailor
I found that some coils would only start to work at about the 9 o'clock dial position (with 7 o'clock or fully CCW being the highest speed). Then by doing careful measurements and reviewing all of Eric's and Reg's postings about coils I managed to better understand coil dynamics as it relates to speed.
I have wound many coils, some with plain magnet wire; AWG 22, 24, and 26 stranded hook up wire; served litz wire; and one of the best coils is made from a 50 ft spool of Radio Shack Kynar coated AWG30 wire wrap. 18 turns will just fit on a 10.5" diameter coil form, which is designed to fit inside a Hays Electronics 11" round coil housing. For shielding, first I wrap the wire bundle with dental floss, then a single layer of electrical tape the a 1/4" OD spiral wrap as a spacer to reduce coil-to-shield capacitance. The final shield of 3M 1190 copper fabric tape is wound around the spiral wrap. I have about 100 pf shield-to-coil capacitance. Here, lower is better.
I only use about 32" of coax between the coil housing and the CS6PI control box. I was lucky enough to have about 25 ft of old "CZ Connector and Cable 8/U" blue coax cable that is not detected by the coil at the fastest PPS speed. Neither is the 3M 1190 copper fabric tape shield able to be detected. I tried many types of coax and lucked out with this.
The real test of coil quality is the free resonance when measured by a pulsed square wave being induced into the coil under test by another coil connected to the pulse generator. Scope probes add capacitance to the resonant circuit, which is very sensitive to even small capacitive loading at resonance. Therefore, measuring the induced coil self resonance, as Eric mentioned, is a more accurate way to do the measurement. Measure the the coil, shield and coax together for self resonance and use this as a figure of merit when comparing coils. I have gotten some of my 320uH coils to be in the 600 to 625khz range. Higher is better! The self resonance then translates into the optimum value of the damping resistor. Again the high value is better, meaning there is less capacitance or stored energy needing to be damped.
One interesting test you can do is to compare the free resonance of the coil to the ringing resonance of the coil connected to the PI machine with the damping resistor removed. Here you can see the effect of the PI circuitry on the coil resonance. MOSFETS typically add the most circuit capacitance.
What is the self resonance of your coil operating at less then 5 uS?
bbsailor
A
Anonymous
Guest
Hi Chris,
What does it look like when you hang an amplifier with a gain of, say, 450x across the damping resistor? There are often low level effects that are not apparent when looking across the damping resistor. My 5uS unit has a spike that returns to zero in 1.5uS, if I just look across the resistor. After amplification, there is an additional 2.5uS of low level ringing, plus the recovery time of the amplifier. I then leave a safety margin of 1uS before sampling.
Eric.
What does it look like when you hang an amplifier with a gain of, say, 450x across the damping resistor? There are often low level effects that are not apparent when looking across the damping resistor. My 5uS unit has a spike that returns to zero in 1.5uS, if I just look across the resistor. After amplification, there is an additional 2.5uS of low level ringing, plus the recovery time of the amplifier. I then leave a safety margin of 1uS before sampling.
Eric.
A
Anonymous
Guest
Here is the unit with the 8in coil.
<img src="http://www.findmall.com/gallery/albums/userpics/10976/Micro 5. 1.jpg" alt="" />
A closer shot, showing the delay control with 1uS per division on the scale from 5uS to 15uS. The unlabelled control is the receiver gain. SAT, filter and audio volume are on the opposite side.
<img src="http://www.findmall.com/gallery/albums/userpics/10976/Micro 5. 2.jpg" alt="" />
Eric.
<img src="http://www.findmall.com/gallery/albums/userpics/10976/Micro 5. 1.jpg" alt="" />
A closer shot, showing the delay control with 1uS per division on the scale from 5uS to 15uS. The unlabelled control is the receiver gain. SAT, filter and audio volume are on the opposite side.
<img src="http://www.findmall.com/gallery/albums/userpics/10976/Micro 5. 2.jpg" alt="" />
Eric.
A
Anonymous
Guest
<img src="/metal/html/biggrin.gif" border=0 width=15 height=15 alt="
">
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A
Anonymous
Guest
Hi,
The coil-decay scope shot of was taken with connected electronic. The amplifire gain was 120.
The coil resonance frequence of my coil, I can't remember for sure, but think it was arround 550kHz...haven't wrote it down. It's quite a while ago when I did the testing. I used Reg's suggested methode to find how to measure the coil resonance frequence.
Chris
The coil-decay scope shot of was taken with connected electronic. The amplifire gain was 120.
The coil resonance frequence of my coil, I can't remember for sure, but think it was arround 550kHz...haven't wrote it down. It's quite a while ago when I did the testing. I used Reg's suggested methode to find how to measure the coil resonance frequence.
Chris
A
Anonymous
Guest
Oops...the last sentens must be ....
....I used Reg's suggested methode how to measure the coil resonance frequence.
Chris
....I used Reg's suggested methode how to measure the coil resonance frequence.
Chris
A
Anonymous
Guest
Hi,
Beachcomber, don't forget the really thin section rings, with the 1ct plus rocks attached. I've found that these rings are about, but very very hard to detect at depth.
I think it may have something to do with the way the ring usually splays out to accommodate especially large solitaires, the "ringing" effect of the ring is markedly reduced.
I'm sure a 5uS unit would get them without relying on the amplification induced by the closed ring geometry.
Great work, and info Eric.
Kev.
Beachcomber, don't forget the really thin section rings, with the 1ct plus rocks attached. I've found that these rings are about, but very very hard to detect at depth.
I think it may have something to do with the way the ring usually splays out to accommodate especially large solitaires, the "ringing" effect of the ring is markedly reduced.
I'm sure a 5uS unit would get them without relying on the amplification induced by the closed ring geometry.
Great work, and info Eric.
Kev.
A
Anonymous
Guest
I agree those thin diamond rings are tough but nice when you do find them!!!!
HH
Beachcomber
HH
Beachcomber
A
Anonymous
Guest
Hi Kev,
Using the 8in coil, I can air detect the single gapped link at 4in at 5uS. At 10uS it is undetectable. Below is the gold chain it was taken from. Not a particularly fine chain as the link wire is 1.25mm diameter, but it shows what a difference a few uS make.
Eric.
Using the 8in coil, I can air detect the single gapped link at 4in at 5uS. At 10uS it is undetectable. Below is the gold chain it was taken from. Not a particularly fine chain as the link wire is 1.25mm diameter, but it shows what a difference a few uS make.
Eric.
A
Anonymous
Guest
Without a discriminator or even with that, we will be digging iron needles or 1 mm radius aluminium foils all day and we'll need sand filters not sand scoops...
Luca
Luca
A
Anonymous
Guest
<img src="/metal/html/cry.gif" border=0 width=40 height=15 alt=":cry">
A
Anonymous
Guest
Now that's what I like to see and hear <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> There are quite a few Diamond Post Earings out there with some nice size diamonds in them!!!! <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> It would be nice to have a PI that could see them. That open link has just a little more metal than the Post Earrings!
HH
Beachcomber
"> There are quite a few Diamond Post Earings out there with some nice size diamonds in them!!!! <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> It would be nice to have a PI that could see them. That open link has just a little more metal than the Post Earrings!HH
Beachcomber