Pulse Detector Depth Proformance

[daverave]

ok. it was Eric answer done in Findmall forum. i repeat it. with text.

[ATTACH]39106[/ATTACH]

Re: Question on coils
Posted by: Eric Foster
Date: May 18, 2005

Here are the curves I have used for many years. The range reaches a maximum when it is equal to the radius of the coil. Coils larger or smaller than this optimum will result in less range. To show how this works, along the bottom axis you see coil diameter, which is obviously 2 x the radius. So for an 11in coil, if we go up the vertical scale to A, we have 5.5in. Also note the diagonal line and the series of ever increasing semicircles. Everything to the left of this line shows increasing detection range up to the maximum where it intersects the line, then decreasing range to the right, where the semicircles are shown dashed.
If a certain metal object is just detected at 5.5in with the 11in coil, then going larger in coil size will cause a reduction (going down the dashed side), and going smaller in coil size will have a similar effect. Initially, it won’t be much, i.e. going from an 11in to an 8in coil will only make 0.5in difference but below 4in diameter, the range will drop rapidly.
Now, suppose with the 11in coil, you can detect an object at about 12.5in (B on the vertical scale. This indicates that the coil is not an optimum size for that particular object. If we carry on up the curve (direction of arrow) we can see that by using a 20in coil, we could gain another 2.5in (C). The curve peaks at 15in with a 30in coil. But the extra inch gained hardly makes such an unwieldy coil worth while.
Other factors come into play of course. The curves assume that the number of turns and the coil current is the same in all cases; which it isn’t necessarily. For the same inductance value, a smaller coil has more turns, which counteracts to some degree the loss in range. Also a smaller coil will pick up less electromagnetic noise, earth’s field noise and ground effect, which make for a smoother threshold.
The end result is, that with a small nugget that can be detected at between 5 and 7in with the 11in coil, so that it is on the top part of the curve, an 8in coil may well give a similar range. That is not to say that smaller coils do not have other advantages. Small coils and probes are very useful in rocky areas or searching in undergrowth. They have less drag too for water hunting, and less pickup from mineralised soil or conductive sea water plus better signal separation on close or multiple objects.
One other point regarding PI, is that the small object sensitivity is largely determined by the sample pulse delay. If an object is so small, or thin, or made of high grade stainless steel, such that all the signal has decayed before sampling takes place, it would not matter how small a coil you made, it would never be picked up.
Eric.

Hi kt 315....thank you for posting this info...lots of food for thought in this artical...the part about small object sensitivity largely determined by pulse delay is very true...ive been trying to get my project up and running again and i noticed at around 15 uS delay to reject salt water my sensitivity to small items has decreased somewhat...so i guess its all swings and roundabouts with metal detectors.

[green]

ok. it was Eric answer done in Findmall forum. i repeat it. with text.

[ATTACH]39106[/ATTACH]

Re: Question on coils
Posted by: Eric Foster
Date: May 18, 2005

Here are the curves I have used for many years. The range reaches a maximum when it is equal to the radius of the coil. Coils larger or smaller than this optimum will result in less range. To show how this works, along the bottom axis you see coil diameter, which is obviously 2 x the radius. So for an 11in coil, if we go up the vertical scale to A, we have 5.5in. Also note the diagonal line and the series of ever increasing semicircles. Everything to the left of this line shows increasing detection range up to the maximum where it intersects the line, then decreasing range to the right, where the semicircles are shown dashed.
If a certain metal object is just detected at 5.5in with the 11in coil, then going larger in coil size will cause a reduction (going down the dashed side), and going smaller in coil size will have a similar effect. Initially, it won’t be much, i.e. going from an 11in to an 8in coil will only make 0.5in difference but below 4in diameter, the range will drop rapidly.
Now, suppose with the 11in coil, you can detect an object at about 12.5in (B on the vertical scale. This indicates that the coil is not an optimum size for that particular object. If we carry on up the curve (direction of arrow) we can see that by using a 20in coil, we could gain another 2.5in (C). The curve peaks at 15in with a 30in coil. But the extra inch gained hardly makes such an unwieldy coil worth while.
Other factors come into play of course. The curves assume that the number of turns and the coil current is the same in all cases; which it isn’t necessarily. For the same inductance value, a smaller coil has more turns, which counteracts to some degree the loss in range. Also a smaller coil will pick up less electromagnetic noise, earth’s field noise and ground effect, which make for a smoother threshold.
The end result is, that with a small nugget that can be detected at between 5 and 7in with the 11in coil, so that it is on the top part of the curve, an 8in coil may well give a similar range. That is not to say that smaller coils do not have other advantages. Small coils and probes are very useful in rocky areas or searching in undergrowth. They have less drag too for water hunting, and less pickup from mineralised soil or conductive sea water plus better signal separation on close or multiple objects.
One other point regarding PI, is that the small object sensitivity is largely determined by the sample pulse delay. If an object is so small, or thin, or made of high grade stainless steel, such that all the signal has decayed before sampling takes place, it would not matter how small a coil you made, it would never be picked up.
Eric.

Thanks kt
Think I'm reading the chart right for increase in distance with a larger diameter coil. Can just detect a coin at 12 inches with a 8 inch coil, can detect the coin at 16 inches with a 16 inch coil. Is there away to determine the detection distance with the 16 inch coil for a nugget that can just be detected at 4 inches with the 8 inch coil?

[green]

Thanks kt
Think I'm reading the chart right for increase in distance with a larger diameter coil. Can just detect a coin at 12 inches with a 8 inch coil, can detect the coin at 16 inches with a 16 inch coil. Is there away to determine the detection distance with the 16 inch coil for a nugget that can just be detected at 4 inches with the 8 inch coil?

Reply #60 has a chart for estimating increase in detection distance when using a bigger coil for detection distances greater than the coil radius. I'm going to try to answer my own question with some data I recorded awhile back. Recorded signal strength vs target distance for a 133mm, 200mm and 300mm coil. Same inductance and current profile for all three coils. Found if I added a line for each coil, for each increase in target distance .41xcoil diameter the signal reduced to 1 tenth. Lines matched the recorded data up to the coil diameter. Lines crossed at(coil A diameter + coil B diameter)/4. Added a line for a 400mm diameter coil crossing a 200mm coil line at (200+400)/4. A nugget just detected at 4 inches with a 8 inch coil would be just detected at 2 inches with a 16 inch coil. Nuggets just detected at 3 inches or less with the 8 inch coil wouldn't be detected with the 16 inch coil.

[daverave]

I was once on a detecting rally where everyone was complaining about lack of finds. There were quite a lot of people there with many different detectors. Towards the end of the day I was talking with an old lady who was using an ancient C-scope. She wasn't able to walk very far, so she had mainly detected by the entrance to the field. In her finds bag she had two lovely shiny gold stators. Needless to say, once she showed her finds to others, they were all over the area like a rash ... but found nothing. It just shows that you don't necessarily need to have the best detector, just a little patience. Many times I've seen the same folks racing around the field like a demon, and then going home 2 hours later empty-handed and saying that the dig was crap. On a few occasions I've dug up roman coins near the field entrance, which literally hundred of detectorists have passed over. You've got to take it more slowly and don't get obsessed by depth.

i guess thats very true "patience" to be honest im a very impatient person and cover a lot of ground too quick whereas when my wife tried detecting she took her time and found things that me and others had missed...i think woman got more patience than men in some cases.

[daverave]

Hi Dave would you mind asking your friend the details and brand of that carbon fibre tape please , sounds like it could be interesting to try out.

Hello 6666...sorry for the delay in getting back to you...i did ask the detectorist and this is what he gave me to tell you...www.fiberglassdirect.co.uk

[daverave]

My Wife

This is my wife detecting on one of our local beaches last summer....she loves detecting...once she gets her hands on detector i have a battle to get the detector back she really liked the white's dfx but we had to sell cause of problems...so its pulse ag ain for her this year.

[daverave]

Dave, and everyone hitting the beach,

Learn to read the beach and find out how deep the hard pack is beneath the loose sand. Most targets in loose sand will gradually sink while the water and waves saturate the sand and then fall down to the layer of the hard pack. Observe the beach wear and first hunt in areas where the loose sand layer is thin or missing due to beach erosion.

In my area, the New Jersey Shore, the spring season brings in more sand and coins get thrown up on the beach. When I find a quarter or heavy coin, I hunt parallel to the water at this point and most coins I find will be quarters or heavier coins. Smaller and lighter coins will be higher up on the beach. The coins seem to be separated by size and weight in lines parallel to the water.

Every beach is different due to:
1. Sand grain size and shape
2. Beach slope and areas of erosion
3. Angle of beach to the prevailing wind and wave action
4. Areas of high beach use in the warm season.

I hope this adds another perspective on finding good things on the beach!

Joseph J. Rogowski

to be honest ive never really been able to read beaches and several of the local guys have become successful at reading the beach and find lots of nice items.

[daverave]

Just had a text from the detectorist who built his own deep pulse detector...he has been out testing it and said he found something called a bun head penny at a depth of 18" and deep bullit shells...and he sounds very hopeful of better finds...for me i can only try and do my best with what detector i have...i guess its best not to take detecting so serious and if one finds something nice then thats a bonus im trying to get out mo re on the beach but sometimes my mood swings get the better of me and i cant function....i have tried hard to continue my projects but ive had very low times lately and ive had to force myself to do anything......ive sorted out the magnetic field of the earth problems with my surf pi's with the help of advice

[Kev]

Dave I used to get very depressed in winter, but I now take vitamin B and D supplements from mid autumn to mid spring and I think they really help me not to sink too low.

I think your associate is using reverse psychology on you, he knows that confidence is the number one prerequisite of a successful hunter, and he is aware that nothing shatters confidence more than the belief that your machine is deficient in some way.

Others here have really hit upon what is more important than outright power, maybe you should note some of them down and give it some deep thought.

I know from my own experience on the beach, that to say that all the good stuff is now too deep is just not always correct. The sand is always on the move, making the depth to hard-pack variable. The different strata-layers are never equally thick across the whole beach, they are angled and taper into each other.
One of the most powerful things you can do to improve your finds is sharpening your observation skills, with respect to beach morphology. There are a number of books dealing with this subject aimed at the detectorist that you may be able to find second hand. I assume your avatar is you and I see you are walking the "wet line" so you already know one area to focus on. There are many more depending upon your type of beach and offshore conditions.

One factor that could change everything tomorrow for you, is climate variability. Perhaps after you get a good unseasonal storm, the whole lot will be stirred up again and good finds will be more regular for you.

In hunting for gold nuggets I regularly bury a target at the threshold of detection and check my settings are optimal on the day. A key factor when seeking very deep nuggets is the response and recovery speed of the machine. One of the biggest mistakes we can make as a detectorist is sweeping the coil faster than the machine can respond to the target. That time is greater the deeper the target sits, since it audibly registers on less of the coil area if it is a small target. This is one of the main factors why many GPZ7000 operators were unable to get the extra depth advertised with this machine, it requires a consistent slow sweep on very deep items.

Swapping between PI and VLF can compound the sweep speed problem, since one is usually processed by an integrative function while the other differentiative where depending upon filters used may have a faster sweep enhancement.

Bottom line Dave, technique and knowledge, they are just as important as your preferred tool.

HH

[Tony2]

24" on an average 9ct ring.......tell him he has the deepest detector ever created by mankind. 40% deeper than my GPX4500 on it's most powerful setting.
Tell him I will pay him $50,000 for this detector...no questions asked.
Sorry but "his claims" are impossible....12" on an average 9ct ring is not bad. I have used just about every PI over the past 15 years (beach and prospecting) and the internet is full of BS "claims".......any pulse delay of 10uS is for dry sand only and not surface dry but dry down to to a foot or so.....top of beach only. These claims should be banned from the internet as it gives newer users a totally unrealistic expectation of what detectors can achieve.
Tell him White's or Minelab will make him an instant millionaire with his amazing new coil.....

He has made a 14" coil using home made litz wire and used carbon fibre tape for the screening and he has got the coil capacitance down...and the coil only weighs about 250g and is ultra light from what he has told me...i saw photo of coil and it looks very good...and he said coil inductance is around 380 uH but has managed to get the coil working at 9 uS...i think he has got good sens on small gold...he claims 24" depth on a 9ct medium size gold ring like a whisper...him and his two friends have the edge now on the beach as one of his friends has a whites tdi sl and has done very well with this machine...with the machines ive made i cannot really compete and some of these guys go out detecting nearly every day.

[daverave]

Dave I used to get very depressed in winter, but I now take vitamin B and D supplements from mid autumn to mid spring and I think they really help me not to sink too low.

I think your associate is using reverse psychology on you, he knows that confidence is the number one prerequisite of a successful hunter, and he is aware that nothing shatters confidence more than the belief that your machine is deficient in some way.

Others here have really hit upon what is more important than outright power, maybe you should note some of them down and give it some deep thought.

I know from my own experience on the beach, that to say that all the good stuff is now too deep is just not always correct. The sand is always on the move, making the depth to hard-pack variable. The different strata-layers are never equally thick across the whole beach, they are angled and taper into each other.
One of the most powerful things you can do to improve your finds is sharpening your observation skills, with respect to beach morphology. There are a number of books dealing with this subject aimed at the detectorist that you may be able to find second hand. I assume your avatar is you and I see you are walking the "wet line" so you already know one area to focus on. There are many more depending upon your type of beach and offshore conditions.

One factor that could change everything tomorrow for you, is climate variability. Perhaps after you get a good unseasonal storm, the whole lot will be stirred up again and good finds will be more regular for you.

In hunting for gold nuggets I regularly bury a target at the threshold of detection and check my settings are optimal on the day. A key factor when seeking very deep nuggets is the response and recovery speed of the machine. One of the biggest mistakes we can make as a detectorist is sweeping the coil faster than the machine can respond to the target. That time is greater the deeper the target sits, since it audibly registers on less of the coil area if it is a small target. This is one of the main factors why many GPZ7000 operators were unable to get the extra depth advertised with this machine, it requires a consistent slow sweep on very deep items.

Swapping between PI and VLF can compound the sweep speed problem, since one is usually processed by an integrative function while the other differentiative where depending upon filters used may have a faster sweep enhancement.

Bottom line Dave, technique and knowledge, they are just as important as your preferred tool.

HH

hi Kev...I think your right when you say about reverse psychology...some people want to make me feel bad by saying their detector go's deeper and makes my detector feel inferior to theirs...in the summer the same detectorist boasts about his finds on the dry sand and shows a handful of coins which makes one want to give up and go home...yes its reverse psychology !!!! maybe i just got to toughen up and ignore these people that try and make me feel ****...i think also with detecting its being in the right place at the right time...my main problem is that im not good at reading beaches...my favorite detector of choice is pulse and i hope in the future to get a pulse detector that will give me some discrimination on iron for the dry sand...for me pulse is the best for the dry and wet sand.

[daverave]

24" on an average 9ct ring.......tell him he has the deepest detector ever created by mankind. 40% deeper than my GPX4500 on it's most powerful setting.
Tell him I will pay him $50,000 for this detector...no questions asked.
Sorry but "his claims" are impossible....12" on an average 9ct ring is not bad. I have used just about every PI over the past 15 years (beach and prospecting) and the internet is full of BS "claims".......any pulse delay of 10uS is for dry sand only and not surface dry but dry down to to a foot or so.....top of beach only. These claims should be banned from the internet as it gives newer users a totally unrealistic expectation of what detectors can achieve.
Tell him White's or Minelab will make him an instant millionaire with his amazing new coil.....

Hi Tony....maybe this detectorist has made these statements to make me feel bad and to make me more depressed !!! ive got to try and toughen up and not listen to no more crap from this guy !!! i have my good and bad days but im really trying to get my act together and be more positive....i didnt mean to make people feel bad on here by saying about the claims of this detectorist but i guess im too naive in believing what people claim

[Nexus]

Hi Tony....maybe this detectorist has made these statements to make me feel bad and to make me more depressed !!! ive got to try and toughen up and not listen to no more crap from this guy !!! i have my good and bad days but im really trying to get my act together and be more positive....i didnt mean to make people feel bad on here by saying about the claims of this detectorist but i guess im too naive in believing what people claim

This is absolutely the correct attitude. Measure everything yourself.
Unfortunately there are those who always want to be on top of everyone else and for that they will claim anything.
And there are those who are simply inaccurate in what they believe.
There will be always someone who's piece is bigger at least in their mind.
Don't pay them any attention.
The moment you feel that someones gibberish makes you feel anything but happy, just walk away.

[woodbob123]

Dave
Having met you and been in your workshop the quality of your builds is excellent, I think you have answered your question when you say the others spend a lot of time out detecting. Learning to "read" the beach is the advantage they have which you can only get by spending time on the beach.
This for many people is a race to win hobby, they must always have the best and deepest machine and get the best finds. A few years ago I saw a guy did up a gold sovereign did the same 2 weeks later second time I saw him calmly walk to where the cameras were, he made no attempt to search the area around the find site??????. Another guy tried to convince me that he could get finds at 18-24" depth based only on the number of shovelfuls of sand he dug.
Has this guy seen the quality of your detectors and the only way to "win" is lie maybe. Bob

[daverave]

Dave
Having met you and been in your workshop the quality of your builds is excellent, I think you have answered your question when you say the others spend a lot of time out detecting. Learning to "read" the beach is the advantage they have which you can only get by spending time on the beach.
This for many people is a race to win hobby, they must always have the best and deepest machine and get the best finds. A few years ago I saw a guy did up a gold sovereign did the same 2 weeks later second time I saw him calmly walk to where the cameras were, he made no attempt to search the area around the find site??????. Another guy tried to convince me that he could get finds at 18-24" depth based only on the number of shovelfuls of sand he dug.
Has this guy seen the quality of your detectors and the only way to "win" is lie maybe. Bob

Hi Bob...it was so nice to meet you in person and have a good chat..i appreciate your kind comments concerning the detectors that ive made...im going to try and not give up so easily in the future..now the warm weather is coming i will try and get out more..maybe keep to myself more on the beach so peo ple cannot come up to me showing off their detected cash and rings