Fiberglass

[Davor]

A picture there shows a "Dugapol set" which I believe is manufactured in Serbia. I never thought of asking - where exactly are you from? Maybe from my neck of the woods?

[Tepco]

Here it is:

http://www.geotech1.com/forums/showthread.php?19121-How-to-judge-shield-effectiveness&p=149061#post149061

[Pin Head]

Hi all, I have still not located the pics of a spiral wound coil that I had printed out in the past. Went through all the sites as told, and still
Not found the pics.no big deal I think I have learned enough from reading through all the threads to figure out what I am doing. Thanks for all the help and input. Rob. ((pin head).

[baum7154]

The information on spiral coils is referenced in the TARGET TIME CONSTANT threads in posts 32 and 33. In 32 I found Tepco comments on spiral coil in the thread 'HOW TO JUDGE SHIELD EFFECTIVENESS'. In 33 there is a post from TEPCO on a spiral coil contained in a paper on building the SURF PI 1.2.

Regards,

Dan

[Tepco]

I never made any thread about flat spiral coils, only mentioned it in some completely unrelated post. Later few people built it with good results and it became popular. Not a problem free design however. Difference in parameters can occur due to different wire type used, internal diameter and insulation thickness etc. Even if you keep physical dimensions (number of turns or ID\OD ratio) constant, wire length may vary for different cable types, affecting coil inductance. Another issue is shielding. Unshielded coil without cable (wire extended some 1.2m and twisted every 2-3cm to connect to control box) is very fast, easily 8uS in unmodified detector, can go below 5 in tweaked version, but shield can affect this significantly. Simple shield (graphite sprayed directly across wire insulation coated with single epoxy layer to keep it compact) and coax cable will slow it down almost twice but this is still fast enough. Using spacers (made of any suitable material to keep shield to coil distance at least 5mm from each side in order to reduce capacitance) can improve speed, in all cases outer coil end and shield goes to GND and inner end to detector connection, otherwise shield will slow it down even more. I prefer this coil type due to simplicity and low cost, without need for machined parts and special material\wire type etc, you can wrap this in 10min. without any tool, but for underwater use more complicated design like described in baum 7154 post can be better. Problem with flat spiral is relatively large surface area, producing lot of drag in water, and if you shield it for high speed you will end up with something at least 1.5-2cm thick, not “hydrodynamical” at all and usually positively buoyant, will tend to float like cork depending on spacer material used.

[baum7154]

Yes you can do it either way i.e. spacer, no shield, glass or glass it then if needed spacer, shield, then glass again. I would be inclined to just try it out without any glass or shield first and then add spacer, shield, and glass if needed.

Yes the white, pink, or black, semirigid, but resilient common foam you see in packing is typically polyethylene and is use 1/4" to 5/16" sheets of it from electronic equipment packaging. For the round sections I use 3/8" backer rod used for crack filling before caulk application. This can be had from Home Depot or Lowes, etc.

If I run across the spiral coil thread I'll forward it to you here.

Dan

[Pin Head]

Thanks for the info. searched SPIRAL WOUND COIL & post by tepco and still have not found it. I am new to this forum and not sure if I am using the search correctly but I will find it again. What are you or they getting polyethylene foam sheet is that just packing stuff? And you said I can glass it first and try it and then if needed graphite and spacer, and glass it again. Should that not be space it then graphite and then glass it again?

[baum7154]

Just looked through the chance pi coil thread again and although that is a mighty fine looking coil that is not the pics I was looking for. The pics I am trying to find was of a flat spiral wound with speaker wire, just can't seam to find it again.

I assume you searched SPIRAL WOUND COIL. TEPCO is a contributor on this forum and I know he has posted on the spiral wound coils. You might do a search on his posts. The spiral coil is quite fast if built with thick insulated wire. It's geometry makes it difficult to shield without losing much of it's speed because it is so broad and flat. For best results the graphite shielding should be spaced at least 1/4" off of the windings with polyethylene foam sheet, I'd probably go 3/8". Then the fiberglass armor can be applied but you can glass it first, try it out, then if needed graphite and spacer, and glass it again. The penalty is more weight in doing it this way.

Good Luck!

Dan

[Pin Head]

Just looked through the chance pi coil thread again and although that is a mighty fine looking coil that is not the pics I was looking for. The pics I am trying to find was of a flat spiral wound with speaker wire, just can't seam to find it again.

[Pin Head]

Yes I have seen your thread and at one time I printed the pics of it (if it is the thread I am thinking of)but my pics got thrown away and I have been unable to find the thread again. I thought it was in the chance PI thread but must have over looked it. So once you build the coil you put the graphite on next? I thought there should be a certain amount of space between the coil and the shield. I will have to find the pics again because I seam to be missing something. I made a spiral coil of 22ga. cheap speaker wire from radio shack. According to several coil calculators I used it should have took around 30 turns with a 5" center. Well after I made it it only had 200uH @ 0.5 ohms. I had to splice back on to it and add another 5 or 6 turns witch made it 304uH @ 0.7ohms. I added about 3 1/2 ft of coax that I got from a old depth find transducer not even sure what coax it may be because their is no numbers on it. It is about 1/8" diameter with a very dense ground and no foil shield. With coax and cheap mic connector it measure at PCB 308 uH @ 0.8 ohms. Not looking for gold nuggets where I live, (close to St.Louis Missouri U.S.A) and the only beaches I get around are river and lake beaches when I can, mostly go to city parks and find maybe a couple hundred dollars of modern day coins a year and maybe some rings on something. Haven't really tried this coil outside yet and not sure if I even need a shield. It seams to work fine so for with out one. I was going to glass it in first for armor as you say and then take it out and try it and if needed add a shield.

[baum7154]

Did a little research on carbon fiber and it looks like it generally has about 100 ohms per inch. This might explain why the CF Fabric was so easily detected by the VLF detector. In my experience this is far too low a resistance to be used on a coil shield for a PI Detector. The research also indicated a negative resistance property of carbon fiber that was cured in resin at a very high pressure leading to a new branch of structural electronic devices.

Dan

[baum7154]

If you look at the CHANCE PI COIL thread it shows a few of my 8" fast coils used for detecting small gold. My chance detector samples down to 8us and there is no problem with the 2 layers or more of polyester resin and fiberglass I have used to armor the coils. I have heard that polyester resin will absorb moisture but the can of it I have says it is suitable for marine applications. There are quite a few pictures in the thread, one with a graphite shield layer in place before the fiberglass was put on. I always put a layer of cloth self-adhesive hospital tape over the coil and profile forming foam before glassing to insure that the air space in the coil is not flooded with resin. Flooding the air spaces slows the coil down.

Have not tried carbon fiber matt or fabric. Does anyone know the resistance per square inch of carbon fiber? If it is in the range of 2K ohms or more it might be a good shield for PI, but if it is less than that I think it will adversely affect sensitivity. The one I shielded with graphite at a resistance of 1200 to 1500 ohms per running inch exhibits some loss of sensitivity.

Regards,

Dan

[Pin Head]

I am from US our sport cars are junk do know of the car you are speaking of.

[Koala]

A few have experimented with carbon fiber but unable to get a reliable connection. I looked into trying but cost was too great.


For Caterham car google is your friend. Very famous brand of sports car in the UK

[Pin Head]

Not to be dumb but what is a Caterham car?