Eric Foster Coil Design

[Goggy]

Hi Altra,

With regards to the Aquapulse and Aquasport, looks like GeoMax is fully up to speed aging them. I'm afraid detectors from the 80's are a little early for my experience with the PI MD's.
I know the Aquasport is essentially the same as a Minipulse. The cylinder Aquapulse however, was a single board when I was servicing and testing them. Still very simple like the Minipulse though.

They used the Ikelite connectors for the Bone-Phone and search coil. All size coils (8", 10" & 15") were 300uH as was the inductance of the 12" and 36" Ferrite Probes.

One common issue we had, was the longevity of (or lack-of) the Hex shaft seals on the control pots.

[Altra]

I also worked on the later Aquapulse 1a and 1b's and like you said they were a single circuit board. The main problems we had with the Aquapulse's were the cable connections to the Ikelite connectors. Once a little water entered, it would wick up the shielding in the orange coax cable and cause false signals and noise. Thank you

[cristiano.sar]

Goggy About the coil shielding, I have seen in some Eric Foster (Ferric Toes) posted photos, that he also used a stripe like the 3M scotch 24, was this used also in production ready coils?
And regarding Vacuum Forming, do you have some photo or detauls of the machines used? (just for curiosity because I also make my own vacuum forming machine and I'm always trying to improve it and get other ideas)

Thanks.

[Goggy]

Hi Cristiano,

The '3M scotch 24' tape was not used in PTL search coils, but Eric may well of experimented and even used this in his own PI designs. The 'Ribbon' cable wrap method and Aquadag coating was definitely used in the Goldscan, Beachscan, etc. designs and maybe a few specials.

With regards to the vacform machine we used: this was the Formech 450 like shown in the picture. It could handle sheets up to 450mm square. We used pin-point vacform molds made by PTL for the simple coils and outsourced molds for the larger vacforms. Hope this helps

[cristiano.sar]

Thank you!
Here is a coil shell I built these days with my diy vacuum former based on the Foster design.

[Altra]

Nice Job!

[cristiano.sar]

Thanks Altra

Goggy , regarding the the potting resins, do you remember the brands or compositions used?

[Goggy]

Hi Cristiano,

The coils were potted in two operations. The first pot was an ultralight resin and the second was a thinner pot which was a sealing and protective layer.

We used Robnor design resins in Swindon for the ultralight and skim pot. I will check my information and see which types were used. I think the skim was type DP147 black polyurethane. The ultralight I will need to check.

Hope this helps.

Chris

[Tinkerer]

Thanks Altra

Goggy , regarding the the potting resins, do you remember the brands or compositions used?

Low viscosity laminating Epoxy resin works fine. The lowest viscosity loaded with a maximum of glass micro balloons can give you a density of down to 0.3. The density of water is 1, so for a diver metal detector the coil will float, which is annoying. For the top coat that wears down when scraping and the sand and rocks, you can use the same resin loaded with milled fiberglass this gives it a lot of abrasion resistance.
If you give me a link to your supplier, I can point out which products will give best results.
Even better abrasion resistance is obtained by adding micro glass flakes. This is a product that they usually add to floor paint to make it abrasion resistant. It is also used in special coatings used to prevent osmosis on fiberglass boats.

[Goggy]

Hello,

Our main supplier of resin was Robnor Design Resins. Type DP147 was the main type. We also used the 'Electrolube' (similar to ER1448 type two-part), available from many electronic part suppliers: Electrolube_Potting_Resin.pdf

The curing times for the resins were any where from 24 hours to 30 mins depending on the type / size / thickness and ambient temperature. The cure could be made much faster (~30 mins to 1 hour) by making the resin 'exotherm' by mixing it vigorously. However, this was not without its issues, as the working time of the resin was also reduced considerably. This meant you needed to work fast and pour it quickly once the reaction started. Another drawback of this fast exotherm method, was that any air caught up in the vigorous mixing, did not have time to work its way to the top of the pot and could create a very poor potting outcome, full of small bubbles and holes.

The Aquapulse coils were potted with two pours of DP147. (As stated by Tinkerer : the density was heavier than water so they did not float).

The Goldscan 2 & 4, Minipulse, SuperScan, Eurodec, etc.. all used Ultralight on the first larger main pot and skimmed with the DP147 resin.

The ferrite probes and SuperScan large coil junction boxes (>4m) were all potted with the cheaper Electrolube type resin. This was down to cost and the fact that it was not seen.

All the resins were selected for there durability, stability and low shrinkage.

Hope this helps,

Chris.

[cristiano.sar]

Thank you. I will try to find something similar here in Brazil.
I have seen other people like Carl Moreland referring the Epoxy Seal 9000 but this seems to be the kind of resin used to pot electronic components and boards but I think it may be is too light to use in the water.

[Carl-NC]

I have seen other people like Carl Moreland referring the Epoxy Seal 9000 but this seems to be the kind of resin used to pot electronic components and boards but I think it may be is too light to use in the water.

Relative density is 0.95. It also has low shrinkage, which is more critical for making DD coils.

[cristiano.sar]

Relative density is 0.95. It also has low shrinkage, which is more critical for making DD coils.

Thanks, so, for a coil that would go underwater, taking into account the weight of the coil windings and an ABS coil shell, will this resin density be ok?

[Carl-NC]

I would think so.