ppo2 meter houssing

Scuba_Vixen · 13th June 2006, 18:54

Quote: (Originally Posted by jaap)

OK

I was confused. So you tried to mold the entire housing out of epoxy?

Another thing, were did you find those threaded PVC-cross unions (croix PVC fileté) you have as sensor holders?

http://www.therebreathersite.nl/imag...ortesensor.jpg

I have been looking around on the internet and in many stores dealing with things like that in the Cannes area.

I went back and re-read the first posts, and that seems to be what he's doing, molding the housing from epoxy, and then having issues with the molded in o ring groove maintaining a seal.

Actually, given the lack of strength and dimensional stability of molded epoxy, I'm not surprised.

Machining out a block of more suitable material than epoxy, and housing an epoxy encapsulated circuit in it would seem more practicle.

Moulding cases for submersible electronics is quite an art and science unto itself. Gene Melton (HSE) might be a good guy to ask if you don't believe me,

If you're really committed to moulding the cases from epoxy, then consider adding some high density filler (like West System 404) to the mix, and plan on machining the surface flatness and o ring groove about a week or two after it's out of the mold.

Darlene

rdmmdr · 15th June 2006, 09:41

replacing the batteries seems to be the only reason to have the oring so why not have a rechargeable battery separately encapsulated. nimh batteries charged slowly can handle the pressure and the epoxy will handle the temp, plus it will act as an insulator for the batteries. i am using a 7.2 volt 2000mahr battery for my unit. it all fits in a 2.5x2.5x3 stainless tube section with ch plugs that are rated to 10,000 psi and 300 volts. no on and off switch, and four days worth of run time. batteries are rated for 1000 charge cycles min. total cost was 100 $.

**jmurba** · 15th June 2006, 22:07

[quote=jaap]OK

I was confused. So you tried to mold the entire housing out of epoxy?

I will try it and was lookinf for guys that had already done to see if threre was no absolute obstacle

Another thing, were did you find those threaded PVC-cross unions (croix PVC fileté) you have as sensor holders?

http://www.therebreathersite.nl/imag...ortesensor.jpg

thank you for the link this is my first gauge i had buit 6 years ago (((-: good memories, still have the silicon mold ... but no way it will never be watertight , this one was, all the other NOT

regards

jean mi

AD_ward9 · 15th June 2006, 22:42

About 20 years ago, I learnt that potting things causes a lot of grief. We had a client obsessed with secrecy, so they scrubbed the markings off chips and potted the whole lot. The fact that I could remove the potting easily using dichloromethane, and see what the chips are by taking the plastic off using hot sulphuric acid and look at them under a microscope, seemed to be lost on them. However, I learnt a few things not to do in this process:

1. Epoxy is expensive, so they tried polyester resins. Polyester resins get really hot when curing, and shrink more, so 70% of what they potted failed and the heat burnt through their waste bins causing a right mess on their floor.

2. Even with epoxy, the shrinkage is quite considerable. If you use surface mount components, then about 30% will fail unless the epoxy has a high elastomer content.

3. Even with through hole components, many do not like being potted.

4. The stress caused by even soft epoxy with non-SMT causes micro cracks. Sea water loves these cracks, so even more fails. Ask anyone with an Inspo handset that has a "crack".

Bottom line: Potting is a messy process and bad idea. Better to design a case properly.

Cases do seal if you follow the rules below:
1. Use internal (piston type) seals, not external or flat compression seals.
2. Seals need a lot of pressure. Be careful on long edges.
3. Use leak detecting paste to check it does not leak. Pressurise it and bash it around with the paste inside, before you put electronics in.
4. I discovered fairly recently that our mech engineers test all their material for helium porosity. This is easy to do. If you case does not leak helium, it will not leak water.
5. Pay particular attention to the seal surface. If it is not really flat, such as if it has machining marks on it, then it will not seal.
6. Make sure you compress the O rings enough, and use PTFE grease.
7. A lot of problems are caused by things bowing under pressure. Fill the box with silicone oil by immersing it in a basin of oil so there are no bubbles at all. The oil prevents the bowing, and reduces the effect of leaks.
8. Spray the circuit board with a silicone conformal coating. These are available from RS. Do not use nail varnish. Quite astonishing what poisons you can generate by burning nail varnish with a soldering iron.

If anyone has any more rules for leak proof design, I am all ears.

Cheers

Alex

**jmurba** · 15th June 2006, 22:59

hello vix

Quote: (Originally Posted by Scuba_Vixen)

I went back and re-read the first posts, and that seems to be what he's doing, molding the housing from epoxy, and then having issues with the molded in o ring groove maintaining a seal.

+++++ exact, I have molded a good 12 houssing using epoxy and it was very difficult to get erfect o ring groove most of the time the groove looked but was not flat making impossible of tightening the o ring properly
I have attached a photo of the houssing, one of the first , afer the bubbles disapeared (vacum chamber) and we used a lumberg connectore.
first we thought that it came from the fact that the cassing was ambiant presure, after making it one atm to gt th oring under pressure , no change , still a bit of water, then we rebuilt the mold using mutch more silicon resin to prevent a deformation of the mold under the epoxy resin...things wer better but not perfect for all units....then we left the resin drying mutch longer to prevent deformation of the houssing getting it out of the mold wnt the resin is too young....but noting perfect...th sale for our sensor holder...

I don't know the secret of gene from hs explorer but he is a hero of epoxy molding, I have 2 explorers and as a matter of fact it is a good job , the cassing was not machined after molding and it is watertight....

my conclusion is that molding an o ring groove is possible but the results are not regular, one cassing will be ok the next leaking....a pain in the neck....grrrrr forget about it.....

on the other hand machining a groove in a sqaure block on metal or delrin is not a warranty ...some that have been machined are leaking...

I have looked the vr3 cassing on the internet, it seems to me that tey machine their units after moulding it at a very simple way, the O ring groove seems to be machined without round curve, they have angular oring curve as if the tool was only making portions of lines conneted one to the other to make the entire groove arround the houssing

obviously building houssing is a problem for everybody.....
electonics are so easay to make comparate to the houssing grrrrr

Actually, given the lack of strength and dimensional stability of molded epoxy, I'm not surprised.

it is strong, no deformation under 15 bars presures (we put modeling clay) in the houssing and immerged it in 150 meter of water (at the end of my reelline , me and the reel on the boat and the houssing at the other end 150 bellow) and mesured the deformation it was ok with the resistence calculation we had done to design the houssing.

Machining out a block of more suitable material than epoxy, and housing an epoxy encapsulated circuit in it would seem more practicle.

yes but i would have liked to built a real and reliable houssing that I could have opened when necessary !!! grrrrr

Moulding cases for submersible electronics is quite an art and science unto itself. Gene Melton (HSE) might be a good guy to ask if you don't believe me,

I belive you, this is a science, maybe are we ging to continue bettering our skills....

If you're really committed to moulding the cases from epoxy, then consider adding some high density filler (like West System 404) to the mix, and plan on machining the surface flatness and o ring groove about a week or two after it's out of the mold.

well analysed...flatness was one of our worst problems....

regards

jean mi

Darlene

Scuba_Vixen · 15th June 2006, 23:34

Quote: (Originally Posted by jmurba)

hello vix

Just to note;
The Explorer cases are injection molded from some type of thermoplastic. Injection molding, which is done under pressure at elevated temperatures is a world of difference than casting epoxy in a mold at room temperature. It's illogical to expect similar results.

Injection molding itself is quite an art and science, the mold and plastic temperatures require very tight control, the rate at which it's allowed to cool, and curing time before removing it from the mold, are majorly important as well. *Everything* has to be spot on, or reliability suffers greatly. ... The big plus is that the strength to material thickness and weight ratio is great.

If you keep trying long enough, you'll either find the Holy Grail of how to do it, or you'll prove conclusively that it can't be done.

Best of luck,

Darlene

**jmurba** · 16th June 2006, 08:24

If you keep trying long enough, you'll either find the Holy Grail of how to do it, or you'll prove conclusively that it can't be done.

hello,
in fact we did manage to do it, but it is not stable in production , I mean some houssing are ok and others not.....
the good point is that we managed to better thing, the black point is that we haven't been able to solve the problem....grrrrrrrr
it is frustating.
regards
jean michel

]

paulraymaekers · 16th June 2006, 11:19

hello jean-mi,

potting can also be done with 2-component PU

why don't you simply get a decent housing milled???? (after all the trouble you went trough..)

regards

paul

(or get yourself a rEvodream :-)

Philippe GERIN · 16th June 2006, 14:37

Quote: (Originally Posted by jmurba)

Hi all,
I'am now thinking of drowning my circuits in epoxy resin.
jean mi

As opposed to drowning them in sea water, Jean-Mi ??

A+
Philippe

**jmurba** · 16th June 2006, 14:55

Quote: (Originally Posted by Philippe GERIN)

As opposed to drowning them in sea water, Jean-Mi ??

A+
Philippe

et tu en sais quelque chose hein (((-:
salut mon bon philippe
jean mi

13th June 2006, 18:54	#11 (permalink)
Scuba_Vixen e/mCCR Dolphin Pilot Current Rebreather/s: Other CCR Home Build Other Rebreather/s: Other CCR Home Build Join Date: May 2005 Location: St. Croix USVI Posts: 557	Re: ppo2 meter houssing Quote: (Originally Posted by jaap) OK I was confused. So you tried to mold the entire housing out of epoxy? Another thing, were did you find those threaded PVC-cross unions (croix PVC fileté) you have as sensor holders? http://www.therebreathersite.nl/imag...ortesensor.jpg I have been looking around on the internet and in many stores dealing with things like that in the Cannes area. I went back and re-read the first posts, and that seems to be what he's doing, molding the housing from epoxy, and then having issues with the molded in o ring groove maintaining a seal. Actually, given the lack of strength and dimensional stability of molded epoxy, I'm not surprised. Machining out a block of more suitable material than epoxy, and housing an epoxy encapsulated circuit in it would seem more practicle. Moulding cases for submersible electronics is quite an art and science unto itself. Gene Melton (HSE) might be a good guy to ask if you don't believe me, If you're really committed to moulding the cases from epoxy, then consider adding some high density filler (like West System 404) to the mix, and plan on machining the surface flatness and o ring groove about a week or two after it's out of the mold. Darlene Last edited by Scuba_Vixen : 13th June 2006 at 18:58.
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15th June 2006, 09:41	#12 (permalink)
rdmmdr designer of death Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Mar 2006 Location: kerman,california Posts: 372	Re: ppo2 meter houssing replacing the batteries seems to be the only reason to have the oring so why not have a rechargeable battery separately encapsulated. nimh batteries charged slowly can handle the pressure and the epoxy will handle the temp, plus it will act as an insulator for the batteries. i am using a 7.2 volt 2000mahr battery for my unit. it all fits in a 2.5x2.5x3 stainless tube section with ch plugs that are rated to 10,000 psi and 300 volts. no on and off switch, and four days worth of run time. batteries are rated for 1000 charge cycles min. total cost was 100 $.
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15th June 2006, 22:07	#13 (permalink)
jmurba wet bear Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: May 2006 Location: cannes france Posts: 920	Re: ppo2 meter houssing [quote=jaap]OK I was confused. So you tried to mold the entire housing out of epoxy? I will try it and was lookinf for guys that had already done to see if threre was no absolute obstacle Another thing, were did you find those threaded PVC-cross unions (croix PVC fileté) you have as sensor holders? http://www.therebreathersite.nl/imag...ortesensor.jpg thank you for the link this is my first gauge i had buit 6 years ago (((-: good memories, still have the silicon mold ... but no way it will never be watertight , this one was, all the other NOT regards jean mi
(Offline)

15th June 2006, 22:42	#14 (permalink)
AD_ward9 RBW Member Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Jun 2005 Location: Scotland Posts: 2,011	Re: ppo2 meter houssing About 20 years ago, I learnt that potting things causes a lot of grief. We had a client obsessed with secrecy, so they scrubbed the markings off chips and potted the whole lot. The fact that I could remove the potting easily using dichloromethane, and see what the chips are by taking the plastic off using hot sulphuric acid and look at them under a microscope, seemed to be lost on them. However, I learnt a few things not to do in this process: 1. Epoxy is expensive, so they tried polyester resins. Polyester resins get really hot when curing, and shrink more, so 70% of what they potted failed and the heat burnt through their waste bins causing a right mess on their floor. 2. Even with epoxy, the shrinkage is quite considerable. If you use surface mount components, then about 30% will fail unless the epoxy has a high elastomer content. 3. Even with through hole components, many do not like being potted. 4. The stress caused by even soft epoxy with non-SMT causes micro cracks. Sea water loves these cracks, so even more fails. Ask anyone with an Inspo handset that has a "crack". Bottom line: Potting is a messy process and bad idea. Better to design a case properly. Cases do seal if you follow the rules below: 1. Use internal (piston type) seals, not external or flat compression seals. 2. Seals need a lot of pressure. Be careful on long edges. 3. Use leak detecting paste to check it does not leak. Pressurise it and bash it around with the paste inside, before you put electronics in. 4. I discovered fairly recently that our mech engineers test all their material for helium porosity. This is easy to do. If you case does not leak helium, it will not leak water. 5. Pay particular attention to the seal surface. If it is not really flat, such as if it has machining marks on it, then it will not seal. 6. Make sure you compress the O rings enough, and use PTFE grease. 7. A lot of problems are caused by things bowing under pressure. Fill the box with silicone oil by immersing it in a basin of oil so there are no bubbles at all. The oil prevents the bowing, and reduces the effect of leaks. 8. Spray the circuit board with a silicone conformal coating. These are available from RS. Do not use nail varnish. Quite astonishing what poisons you can generate by burning nail varnish with a soldering iron. If anyone has any more rules for leak proof design, I am all ears. Cheers Alex Last edited by AD_ward9 : 15th June 2006 at 23:00. Reason: Put in some smilies
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15th June 2006, 23:34	#16 (permalink)
Scuba_Vixen e/mCCR Dolphin Pilot Current Rebreather/s: Other CCR Home Build Other Rebreather/s: Other CCR Home Build Join Date: May 2005 Location: St. Croix USVI Posts: 557	Re: ppo2 meter houssing Quote: (Originally Posted by jmurba) hello vix Just to note; The Explorer cases are injection molded from some type of thermoplastic. Injection molding, which is done under pressure at elevated temperatures is a world of difference than casting epoxy in a mold at room temperature. It's illogical to expect similar results. Injection molding itself is quite an art and science, the mold and plastic temperatures require very tight control, the rate at which it's allowed to cool, and curing time before removing it from the mold, are majorly important as well. Everything has to be spot on, or reliability suffers greatly. ... The big plus is that the strength to material thickness and weight ratio is great. If you keep trying long enough, you'll either find the Holy Grail of how to do it, or you'll prove conclusively that it can't be done. Best of luck, Darlene
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16th June 2006, 08:24	#17 (permalink)
jmurba wet bear Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: May 2006 Location: cannes france Posts: 920	Re: ppo2 meter houssing If you keep trying long enough, you'll either find the Holy Grail of how to do it, or you'll prove conclusively that it can't be done. hello, in fact we did manage to do it, but it is not stable in production , I mean some houssing are ok and others not..... the good point is that we managed to better thing, the black point is that we haven't been able to solve the problem....grrrrrrrr it is frustating. regards jean michel ]
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16th June 2006, 11:19	#18 (permalink)
paulraymaekers rEvo's daddy Current Rebreather/s: rEvo Other CCR Home Build Other Rebreather/s: rEvo Other CCR Home Build Join Date: May 2005 Location: belgium Posts: 1,638	Re: ppo2 meter houssing hello jean-mi, potting can also be done with 2-component PU why don't you simply get a decent housing milled???? (after all the trouble you went trough..) regards paul (or get yourself a rEvodream :-) __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...
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16th June 2006, 14:37	#19 (permalink)
Philippe GERIN ccr apprentice Current Rebreather/s: Megalodon Classic Kiss Other Rebreather/s: Join Date: May 2005 Location: Strasbourg, France Posts: 647	Re: ppo2 meter houssing Quote: (Originally Posted by jmurba) Hi all, I'am now thinking of drowning my circuits in epoxy resin. jean mi As opposed to drowning them in sea water, Jean-Mi ?? A+ Philippe
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16th June 2006, 14:55	#20 (permalink)
jmurba wet bear Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: May 2006 Location: cannes france Posts: 920	Re: ppo2 meter houssing Quote: (Originally Posted by Philippe GERIN) As opposed to drowning them in sea water, Jean-Mi ?? A+ Philippe et tu en sais quelque chose hein (((-: salut mon bon philippe jean mi
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