I have seen various request for information pertaining to assembly, potting etc. I don’t work for Blue Robotics; however, I do have video/audio equipment and software to create guides.
If you were able to access a video or fabrication guide, what would you be interested in seeing?
I have extensive experience in molding with various polymers and epoxies (20+ years professional) and I am an Electronics type by degree / trade. I have been doing J-STD-001 soldering for since the standard came out and the list continues.
So this is your chance to speak out. I don’t have a swimming pool etc. so don’t ask me to drive an ROV around for you, I would like to concentrate on fabrication issues. I believe that is where I could best create documents with some insight of doing similiar things for a long time.
Bottom line is that I am bored.
Great idea Harold. I think one critical area that could use a video tutorial is how to pot / seal a hull penetration using B.R. cable penetrators and epoxy. There are lots of ways to screw this up and I think a video tutorial would really help a number of people.
Thank you for offering your time and expertise. Yup, underwater sealing, especially cable entries is the holy grail here. An article or video series showing a few options for sealing would really be appreciated.
The attached document is my first go at putting together a guide for performing the potting on the Cable Penetrators. This is pretty short, but there isn’t a lot to say about this subject. Please post whatever feedback you have for improvements, comments etc.
fabrication-guide (287 KB)
Thanks a lot Harold, beautifully made and very insightful.
Thanks JL, no rotten food tossed at me on my first review!
As with most new polymer systems that I have played with in the past … how you think it is going to work out doesn’t always end up as planned. The epoxy just didn’t want to follow my directions I am used to thicker two part epoxies getting less viscous when the hardener is mixed in … not this time! The viscosity didn’t appear to change at all. On a larger volume opening, it might have behaved differently, but on the penetrator it was a pain in the rear.
Any suggestions for another topic matter to do a guide for? I am back to being bored again … the slave drivers at work don’t keep me busy enough (just don’t tell them that).
A lot of topics will depends on the technical choices made by each ROV builder… you’ve probably already nailed the most universal and immediate issue that all BlueROV users may eventually encounter
There are many interesting topics to discuss and share… Some other “universal” ones:
- Wet-testing your BlueROV (checklist, warnings, recommendations, etc.)
- Balancing your BlueROV (buoyancy, center of buoyancy / gravity)
- Building a tether (OpenROV is selling a neutrally buoyant tether on their latest kickstarter, but gosh - USD 250 for 100m !)
- Building a battery pack / power solution
- Step by step installation of BlueROV ROS package on [your favorite micro-computer / micro-controller here]
- Waterproofing / "depthproofing" a servomotor (I've made some interesting progress on this with epoxy, but still need to test the result at depth)
- Re-programming the BlueESC to change the I2C address
Thank you for the well written and easy to follow instructions for potting, I do have a question
I read somewhere, I believe it was on the HomeROV site that you should also strip/remove a small section (1/8") of the individual wire sheathing to allow the potting material to bond with the wire strand. The reason for this is to stop any water that may have gotten to the wire due to a cut in the sheathing somewhere in the tether and is now traveling between the wire strand and the sheathing.
I was wondering yours or anybody’s thoughts on this
Another subject that was not mentioned above and I would like to know more about is cameras (maybe this would be better as a separate post)
What I am thinking would be a list of quality’s that make a good ROV camera, what to look for as far as Field of View, Aperture (f-stop) also the latency of IP Cameras. What would be great is a footage comparison of all the different parameters showing the differance.
Again Excellent job on the first tutorial and thank you for taking the time to create it
I understand what you are talking about and there are reasons that you might do this but 99% of the time, leave the insulation alone. I have built and tested litterlly 10,000+ cables over my almost 30 years with my current company and have looked a lot of failure modes invovling leak pathways. We pressure test all of our cables in water to usually to 1000 psi … so I can give you plenty of sea stories on how they can die. I will discuss the particular cable that is currently used for the Thrusters etc.
This cable has a thermal plastic urethane (TPU) jacket and has individual conductors with Teflon insulation. There is no “water block” material between the conductors. Frankly for a system that is only going as deep as everyones units will go without never coming back … it is not a bad choice of cable at all.
The TPU jacket is friendly to a lot of compounds with the exception of certain epoxies. That is one reason I roughed up the cable jacket. This weakness could be countered by using a primer made for TPU material that will greatly enhance bond strength.
The Teflon on the conductors is a great choice from an insulation standpoint and Teflon is a outstanding dielectric. I haven’t calculated what the impedence is on this cable and I really don’t expect people to be using it for stuff that required a particular impedence value. If so, time to come up with your own cable. Teflon will normally not bond to about anything; however, you can get Teflon wire that has been “acid etched” and it will bond with various potting compounds.
So here is the bottom line with the Thruster cable. If it is cut through the jacket, water will travel down the wire and will have a high chance of a flood out. The cost for Blue Robotics to go out and have custom “water block” cable would be a little more expensive than what your typical at home ROV type will want to pay. I pay from on average $2.50 to $1000.00 per foot for cable at work.
So what is your trade off? Cost, added weight and the list goes on. One thing you can do is what is called a “dam blok”. There is a company that actually sells it. Checkout PMI’s info on the kits
This is one of the things we use in the Submarine world to handle issues with cable that is not water blocked and to enhance splices etc. Again, most at hom ROV types don’t want to incorporate stuff like this.
The bottom line is if you have money, you can engineer your ROV to go to Hades and back … but it is going to cost a TON of money. You can go the cheap route and put protective sleeving over vital cable runs. This adds cost and weight. Everything is going to cost you in some manner.
Another great and timely post, Harold. Wouldn’t stripping a short section of the wire’s insulation, such as Joe suggested, accomplish the same thing as a “dam blok”?
A few reasons why I wouldn’t do it. First from a chemical standpoint, if I don’t know what is in the chemistry for the epoxy I don’t know if there is something in there that might chemical etch the conductor standing as part of its cross linking during cure. A lot of times you have carbon dioxide, water and the list continues as by products in various curing chemistries. Anything that might degrade the conductor standing is a bad thing.
Second, stress … if stress, as in sea pressure, is translated into the epoxy the insulation on the wire can help mitigate that. The individual stands by themselves will feel the strain if there is no insulation. I have had wires shear right at the back of a connector as the wires entered a potting / backfill area because some potting materials will “cold flow” under stain. Please see link for basic defnition. https://en.wikipedia.org/wiki/Creep_(deformation)
Impedence changes can happen if the epoxy has different dielectric properties than the original insulation.
Ok, enough for all of that …
The Dam Bloks are hermetically sealed disk that are designed to provide a low creep (cold flow) scenario for a splice and even if fluid wicks down the wire, it will not go through the dam blok. The way the splice kit is designed and the polymers that they use … these kits are good to the bottom of the ocean. PMI has been developing and upgrading these kits for at least 15+ years. They know what they are doing.
Please remember, the procedure I wrote is for a pressure proof penetrator … not a final termination. If you want a better flood out resistance electrical pass through into your component compartment, then you go with receptacle and plug design. Your receptacle would use hermetic seals around the pins so you never experience a flood out into the compartment … and if your plug floods out, you replace that cable area.
Rusty and I have talked about receptacle and plugs but for the application on these shallow units it is way to expensive. If you want to go to a half mile down and you are using metal casing etc. then I can help you make stuff that works. The other thing that you can use vice potting a penetrator shell is to use a packing gland. I have seen packing gland designed that will handle explosive underwater shock; however, the gland had “water blocked” cable going through it. If you are up to this level then you have a deep pocket book and you can afford to do things the correct way. Everything on here is hobby level to keep the price down.
Harold and Paul,
This is great information. Just for the record, we are slowly making our way towards shifting to waterblock cables so that this issue will be minimized.