The PUR cable doesn’t appear to be water blocked, so water could follow along the wires beneath the cable jacket and possibly get past a penetrator and into the enclosure. Is the cable at least water blocked on the thruster end?
Hi @Kansas -
No, it is not water-blocked, and that is definitely a known failure mode. Connectorizing the thruster is the only way to get around this!
The cable is epoxy potted at the motor though, isn’t it?
@tony-white What exactly is the failure mode? Is it damage to the jacket or water seeping down between the cable jacket and the potting? I guess in either case, the thruster is toasted anyway.
Hi @psupine -
If the cable is nicked so water can enter the outer jacket, it can flow into the housing through the WetLink Penetrator. Alternatively, if taken to extreme depth (>400m) water can find its way past the potting located within the thruster where the cable connects to the stator, and flow back along the cable from there.
If a thruster has a connector put on the end of the cable, it can then work at full ocean depth. The water may still make its way past the potting and into the cable, but as it isn’t able to reach the housing, no problem! This will likely result in voltage from the motor phases reaching the water - this will not cause the thruster to stop functioning, but may accelerate corrosion that leads to it (or other) devices failing…at least in the long term!
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@tony-white (… so picking up on the second case …)
I think you’re saying that at extreme pressure, the cable and jacket will compress more than the epoxy, and that will open up a (micro) gap to water ingress.
In this case would top-coating the epoxy/cable interface with something more compliant, like a urethane sealant, address this? I’m thinking that the softer sealant would compress around the cable and bridge the epoxy/cable gap.
Thanks Tony. Perhaps BR should mention it on the website. The way BR sent me my T200 is with several feet of cable with the jacket stripped 6 inches from the end and the three wires hanging out - suggestive of being put through a penetrator and into a housing ready for connection inside, not cut short and ready to be terminated with a connector. I believe a specific penetrator was even suggested.
Hi Kansas,
All the thrusters are like that. Originally, the “penetrators” were just a kind of hollow bolt and the cable was potted in epoxy. (Maybe you already know this).
I think the current type, with the compression collar, is better in all sorts of ways. One of which is that the cable compression will go at least part of then way to reducing the degree that water might creep up inside the jacket, in a way that the epoxy potting would not have achieved at all.
The compression collar ends up sitting about 5mm from the end of the jacket. It would be nice if the jacket could fit all the way through the threaded end of the penetrator, but it doesn’t. The three internal wires end up running over a bit of an edge where they emerge at the threaded end.
Personally, I’m not worried about the potential for cable nicks. There is always the possibility of failure around the epoxy potting, but there’s also the possibility for failure around any of the other seals around the place. Any of those o-rings could have a problem that passes the vacuum test but fails at pressure.
You can see the epoxy potting at the thruster end of the cable by undoing the two screws at the front of the thruster. That’s the bit where I’m thinking I might smear some soft sealant over, but it’s probably overkill.
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Hi @psupine -
Adding a compliant epoxy there might help, but generally it’s tough to get good surface preperation and a bond that can withstand the pressures at extreme depth.
@Kansas Blue Robotics sells both the T200 with bare cable end, as well as ROV spare version that come with WetLink Penetrator already installed…
@tony-white You’re right about BR selling the T200 with the penetrator installed, which definitely indicates they are saying it is OK to do. If you are also right about the thruster-side water-blocking failing at ~ 400 m, then they seem to be selling a combination of products (T200 + enclosure) that can fail well below the stated depth specification.
Anyway, yes, I’ll purchase some connectors. Thanks for the thoughts.
Hi @Kansas -
It is definitely ok to use the T200 with WetLink penetrators. Blue Robotics doesn’t give a specific depth rating for the T200 (for now), but suggests it should be fine to 500m. The WetLink Penetrator, when properly installed, is good to1000m, except in the case of water moving along the inside of the cable.
The depth rating of the Blue ROV2 is 100 or 300m, depending on acrylic/aluminum enclosures. I’m a bit confused as to how you’re getting the idea that products are being sold “that can fail well below the stated depth specification.” Any product can fail below the rating if abused - hitting an ROV hard enough with a hammer to the dome at a meter deep in a pool would also result in a failure after all! In normal use, Blue Robotics stands confidently by depth ratings provided…
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Hi @tony-white
By “below specification” I mean below in value, not in depth. I am not working with the Blue ROV. And I am specifically talking about the case of water migrating along the cable.
Depending on size and endcap ratings, some of the enclosures are rated for 500, 750, 900 and 1000 m. If you connect a new, stock, perfect condition T200 and put its cable through a new, stock, perfect penetrator into a new, stock, perfect housing, and that housing fills with water migrating along the cable, that is a failure. The product suite did not meet the specified depth rating.
Hi @Kansas -.
I’m afraid I’d have to disagree with your assessment. The depth rating of any assembly is determined by the “weakest link” or shallowest depth rating. While a housing may be rated for 500+ meters, the depth rating of other components may be lower, and so determine the maximum allowable depth.
The BlueROV2 depth rating is determined by the acrylic housings, and then the Bar30, and finally the acrylic 4" dome as you replace each limiting component.
Blue Robotics doesn’t have an official depth rating for the T200 yet, but we are working towards a potting-less design that will have an official depth rating. If this depth rating is less than components it is used with, it will limit the overall assembly.
Due to inherent variability in potting, no official depth rating is present. If you’d like to use a T200 deeper than 500m, connectorizing as mentioned is the best approach!
In summary, depth ratings are only for the enclosures, and not a “product suite” of anything you may attach to it!