I’m spinning up a student-friendly tethered ROV to reach 30 m with a 100 m tether, four T200s, a Pixhawk on ArduSub, and an acrylic pressure tube with dual O-rings, and I’d love hard-earned tips on leak detection and buoyancy trim before its first cold dunk in Yaquina Bay next month. If you’ve battled silt, thermoclines, or fogged domes on budget cameras, what worked for you — desiccant, pressure equalizers, or biting the bullet and going aluminum?
I vacuum-test the tube at -5 to -7 psi with a hand pump (Mityvac or the BR kit: Hand Operated Vacuum Pump for Leak Testing ROVs and Enclosures), stick it in the fridge and rotate it — if the gauge drifts or you hear a “tink” as an O-ring slips, fix that before the bay; small caveat: grease swelling can fake a slow drift, so run a second cycle. For trim, stay slightly positive and put buoyancy as high as you can — one thin lead strip under the battery tray cured my pitch bob in silt; you seeing thermocline-induced fogging or just ambient chill?
One cheap win for leak detection: tie a tiny paper-towel “tail” under each cable penetrator so the first drip wicks straight to a low-mounted leak sensor and trips early. Building on @erikV’s cold-test, swap the tails after each session so they don’t hold moisture and fog your acrylic.
Building on @bellaO90’s early-trip idea, drop a 30/40% humidity indicator “canary” card and a tiny desiccant pack inside, then power the lights/ESCs for 15–20 minutes on the bench; if it flips color, you’ve got vapor ingress and a fog risk before any droplets. For trim, chill your test tub to Yaquina temps and set it just positive with the battery sled low — the thermocline will rob a few grams and a warm-room neutral can slowly sink.
Trim for depth, not the bench: with an acrylic tube and typical buoyancy foam you’ll lose lift below the first thermocline, so I clip a 120 g drop weight under the keel and peel grams off after a 10‑minute soak at 25–30 m until it sits just slightly positive. Keep the battery as low as you can and foam high to lock in self‑righting; the four T200s will hide bad trim but you’ll pay in amps. If it helps, this stability primer is dead on: https://bluerobotics.com/learn/buoyancy-ballast-and-stability/.