After some analysis of our manufacturing technique for making the main electronics tube endcaps, we have found a flaw that affects some of the o-ring grooves. The 6mm piece of cut acrylic that the o-ring is initially pressed against might have an imperfection at the point where the laser cut starts and stops. The o-ring can compensate for this imperfection in most cases, but if the amount of o-ring doesn't fit snug enough inside the electronics tube there can be a leak. If you have having problem with water leaking into your main electronics tube along this may be the ingress point.
There is an easy fix for this that we're doing on our own ROVs that is much quicker than rebuilding the endcaps.
The imperfection on the 6-mm thick piece of acrylic on the main electronics tube endcap looks like this.
It usually occurs around the hole where the wires pass through or 180 degrees from it.
You will need the following items to fix the problem:
Acrylic Cement: We use Weld-On 16 Cement
Micro Tips for Tubes: We use these
Sandpaper: 150 grit, 400 grit, 1500 grit (or around these numbers)
Steps to correct imperfection
1) The first step is to apply a layer of acrylic cement to the area of the imperfection. Make sure to fill in the divot and that you will from edge to edge so that the entire line is filled. Try to avoid getting the cement onto the facing pieces (resulting in a fillet).
2) Let this dry for 20-30 minutes
3) Once the cement has had time to dry you are going to use the sandpaper to smooth the area back to being smooth. Start with the 150 grit and then move to the 400 grit and finally to the 1500 grit. When sanding try to avoid scratching the sides of the o-ring grove (the acrylic pieces on either side of the 6mm piece where the imperfection is).
4) When you are finished sanding you should have a smooth surface for the o-ring to seal against