The Cap
My first idea was a cap like the yellow thing in the drawing:

The cap rests on the top of the tube. In the cap there is room for an O-ring. After some simulation (a collegue did it in 15 minutes) I had the following results:

The drawing shows half of the cap.

At 100m depth (10 bar) the cap is pressed in for 5.7 mm!! The cap is 15 mm thick. The presure is about 3300 kg. Because the cap is pressed, the presure point, where the cap is pressing on the tube is at the inner part of the tubewall. 

In the figure right the pressure inside the cap is shown:

In the middle of the cap, the pressure inside is about 50 MPa. PMMA can handle this. But at the pressure point it is 68 MPa. This is too high. The cap wil start too scratch or even break here...... 

Soo I choose another solution:

The ring is sawed from the tube. It is 3 cm wide. The ring is mounted with 8 M6 bolts through the ring and the tube. I tapped a thread in the tube and a nut inside the tube (I can make a drawind, but ....) I turned the bold from the outside in the tube, throug the ring and placed the nut. I used Loctite 572 for sealing.

The cap with the o-ring: The O-ring is already mounted, the protective sheets are still present.

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