pressure-design-dat

• submarine-dat

Normal direction and force distribution

This is a fluid‑statics + structural mechanics question. Conclusion first, details after.

1) Hydrostatic pressure itself: no difference

For a flat surface and for a curved surface (at the same depth h):

p = ρ g h

Pressure depends only on depth, not on shape. The pressure magnitude per unit area is identical for a given depth.

The difference is not in the pressure magnitude, but in the local normal directions and how the pressure vectors combine.

2) Difference in normal directions (the key point)

• Flat surface

  • The entire face has a single fixed normal direction.
  • All pressure vectors point the same way.
  • Resultant force = pressure × area (single direction).
  • Example: observation windows, flat end caps.

• Curved surface (cylinder)

  • Each small surface element has its own normal direction.
  • Local pressure always acts along the local surface normal (radial inward for a cylinder).
  • Example: pressure hulls, submarine shells.

3) Resultant force and stress consequences

• Cylinder sidewall

  • Radial pressure components around the circumference largely cancel each other.
  • Net lateral resultant ≈ 0.
  • Principal stresses produced are hoop (circumferential) stress and axial stress (if end caps are present).
  • Therefore cylinders are very resistant to external hydrostatic pressure.

• Flat plate

  • All pressure vectors add in the same direction.
  • The resultant force accumulates and causes bending, bulging, or fracture.
  • Flat faces are typically the weakest parts of pressure designs.

4) Intuitive picture

• A flat plate feels like it is being "pushed" inward by a block of water.
• A cylinder feels like it is being "squeezed" evenly from all sides; the water "hugs" it rather than pushes it off.

shape design summary

Conclusion (same internal volume):

Shape	Surface area	Stress concentration	Pressure efficiency
Flat box	Largest	Very high	❌ Worst
Cylinder	Medium	Low	✅ Good
Sphere	Smallest	Almost none	✅✅ Best

For the same internal volume, the rounder the shape, the more depth a given amount of material can resist.

Acrylic (PMMA) hemispherical container for 100 m depth

Summary and key engineering recommendation

• Design safety factor: multiply theoretical thickness by 4–6.
  • Example: 2.2 × 5 ≈ 11 mm
• Conclusion (ready-to-use): For a 130 mm diameter hemispherical acrylic (PMMA) viewport at 100 m depth:
  • Recommended thickness: 10–12 mm
  • Absolute minimum (not recommended): 8 mm

Flat window comparison

• Under the same conditions, a flat acrylic window would require 25–30 mm thickness or more and still carries a risk of sudden brittle fracture.
• A hemispherical window is approximately 3–5× stronger than a flat window.

Practical construction advice (important)

• Use a single-piece thermoformed hemisphere (do not bond halves together).
• Do not glue the hemisphere in place.
• Use an O-ring with a floating clamping arrangement; avoid rigid clamping.
• Make the inner diameter slightly larger than the outer diameter seating to prevent the window being "pulled out" by differential pressure.

One-line summary

For a 130 mm diameter hemispherical acrylic viewport operating at 100 m depth, 10–12 mm thickness is a reliable, engineering-grade, safe choice.

Metal cylinder with transparent end windows (100 m target)

Context: If the pressure vessel is a metal cylinder and the two ends are transparent acrylic observation windows, at 100 m depth (≈10 bar / 1.0 MPa external pressure) the failure mode shifts from cylinder buckling to transparent end-window deflection and seal failure. Below are practical, engineering-focused parameter recommendations that include an overall safety factor of ≈2.

Overall conclusions

• A metal cylinder easily reaches 100 m and even 300 m.
• The real limiting factor is the transparent acrylic end windows.
• To reliably reach 100 m, the windows must be: thick, spherical or domed, and sealed with a face (axial) O-ring.

Recommended standard design (most robust — strongly recommended)

This is a common, engineering-grade approach for 100 m observation housings. It is simple and has a high success rate.

Cylinder (main pressure hull)

• Material: 6061-T6 aluminum or 304 stainless steel
• Outer diameter: 200 mm
• Wall thickness:
  • Aluminum: 4–5 mm
  • Stainless steel: 3 mm
• Length: 600 mm
• Construction: minimal welds or full welds with annealing

-> For 100 m this provides a large strength margin.

Transparent end windows (critical)

• Material: cast acrylic (PMMA)
• Shape: outward convex hemisphere / spherical cap (not flat)
• Diameter: ≈180–190 mm (embedded)
• Minimum thickness: 20 mm
• Effective radius of curvature: ≥ 90 mm
• Loading behavior: external pressure clamps the dome and improves sealing as depth increases.

Sealing

• O-ring material: NBR or FKM
• Hardness: 70–75A
• Compression: 20–25%
• Sealing style: axial face seal (recommended); radial seals are not recommended
• Window mating surfaces: chamfered and polished (Ra ≤ 0.8)

Engineering assessment

• Theoretical window pressure capacity: ≈200 m
• Recommended operational depth (with safety margin): 100–120 m for long-term service

Secondary (lower-risk) option (higher risk, but lighter)

• Window material: acrylic
• Shape: shallow dome
• Thickness: 15–18 mm
• Constraints:
  • Window diameter ≤ 160 mm
  • Use dual O-rings
  • Use a metal clamp ring to load the window evenly

This may reach 100 m for short-term use but is not recommended for repeated long-term operation.

Absolutely unacceptable end-window options (will fail at 100 m)

• Flat acrylic windows (no matter the thickness)
• Transparent windows ≤ 12 mm thick
• Large-diameter (≥ 180 mm) flat windows
• Gluing the window directly (no mechanical seal)
• Unsupported "clamped glass" without metal backing

Quick risk summary (100 m class)

• Cylinder structural strength: ★★★★★ (very safe)
• Transparent window risk: ★★★★★ (the single critical item)
• Seal failure risk: ★★★☆☆ (controllable)
• Manufacturing tolerance importance: ★★★★★

One-sentence version

Metal cylinder + thick domed acrylic observation windows = standard engineering approach for 100 m.

Recommended parameters: metal cylinder 4–5 mm wall + 20 mm domed acrylic end window + O-ring face seal.

General pressure design notes

Using tubing for deep water

If you need to reach 100 m underwater, follow these recommendations:

1. Industrial thick-wall PVC (PN16 or higher) can be used, but plastics are still not recommended for deep external pressure environments.
2. Use a flange connection with an O-ring seal — this is far superior to bonded joints.
3. Using a metal pipe (e.g., 316L stainless steel) is more reliable.

Most important note: Never rely on plastic adhesives for deep-water pressure seals. Always use mechanical sealing (flanges + O-rings), metal housings, or purpose-built deep-water equipment.

References

• waterproof-dat