Screen or Perf?
Both screen and perforated sheet are used in PMDs. They both can prevent gas penetration while allowing liquid flow.
Perforated sheet's main advantage is that it is more easily welded and handled in manufacturing. Unfortunately it is often the poorer performing porous element.
Screen is more difficult to weld and handle but offers four potential advantages over perforated sheet:
1) Screen comes in finer pore sizes and thus higher bubble points. If a higher bubble point is required then screen must be used.
2) At the same bubble point, screen flow losses are almost always lower. This does not preclude perf sheet but more area is required.
3) At the same bubble point, screen transient capability is almost always higher. This does not preclude perf sheet but more area is required.
4) Screen wicks and perforated sheet does not. Anywhere where either gas penetration may occur prior to depletion (e.g. a trap entrance) screen is highly desired since it will reseal itself without other PMD components. In addition, anywhere gas may be in contact with both sides of a porous element, screen is required. Perf sheet cannot hold its bubble point with both sides exposed to gas.