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To be legally called a 'purifier,' a unit has to meet the EPA specifications for blocking the passage of protozoa (large), bacteria (medium), and viruses (small). The industry term 'microfilter' does not have a legal definition, but is generally taken to mean a filter which meets the EPA specifications for bacteria and protozoa, but not the specifications for viruses. In general, viruses are just too small for small filters.

Most microfilters use either a labyrinth filter (a maze of twisty little passages, all the same) or an absolute filter layer (holes no larger than the filter rating). The latter may be referred to as a membrane. Both sorts have the disadvantage that they block up fairly quickly when handling murky water, and you have to either mechanically scrape the filter cartridge surface clean or replace the cartridge.

This MSR Hyperflow filter is a little different. Instead of having a single cylindrical membrane, it has a mass of fine 'micro-porous' tubes all bundled together. The tubes have tiny 0.2 micron holes in the walls for the water to flow through: holes small enough to block the bugs. Water is pumped from the outside of the tubes into them, to emerge at the open end. MSR calls this a 'Hollow Fiber Membrane'.

The photo above shows the ends of the little tubes. They look as though they are packed very loosely in the block of epoxy at this end, but this is a bit deceptive. The tubes do not go down to the other end of the cartridge and stop there: instead they are actually folded over at the bottom so that both ends are at the top face shown here. If you peer inside the other end of the filter cartridge you can just see the fold of the tubes.

The filter cartridge seems fairly robust, although MSR cautions against dropping the cartridge by itself onto hard surfaces. If it is dropped, it can be tested by the user: instructions are included for this. MSR also cautions against letting the wet cartridge freeze. Ice is likely to rupture the fine tubes, so this unit is not suitable for winter trips.

So far, this filter unit has strong similarities to other membrane filter cartridge units. But this MSR Hyperflow cartridge can be 'back-flushed' with clean water. The idea is that back-flushing causes most of the stuff sitting on the surface of the tubes - dirt, algae and bugs, to be dislodged from the surface. This should leave a clean surface and restore the filter to nearly full capacity. You can also disinfect the filter with a very dilute solution of household bleach for storage.

For those curious about the details of the mechanism, it seems that when the filter is back-flushed, the pressure inside the tubes increases, which forces them open just enough that particles wedged in the very small holes should fall back out of them and go back out the inlet port of the filter. Of course, you must use clean (filtered) water to do this!


  • Technical Background
  • Product Details
  • Product Operation
  • Formal Laboratory Tests Results
  • Initial Assessment
  • Field Testing
  • Summary
  • Specifications: Manufacturer:, Year/Model:, Manufacture:, Materials:, Filter specification:, Weight:, MSRP:
  • What's Good
  • What's Not So Good

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