MSR AutoFlow Gravity Filter Review

Overview

The MSR AutoFlow is a 4-liter gravity filter: fill the roll-top reservoir, hang it from a branch, and a hollow fiber cartridge does the rest at roughly 1.75 liters per minute. It suits groups, basecamps and anyone who would rather set up the tent than crouch at a stream working a pump handle. The trade-off is weight and bulk. At 402 g it is a camp appliance, not a solo ultralight filter.

Key Specs

Spec	Detail
Weight	402 g (14.2 oz)
Dimensions	25 x 9 cm (10 x 3.5 in)
Reservoir capacity	4 L
Flow rate	~1.75 L/min (varies with filter condition and water quality)
Filter media	Hollow Fiber Membrane, 0.2 micron
Filter life	~1,500 L
Removes	Bacteria (99.9999%), protozoa (99.9%), particulate
Does not remove	Viruses, chemicals
Test protocol	NSF P231
Storage	Keep above 0 °C (32 °F); freezing damages the fibers

Performance

Speed. Gravity filtering is slower than the best squeeze filters in a single bottle, but it runs without anyone attending it. A thru-hiker on Trailspace who upgraded from a Sawyer Mini reported filtering 4 L in about two minutes, and called the difference striking. Another owner took it on a 35-mile trip with a group of 12 in Colorado’s South San Juan Wilderness. By the second day, that group was convinced, and hikers from outside the party reportedly gave up their own pumps to borrow it. Outdoor Gear Lab scored it 9 out of 10 for both speed and treatment capacity in its testing.

Ease of use. The wide, roll-top reservoir is the quiet star. Outdoor Gear Lab’s testers found it easier to fill and easier to seal than the zip-style closures on the competing Platypus GravityWorks bags, a gap that widens in cold weather. A clear window with volume markings shows how much water remains. The kit includes a 12-inch inlet hose, a 48-inch outlet hose, a shut-off clamp, a universal bottle adapter, a clean-side cover and a storage sack. The adapter fits wide-mouth Nalgene-style bottles and most reservoirs, so water goes straight into whatever you are carrying. There is no clean-water bag in the box. Anyone who wants to fill the system and walk away needs to bring a second container.

Shallow and silty sources. Gravity needs depth. One owner who used it on hunting and backpacking trips described digging out shallow creeks by hand and stacking rocks to get the bag fully submerged, and topping off with a cup when digging wasn’t possible. Outdoor Gear Lab flagged trickling or goopy sources as the one place a pump or a straw-style filter makes more sense. A hanging point also helps, and above treeline that can be the hardest part of the process.

Maintenance. The cartridge is rated for about 1,500 L and can be backflushed in the field without tools. Outdoor Gear Lab recommends backflushing about every other use. In heavy group use, its testers found that pushing a full four liters backward through the cartridge restored flow best. One Trailspace owner found the first backflush a bit fiddly. Another reported about 40 gallons through the filter with no slowdown, then flushed it with warm water and white vinegar at home. A separate owner who logs 25 to 30 field days a year said the system held up through four seasons.

Durability and hygiene. There are no moving parts to fail, which Outdoor Gear Lab cited in giving it a 9 for reliability. Its testers also noted that the bag material is less tough than some rivals, and that the bottle adapter is awkward to keep clean. The care rules for hollow fiber apply. The cartridge’s internal fibers can be damaged by a drop or by freezing, and the damage can’t be seen. MSR’s manual says to follow the filter test guide. Cold-weather campers need to sleep with it or leave it at home. After a trip, the filter should be washed and dried before storage.

Viruses. The cartridge handles bacteria, protozoa including Cryptosporidium, and sediment. It does not treat viruses. That is acceptable for most North American backcountry water. For international travel it needs a chemical or UV backup.

Pros & Cons

Pros

• No pumping. Hang it and set up camp while it works.
• About 1.75 L/min, fast enough to keep a group supplied
• Wide roll-top bag fills easily and closes without the struggle of zip-style seals
• Tool-free field backflushing, with a cartridge rated for about 1,500 L
• Universal adapter filters directly into most bottles and reservoirs
• No moving parts

Cons

• At 402 g, heavy for solo ultralight kits compared with squeeze filters
• Needs a hanging point and enough water depth to fill the bag
• No clean-water reservoir included
• Does not remove viruses
• Freezing or drops can silently ruin the cartridge
• Testers rate the bag as less durable than some competitors, and the adapter is hard to keep clean

Who Should Buy This

This is a filter for duos, families, scout troops and basecamp trips, and for hikers who have decided that pumping is no longer worth their evening. It also suits thru-hikers who cook and filter in camp rather than at every stream. Solo hikers counting every gram, shoulder-season mountaineers who can’t guarantee the filter stays above freezing, and anyone who mostly drinks from trickling seeps should look at a squeeze filter or a small inline option instead.

Verdict

The AutoFlow does one job, turning a stream into 4 liters of clean water with almost no effort. It does that job quickly and reliably. It is too heavy and too tied to camp life to justify a place in a gram-counting solo kit. For a group, it is among the more sensible filters to carry.