Yes, but the filter has thousands of holes. Water molecules are only slightly smaller than air molecules.

A liter of water is 1kg. A liter of pure O2 (liquid oxygen) is 1.14kg.

If water can get through, so can air.


The person who said it was likely asking the person to blow through it to determine if the filter was CLOGGED, not ruptured.

Edited by mdilthey on 01/29/2013 15:10:55 MST.

When I just tried a 1 mm hole, the air flow was noticeable.

I don't know what the combined cross section of all the pores is in a Squeeze, but I'm guessing something less than 0.5 mm, just based on me blowing through Squeeze and through 0.5 mm hole.

Yes, there is air flowing through the Squeeze when I blow into it, but it's just not noticeable.

I'm pretty sure the person said blowing through the filter was for FREEZING, not CLOGGING : )

Because the discussion was the exact same question

But at my age, I start forgetting things : )

When I blow through my clean Squeeze - no noticeable air flow - so that wouldn't be a good test for clogging

Case and point here being, someone long ago confused the idea of blowing through an oil filter or a Brita filter to see if enough contaminants had built up in it that it needed to be replaced with the idea that a Sawyer filter was small enough not to let air pass through. There's no scientific basis for the claim that air cannot pass through a Sawyer Squeeze.

I challenge ANYONE on this forum to provide a scientific basis for the "blow test," and if none arises, I suggest we put it to death and stop believing everything we read. If air cannot pass through your Sawyer, it's time to clean it.

I think the scientific basis for needing water treatment is questionable

That is, the scientific data and interpretation is inconsistent - you can make an argument either way - again, enumerable threads and articles about this

But, Travis, if you can blow through your filter and notice air flowing, I'm pretty sure it's no good. Did it ever freeze?

If you don't notice air flowing, then maybe the filter is good and maybe not

Jerry, I didn't ever leave it out to freeze, but that doesn't mean ice crystals didn't begin forming with frigid water while it was out during the filtering process.

The instructions for the Gravity Works system by Cascade Designs, which like the Squeeze uses a hollow fiber filter media, describes:

Learn the very easy filter integrity test. If the cartridge freezes or is dropped hard, test it to be sure it is not damaged.

Filter Integrity Test:

A. Backflush a minimum of 1/2 liter of water.
B. Remove clean hose.
C. Blow air and check.
-cannot blow through filter = fiber OK
-see steady stream of air bubbles = fiber broken
-STOP!Replace cartridge.

Click here for a PDF of the instructions

Well I'll be dammed! Thanks Alex.

You're welcome, Travis. I'm not sure if this directly applies to Sawyer though. I seem to recall having a similar question and e-mailed Sawyer about it. I'll see if I can find it in the archives.

Rusty I doubt freezing was the problem with your Sweetwater filter. MSR says, "FOR LONG TERM STORAGE, place filter cartridge in a zipper-lock bag in freezer to retard bacterial growth. Discard your first few quarts of water after extended storage."

I wouldn't be too surprised if the crack occurred when sawing the cartridge in half.

"Jerry, I didn't ever leave it out to freeze, but that doesn't mean ice crystals didn't begin forming with frigid water while it was out during the filtering process"

I don't think it would freeze during filtering process, it takes a while for it to cool down and freeze. The plastic in the housing has a bit of thermal mass.

Regardless, if you can blow through it, I don't think it's any good.

Yeah - that makes sense Alex - I bet that applies to Squeeze too

Too bad you can't replace the filter in the Squeeze - except the Gravityworks filter cartridge costs a little more than the Squeeze - the Squeeze isn't much more than a filter

I have a Sawyer Complete 2 litre and another inline filter which I have the sneaking suspicion may have froze on trips last year even though I took all the precautions above.

One big mistake I made with the Sawyer was to try to filter it backwards by mistake (had filter around the wrong)
Rather than replacing them I think I will use some Aquamira with them to play itbsafe if I think the water is dodgy.

"I challenge ANYONE on this forum to provide a scientific basis for the "blow test," and if none arises, I suggest we put it to death and stop believing everything we read. If air cannot pass through your Sawyer, it's time to clean it."

Sorry but a freshly flushed easy flowing Squeeze does not seem to pass air to a noticeable degree. So I have to think you are wrong. The Gravity Works (also a hollow fiber filter) instructions seem to bear that out. I'll pass on trying to explain why.

"Oh crap. I can blow air through mine both directions easily. But it's been dry for almost a year, maybe this would account for that?"

I'd try it when wet out of curiosity if nothing else.

"Rusty I doubt freezing was the problem with your Sweetwater filter. MSR says, "FOR LONG TERM STORAGE, place filter cartridge in a zipper-lock bag in freezer to retard bacterial growth. Discard your first few quarts of water after extended storage." I wouldn't be too surprised if the crack occurred when sawing the cartridge in half."

Ah, yes, Pete. Thanks for jogging my memory. I do recall that when I got my first Sweetwater back in 95 or 96...before MSR bought them.

I don't know how that crack occurred...but I'm reasonably confident it was not during the sawing. I used my band saw w/ a fine toothed blade. It went through very smooth and easy. I inspected carefully afterwords and the crack was the only visible defect where water could have flown through unimpeded. The water I had filtered was full of sediment...and I remember thinking, "That's why it was pumping easier". In other words, the filter wasn't filtering. I noticed by pure chance. I happened to look in my stainless bottle at just the moment the sun shone in. Thought I was in deep poo with all the cows (major grazing area in the desert...typical silty desert water). Knock on wood. Didn't get the p00ps or any other side affects.

Edited by rustyb on 01/29/2013 18:28:33 MST.

“Yeah, if you blow on it and air flows through, it's obviously no good”

And vice versa; if you can’t blow air through doesn’t mean it’s still good.
Say the 0.1µm hole became 5µm because of freezing (or drop) damage. I don’t think you can feel/tell air is blowing through. Likewise can a 5µm hole make bubbles in water that are discernible with the human eye? It sure doesn’t seem like it. Is there a liquid expert on board?
Thanx,
-Barry
-The mountains were made for Tevas

My sawyer blew just fine when I got it. But, it seems, after filtering a couple liters of clean water in it, I can no longer blow through it. It baffles me; I thought I had all the science to it, but I am most definitely wrong in this instance.

So, this leaves me kind of wallowing in misery, because I was really proud of all that science I got to do for my last few posts... I wonder if it has something to do with the viscosity of the water trapped in the fibers?


I, for one, would LOVE to know, for sure, if A) the blow test works and B) what can be expected from a Sawyer after a full freeze.

"I, for one, would LOVE to know, for sure, if A) the blow test works and B) what can be expected from a Sawyer after a full freeze."

Same here

Rather than their pathetic statement from the original post

Max, a waterproof breathable membrane like goretex will allow gases to pass through it, but won't let you blow through it. The ability for fluid to flow does not mean that it will let it flow at a high rate. Blowing through a filter is "high rate" and seeping water is slow. You confused rates with absolutes. Look at all the technical reports and threads on this site relating to breathability and air permeability, similar concepts hold true for water filters.

If you want to explore the fluid dynamics it has a little to do with laminar flow and boundary layers. Fluids are still immediately next to a surface and flow faster the farther from the surface you move...so tiny tubes/holes reduce the area that air can flow quickly compared to larger diameters. Hence more efficient to have big water pipes than a bunch of tiny ones. Also viscosity of fluids has a damping effect related to the velocity of movement. The harder you blow the more resistance you'll have. Hence why you can't blow through a good filter but water will seep through it. I'm sure sealing dynamics play a role, IIRC the "effectiveness" of a seal between two surfaces is proportional to the square of the distance between the surfaces, ie diameter of tubes, and linear to the length of the tubes. That's why it's harder to blow through a long pipe than a short one.

Just because there are millions of tiny holes does not mean there are enough gaps to allow a "breath test" to work regardless of integrity. Now if we know that a sawyer squeeze does not allow air flow when new, and it starts allowing air flow after damage (freeze, drop, etc) we can say that a gap large enough to allow breath through has been created. This gap is most likely far larger than the items we are trying to filter so replacement in necessary.

This test is highly accurate, unfortunately not very precise. There could be micro damage to the tubes that don't allow breath through, but do allow water borne pathogens.

The MSR and similar filters that are "hollow tube" in design basically work by having thousands of tiny ceramic (ie rigid) straws glued together. When they freeze the ice expands and due to the inelasticity of the ceramic, it can fracture. You have a hard enough freeze and it can shatter enough of the tubes to ruin the filtering capabilities. The Sawyer Squeeze uses a different technology. First they use flexible tubes that can stretch. This should mitigate much of the "cracking" traditional filters suffer. Second they do not filter through the tube directly. Each tube is a permeable membrane with micro pores smaller than the specified micro filter sizing. Think of making loops with straws, then poking pinholes all along the length of the straw. You fill with water from both ends of the straw and the water seeps out the pinholes on the sides.

Unfortunately we don't know, and Sawyer hasn't studied, how their material behaves when frozen very well. Just because it is flexible doesn't mean the ice can't rupture or stretch out the micropores. Also depending on where the ice forms it could cause mechanical failures in the attachment of the tubes, similar to severing a tube that could allow dirty water to flow through easily.

It's just a matter of your risk tolerance. I slight freeze and you may be fine or willing to risk the filter on cleaner water source but a hard freeze could likely be problematic.

If you REALLY want to know, freeze your filter, study it under a microscope and then, sterilize a jar, filter some cow dung riddled water and culture the "cleaned" water on petri dishes. Repeat for about 30 filters (more if you're worried about contamination errors or are OCD) and you'll have a better idea of whether the filters survive. Or just keep it in your sleeping bag at night ;)