Sawyer Mini rocks!

@Colin- "At the filter, no matter where it is on the tube (the top, the bottom, or somewhere in the middle) the positive pressure from any water above the filter and the negative pressure from any water below the filter will collude to compel water through it at a pressure of 1.3 psi."

OK, I grok positive and negative pressure. But for this negative pressure doesn't that assume that the tubing beyond the filter is narrow enough that it is flowing freely with a water column its full diameter, as opposed to water just running down the inside of the tube? I suppose that's what you mean when you talk about air entering the tube from below. In my head I was picturing a rivulet running down the inside wall of the output tubing rather than filling it completely, since that's what mine seems to do. When using tubing as wide as the drinking hoses on most hydration systems this is what seems to happen if the flow is as slow as a Sawyer gravity rig, and you certainly avoid this possibility if you put the filter on the end of the tubing rather than between tubing and reservoir. Or is it just me?

Because it might just be me… :)

Ah. Yes, we were totally making different assumptions about the outlet tube. That explains it.

It didn't occur to me that you might be imagining a trickle of water through an air-filled outlet tube. You're right that a gravity setup with the filter at the top will only work if the outlet tube is full of water. I use 3/16" outside diameter tubing for mine to reduce weight and bulk, and it is always full of water during filtration.

If you want to use large-diameter tubing in your gravity setup because you already carry some for your hydration bladder, you can still put the filter at the top if you purge the air from the outlet tube by pointing the outlet end up and lowering it while it fills. If you use bottles for water storage so you don't already carry large diameter hose for a bladder, I don't see a reason to use tubing larger than 3/16".

"I don't see a reason to use tubing larger than 3/16"."

I assume that is outside diameter. However, all of my filter fittings go to the inside of the tubes. What is your inside diameter?

–B.G.–

The tubing I use is 3/16" outside and 1/8" inside (wall is 1/32"). It is NSF/ANSI 61 certified drinking water grade clear polyurethane tubing from Smallparts.com. They sell 25 feet of it for about $11:

http://www.amazonsupply.com/superthane-clear-polyurethane-nsf-61-tubing/dp/B003TJ9YN8/ref=sr_1_3?sr=1-3&qid=1381265976

I cut a 2" length of larger tubing that fits on the filter, and the smaller tubing fits inside it.