I was looking the clean Stream

http://www.platypushydration.com/product_detail.aspx?ProdID=29

I noticed in the photos that the 'clean' bag was used in an upright position - Would this not slow the flow? It appears that the water is filling the bag from the bottom up and would have the weight/pressure of the water that has already filtered above the in flow point.
Can those bags be flipped upside down? it just seems to make more sense to fill the other way - but then there would have to be no air in the bag.

Side question - can the ceramic filter from an msr miniworks be used to gravity filtering without having touse a super long hose and great distance between the bags toprovide enough pressure to force water thru the filter? I just want to know about the filter - I have already solved the problem of having a housing for the filter cartridge itself.

Share the secret, how are you enclosing your filter cartridge to make a gravity filter ?
You should be able to have the bag any way you want, fill bottom up or top down.

No secrets here - I chose to replace my old one instead of paying 40 to just get the ceramic filter- that left me with the original housing. The seals are still in perfect condition, so all I need to do is remove the handle to convert from pump to gravity. If this works I will search for some type of light plumbing pipe of the correct diameter to enclose the ceramic cartridge in an effort to drop the weight. Actually the gaskets are what provides the seal - so the width of the container is not as critical as the length. The main problem is that it may be hard or impossible to find something that weighs less than the original.

Just tested a nalgene lid on the body of the filter - no mods needed to the pump handle - just fit the feed hose to the nalgene lid and let it drip and leave the handle at home - weight of filter 8 ounces with nalgene lid. Add to that the hose, and a dirty water bag (platypus).

I just bought a new gravity filter and did some research reading on this subject recently. My guess would be that the position of the lower bag is all for photo opps and that you would want to most likely lay it flat on the ground or upside down. A few people mentioned that as the clean bag fills up with water, that you need to keep an eye on it as it will need to be burped. It seems that there is a fair amount of air that accompanies the clean water. So it may not be a simple case of set it up and walk away. Since I use a MSR Dromedary bag, I took the plastic universal filler cap off of my old Sweetwater filter and use it with my new gravity system on the Dromedary bag as it has the same size mouth as a Nalgene. It has the little hole in it already so it self burps. There were quite a few people that mentioned using a Sawyer brand inline filter available for around $30 with good success. So if you already have the Platy bags, you could do it cheaper yourself by just buying the filter and hose or simply place the filter inline between the hoses of the 2 existing bags. Sorry, I did not read anything about being able to use the ceramic filter in a gravity system. The Pur Hiker Pro is used in a few. It’s my understanding that both the Platy and MSR systems use the same inline filter (just different colors).

Spey - which gravity filter did you get?

I went with the ULA Amigo which uses a Hiker Pro filter. I had thought about making my own after I read this thread on a DIY one. But after I priced the replacement filter cartridge ($40) and the rest of the pieces I needed, it just made more sense to buy the Amigo. It just came last week and I've only tested it in the bath tub. I wasn't sure if I would like the bag as it is made out of one big flat piece of 1.9 oz rip stop. But it actually fills up pretty easy in only a couple of inches of water.

I like the idea of double Platy set up because you could always use the dirty bag in a pinch by just sterlizing it out with some Aqua Mira or similar chemicals. That way you always have a back up bladder and even the ability to use both at the same time if warranted.