Where in the line should I put an inline filter? I just don't know if one place is better than another.

What is it that you are trying to filter?

What brand or type of filter is it?

--B.G.--

Water from a 3L Platypus Big Zip SL through a Sawyer 3-in-1 to whatever.

Gravity filters are all a little different. Some need more water pressure above, and some do not. That depends on the pore size of the filter. Mine works best if I have the upper bag, then about 2-3 feet of tubing, the filter, and then directly into the lower bag.

--B.G.--

For highest flow rate, place the filter near the bottom of the hose. And, the longer the hose, the better. This creates pressure provided by the height of the column of water, to push the water through the filter.

However, all filters are not created equal. Some do not require much pressure while others do. Therefore, some can be placed inside the "dirty" bag and work just fine. I use a Sawyer filter that is so fine that it can filter viruses, so it takes a lot of pressure and would never work up that high.

Depends on where you put your bladder, among other things. I would splice the filter fairly close to the bladder. No advantage leaving the black, ugly filter halfway up and exposed.

Back when I was using the Sawyer, my backpack's side pocket was spacious enough to accommodate both. I spliced 4-5 inches of tubing between bladder opening and filter -- to allow for a gentle "U" when inserting both into the side pocket. But you may well do differently if you've got a narrow but deep side pocket... so depends...

I use my Sawyer and Platypus by drinking directly from the hose most of the time but use it as a gravity filter for other water needs. I wanted to make sure the filter would travel inside my backpack but still maintain pressure and effectiveness when I used it as a gravity filter. The result is the Sawyer is located somewhere close to the middle of the hose. I dont know the precise flow rate but it works well enough as a gravity filter in this location.

Edited by Davidpcvsamoa on 07/02/2010 17:55:28 MDT.

I do want to use it for drinking from the 3L through the filter and filling 1L bags also.

of the filter on the hose, of course.

The claim is that the longer the hose before the filter the more pressure at the filter.

Why is this so? The size of the conduit doesn't change, and the amount of water pressing down on it doesn't change either.

To me it seems intuitive that 1 inch of hose would create as much pressure as 10 feet of hose. Is there something about the physics involved I'm not understanding?

Edited by hankj on 07/02/2010 13:50:31 MDT.

Henry, in the old days we used to fill a water bag and attach a long tube out of it. We would hoist it up over a high tree branch so that there was 15 feet of tubing running down to the filter on the bottom. With one particular kind of filter that we had back then (a VW gas filter), it took that much "head" or pressure of water to force the water through the filter.

It is a little different now with modern filters, but the principle is the same. A 15-foot column of water will put more pressure on the filter than a 2-inch column of water.

With my rig, I get very good results with a 2 or 3 foot column of water. Filters vary.

--B.G.--

I really have to think more on this I guess of just gut it after 2' but I am still wary.

Putting the filter higher up would create a negative pressure area below it. That's not bad in the ideal situation, but practically speaking the hose could collapse and block flow, or contamination could be drawn from outside the hose through a leak.

"Water from a 3L Platypus Big Zip SL through a Sawyer 3-in-1 to whatever."

Brett,

The box your Sawyer Filter came in should have a "flow" rate on it. The filter is designed for a specific flow rate.

It also doesn't really matter where on the line you put the filter. For my gravity filter I like it in the bag to help keep it clean and ready to use. If I want to dry it out I just hang it with the bag open.

If you want to verify what I have said you can call Sawyer. I just called them to confirm what I have just posted about the fixed flow rate. At about 6:15 CST - Friday - this number was still working.

1-877-260-1657

Well, on my 3-way filter box I can not find anything about a flow rate. It just has on it a few arrows on the unit pointing which way it is supposed to flow.

Theoretically, in ideal conditions, it makes no difference where you put your filter in a gravity water filter system. However, placing the filter at least a few feet below your water source has practical advantages.

The difference in pressure between the filter inlet and outlet is what forces water through it.

In a ‘bottom filter’ setup, each foot of elevation between the water source and filter inlet creates .434psi of hydrostatic head at the filter. Ignoring atmospheric pressure, the filter inlet pressure is .434psi for each foot of head and the filter outlet pressure is 0.

In a ‘top filter’ setup, for each foot of elevation between the filter outlet and the lower end of the tube, there is also .434psi of hydrostatic head. The head creates a ‘vacuum’ effect at the top of the tube equal to .434psi. Ignoring atmospheric pressure, the filter inlet pressure is 0 and the filter outlet pressure is negative 0.434psi for each foot of head.

In a top filter system, head between the water surface of the source vessel and the inlet of the filter may be sufficient pressure alone to force water through the filter.

However, for the ‘vacuum’ effect of the top filter system to work effectively, the tube must be primed and handled carefully so that prime is not lost. This is where the bottom filter setup has an advantage.

In the following pictures, I try to demonstrate the theoretical pressure differences.

In this picture, a tube is filled with water, both ends leveled and a pressure gauge attached. The gauge reads zero.


In this picture, the gauge is at the bottom with approximately 34” of head. (34” of water is approximately 63mm of mercury). Gauge shows about 60mm hg.


In this picture, the gauge is at the top with approximately 38” of head below it. (38” of water is approximately 71mm of mercury) The gauge moved from 0/320 down to 250; a decrease of 70mm hg.


These two pictures show the overall setups.


My observations and calculations confirm that the pressure differences across a filter should be the same in theoretical top filter and bottom filter setups.

My personal experience with Sawyer, Seychelle and Frontier Pro filters is that several feet of head between water source and filter work best because of the priming issues mentioned above.

Conversion factors used:
1 inch = 25.4 mm
Specific gravity of mercury = 13.6

Edited by Lancem on 07/03/2010 01:18:11 MDT.

Hank wrote:
"The claim is that the longer the hose before the filter the more pressure at the filter.

Why is this so? The size of the conduit doesn't change, and the amount of water pressing down on it doesn't change either.

To me it seems intuitive that 1 inch of hose would create as much pressure as 10 feet of hose. Is there something about the physics involved I'm not understanding?"

Pressure P=rho*G*H

where rho = density of fluid
G = gravitational constant
H = height of fluid column (measured parallel to the gravitational field)

Diameter does not enter the picture.

Science is NOT common sense.

We have had common sense and smart people for several thousand years, and if common sense were that important to science, we would be much further along.

A great failure of American education is for elementary teachers to apply "common sense" to teaching the sciences that they don't understand.

The penstocks on a hydroelectric dam seem like a good example to illustrate this principle. Steeper/longer penstocks have the potential to generate more energy.

Brett, the further you place the filter from the dirty water (in vertical feet) the faster you'll be able to filter water. This is b/c (as many have described) of an increase in pressure accross the filter.
The filter flowrate spec'd out by the mfct was/is likely determined for a specific pressure - that flow rate will be different for diffenent pressures. The more pressure (provided by static head in this scenario), the higher the flow. The height from the bottom of the filter to the clean bag will have a negligible impact on flow rate.

James

**EDIT: After considering/debating Lance's posts I agree that, theoretically, it doesn't matter where in line the filter is placed.

Edited by jnklein21 on 07/06/2010 07:47:31 MDT.

Henry, pressure is the distribution of force over a unit area.
Imaging an open bag of water with a long tube exiting the bottom and a valve at the bottom. All nonvertical surfaces in contact with the water contribute to holding the water up. If you increase the volume of the bag but keep its height the same (make it wider) there will be a greater force exerted upward on the water but it is also be exerted over a lager area - the net result is no change in pressure for equal heights on the two setups.
Now if you just made the bag taller (not wider) there will need to be an increase in force but no additional additional area to spread it around so the force intensity (pressure) has to increase.

James

Lance,

I was thinking the same thing you demonstrated. Thanks for the photos and explanation.

Brett