Where to place an inline filter in the line?

[Brett Peugh]

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

[Bob Gross]

What is it that you are trying to filter?

What brand or type of filter is it?

–B.G.–

[Brett Peugh]

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

[Bob Gross]

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.–

[Michael Davis]

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.

[EndoftheTrail]

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…

[David W.]

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.

[Brett Peugh]

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

[Henry Laufenberg]

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?

[Bob Gross]

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.–

[Brett Peugh]

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

[Kendall Willets]

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.

[Bill Fornshell]

"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

[Brett Peugh]

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.

[Lance M]

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

[James D Buch]

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.

[David W.]

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.

[James Klein]

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.

[James Klein]

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

[Brett Rasmussen]

Lance,

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

Brett

[James Klein]

Lance the vacuum effect you described/demonstrated is nescessary to hold the water column static (the suction at the top balances the weight of the water – keeping it from falling). If the water were flowing freely through the tube there would be no vaccuum at the top.

James

[Lance M]

James, the 'vacuum' effect is maintained by the velocity of moving water. Potential energy is replaced by kenetic energy. The 'vacuum' continues.

[Henry Laufenberg]

Lance thanks for the demonstration.

James K thanks for the clear explanation of the science.

James B thanks for staying away from a career in education.

[James Klein]

Lance, I may be missing something….here is my understanding:

Static head builds b/c you are keeping the water from accelerating against gravity. There would be no pressure gradient from top to bottom in the dirty bag if it were falling off a cliff. If you have water flowing freely through a vertical tube of water the pressure at the top would equal the pressure at the bottom. The water would not be accelerating but that is because of the friction in the tube.
Keeping w/ flow through a tube…whenever the flow rate being supplied to the tube is less than the rate would be for free flow a vaccuum would occur. The magnitude of the vaccuum would increase with volume of water in the tube and decrease (toword zero) as flow approaches free flow. It would max out at the static head value for zero flow(but never get too close to 0psi).
Yes potential energy is converted to kinetic energy but this is when elevation of water is traded for velocity of water. The vaccuum comes about if the supplied flow isn't sufficient to let the water flow freely against gravity.
James

[James Klein]

Actually, after reasoning through (or trying to) my previous post, I've decided that (as you and a few others have mentioned), where the filter is placed inline isn't important – just the overall height from dirty bag to clean bag. This is assuming proper priming and tubing of same diameter.
The static pressure that builds on top of a the filter from a length of tube will be a function of water velocity – just as the vaccuum on the discharge side is. The effect tube height has on pressure (and thus flow rate) on either side of the filter is equivalent.
Sorry for whatever confusion I may have added to this thread. I am editing my original post to reflect my current understanding.
James

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