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In these units: cm³.20μ/m².24hr.atm - (definition) cubic centimeters for a 20 micron film thickness per square meter per 24 hours under a pressure difference of 1 atm.

My question is what does the "Pressure difference of 1 atm" mean?

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One atmosphere pressure difference means the pressure one one face of this membrane is higher than the other face by one atmosphere.

It is the driving force for tests to measure the permeability of membranes for certain gases under constant pressure gradient and constant temperature. check here. Pressure difference is to test static mechanical permeability. Many other test are needed for each chemical or substance.

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  • $\begingroup$ So, if I wanted to determine how long it would take for a container to be emptied of its gas contents through permeation (assuming a higher pressure on the inside than on the outside), how would the 1 atm pressure difference play into the calculation? Would it be ignored as it being only a specific of the conditions of the measurement and instead only work with cm³.20μ/m².24hr ? $\endgroup$ – Hypnos Stratagem Aug 20 '16 at 2:48
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    $\begingroup$ it would take forever because as the gas passes through it's pressure decreases until it is equal to the ambient pressure. Hypothetically if your container would shrink to maintain the pressure difference then you would need the permeability index of that particular gas for that particular membrane and treat it as T= Volume in cm^3/permeation index while the temperature is held as specified. then you can convert the result to your desired unit of time. $\endgroup$ – kamran Aug 20 '16 at 4:13
  • $\begingroup$ The pressure inside the container will be about 10 atm and will be filled with hydrogen. This is the reason that permeation of gas through the membrane is something to be considered. I think that the pressure should be considered to be held constant throughout permeation (I am just trying to get rough estimates). I believe that the index on page 5 of this pdf should be helpful in the calculations (I could not post the screen cap of the page for some reason). eval.eu/media/38110/hydrogen%20fuel%20cell%20brochure.pdf Please note the hydrogen gas transmission rate part. $\endgroup$ – Hypnos Stratagem Aug 20 '16 at 19:35
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    $\begingroup$ I looked the chart, it says the thickness is factored linearly inverse but in the absence of more clear date about the pressure by just assuming permeability index of .3 by eye-balling the chart at 23 centigrade and zero relative humidity. so your thickness is .2 mil 1/ .2 =5. 5* .3= 1.5 this is our index. This means we lose 1.5 cubic centimeter of hydrogen across one sq. meter of this barrier per day. if you need to convert it par hour dived by 24 1.5/ 24 = 1/120 cubic cen. per hour. please check the numbers, I did not use a calculator or paper. $\endgroup$ – kamran Aug 21 '16 at 2:10
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    $\begingroup$ I picked the .3 from the second chart below the one you picked 3. By curve fitting the given sets of data at 1 and 50 and 100 bar which is near one atmosphere I think. Then I multiplied it by 5 for the thin 20 micron film. All by just eye and rounding to get fast result. $\endgroup$ – kamran Aug 23 '16 at 6:40

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