here's my interpretation of it... what the permeability coefficient tells you, is that net flux (loss) is as follows (from fick's law):
- proportional to the area of the membrane
- proportional to the difference in partial pressures inside and outside of the membrane
- inversely proportional to the thickness of the membrane
or net flux = permeability constant x area x deltaP / thickness
essentially... for a given tubing thickness, ID, length of tube, and operating pressures, the silicone will lose about 20132/90 times more CO2 than polypropylene (224x) tubing.
Example calculation:
1) area = PI x diameter x length of tubing
= PI x 0.47625cm x 30.28cm
= 45.60 cm2
if we assumed the following
ID = 3/16 inch = 0.47625 cm
length = 1 foot = 30.48 cm
2) deltaP = pCO2_inside - pCO2_outside
= 77.572 mm-Hg - 2.386 mm-Hg
if we assumed the following
0.0314 = CO2 mole percent in dry air
76 cm-Hg for std atmospheric pressure
pCO2_outide (Dalton's law) = 76 cm-Hg x 0.0314
pCO2_inside = 15 psi = 77.572 cm-Hg (pure CO2)
3) thickness = 1/32 inch = 0.79375 mm
4) permeability constant for silicone = 20132e-10 cc-mm/(sec-cm2-cmHg)
Therefore,
net flux_silicone = 20132e-10 x 45.60cm2 x 75.2cm-Hg / 0.79375mm
= 0.0087 cc/sec (Loss)
Assuming you are bubbling at 2 bubbles per second:
Let each bubble be around 0.5cm in diameter (SWAG)
volume (sphere) = 4/3 PI R^3
= 0.065 cc
Bubble_rate = 2 bubbles per sec x 0.065 cc
= 0.13 cc/sec
Net CO2 loss would be around 0.0087 / 0.13 = 6.6% per ft of tubing. so, a short run of silicone tubing (i.e., several feet could mean a non-trival amount of CO2 loss).
-snafu
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