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CO2 loss through silicone tubing (was multi tank CO2)



It has been suggested that I modify my previous post in the following way:

ďThe pressure difference you need to calculate is the difference between the
CO2 pressure in the line and the partial pressure of CO2 in air.  And 15 psi
is a more realistic pressure.  The gauge is reading the pressure *above*
room pressure.Ē

The CO2 permeability of silicone tubing is 10132 (units:  10^-10
cc-mm/sec-cm^2-cm Hg).  A better choice is FEP (fluorinated ethylene
propylene) tubing (CO2 permeability:  5.9).  So, is it worth it to use FEP
instead of silicone tubing?  Iíve wondered about it myself, so hereís my
shot at a calculation (Iím sure someone will let me know if Iím wrong):

CO2 flow rate		1 bubble/sec		0.06 cc/sec (my measurement)
Room pressure		14.7 psi		76.0 cm-Hg
Partial pressure CO2 in air				  4.0 cm-Hg
CO2 Line Pressure		15.0 psi		78 cm-Hg
tube wall thickness		1/32 inch		0.8 mm
ID of tube			3/16 inch		4.8 mm
Area 1 ft of tube		7.07 inch^2		45.96 cm^2

Fickís Law:
Flow = Permeability * Area * (Pressure Difference)/ Thickness
= 10131*10^-10*45.96*(78+76-4.0)/0.8

Tube length		CO2 loss (silicone tubing)
1 ft			0.0087 cc/sec (ml/sec)
2 ft			0.017
4 ft			0.035
8 ft			0.070
10 ft.			0.087
100 ft.			0.87

So, you lose about 58% (!) of your CO2 with 4 ft of silicone tubing per tank
and virtually none with FEP.  With multiple tanks, the percentage loss is
the same, but the total volume lost is greater.  CO2 is cheap (I just paid
$13.65 for a 10# refill) -- but youíre also saving the cost (annoyance) of
getting your tank refilled more often.

Well, thatís a huge lossÖ  Iím I still doing something wrong?  Wrong
equation, wrong assumptions?

Joe K.