# RE: Final end-of-tank-dump comment (from me)

```I'm sure no-one is interested in this anymore ... but I'll post one final
bit of information.

As below, yesterday I tried filling a 6ft length of airline with CO2.  This
time I stood a jar of water on a high place with the open end of the top of
the tubing held below the water level.

The bottom end of the tube (~ 6ft lower) remained tightly clamped.

(If anyone's really interested I can email you a photo.)

After 30 minutes the water column had risen up over the lip of the jar (up
about 3") & progressed down the airline by about 3ft

After 6 hours (overnight) only about 6" of airline remained without any
water at the lower end of the tube.

So I guess the strong message is - use a checkvalve.  If not, as soon as
any positive pressure from flowing CO2 stops (tank empty, solenoid closed
etc.) the water at the end of your airline will absorb CO2 & be pulled back
towards your regulator & cylinder.

Regards, Kevin

-----------------------------------------
For anyone who's still interested in this thread ...

Last night I did a little experiment.

- Take a 6ft length of silicone airline.

- Fill it with CO2 & put a clamp on both ends.

- Suspend it from a suitable high place with the bottom end submerged in a
container of water.

- Remove the clamp from the bottom end.

What happens is that a column of water slowly starts to climb up the
airline.

After 30 minutes it will climb up about 3 feet.

After an infinite amount of time (well ... overnight anyway) it climbs up
to around 4-1/2 feet.

Interestingly, it doesn't collapse the remaining, water-free tubing -
unlike what I see in my aquarium CO2 set-up..

I assume that the reason the water column only goes up to 4-1/2 feet is
because the weight of water causes a pressure reduction in the remaining
1-1/2 feet of CO2 which balances the waters ability to further absorb any
more CO2.  So in a real system, with the cylinder beneath the aquarium &
where the water would enter at the top & the weight of water is pushing
DOWN on a static column of CO2, I guess the water would eventually
completely fill the tube?

I suppose that tells me that anyone with a CO2 system which, at any time,
has the CO2 shut-off (like in a pH controlled system or if the cylinder
runs empty) is at risk of having water pulled back into the regulator &
cylinder unless they have a check-valve.

I might test that hypothesis tonight, if I have time, by swapping the
location of the container of water to the top of the airline.

Regards, Kevin

```