Dear All, firstly many thanks to everybody who replied.
What I learned was that others have this problem too. The oddest story was
from someone who said he had the suck-back problem in the quarantine
set-up, but not in the separate fly lab.
Just to remind you, here is what I wrote:
> Can anybody help with our strange problem with our CO2 anaesthetising
> set-up? We have a dual-stage regulator fixed to a CO2 cylinder, and run
> two lines off it. Each line bubbles through a separate side arm flask of
> water to humidify the gas. The problem is that when the cylinder is
> off, we get water sucking back into the tubing from the flask, sometimes
> nearly all the way back to the regulator.
>> This happens whether or not the cylinder is also turned off at the needle
> valve. It also happens in both lines, even if one of those lines is
> switched off. We can solve the problem by breaking the line at the
> branch-point, but we'd rather not have to do that every time we turn the
> gas off!
There were a few suggestions as to what could be causing it, but only one
seemed to apply to my particular system.
>Some tubings are CO2 permeable. Yours probably is. What happens is
>that after using CO2, you turn off the system. The CO2 leaks out of the
>tubing, but air cannot get in and you get a vaccuum that sucks up the
>water. One solution is to tip the water bottle when you are finished so
>that the bottom of the tube is exposed to air and not water.
I don't know whether my tubing is CO2 permeable, but it sounds plausible.
Suggestions as to what to do were all very good. The solution I am using
for now is to turn off the stopcocks between the flasks and the regulator.
We get water all the way back to the stopcocks, but the regulator is
protected. When we turn the gas on and open the stopcock, the water is
blown out back into the flask.
Another suggestion was to put an empty flask upstream of the water flask.
This would work, but the shear volume of water that gets sucked up would
mean we'd have to empty the upstream flask probably every day - we have not
yet tried it though.
One person said they do not humidify their CO2, but the letter implied that
their flies die after 10 - 15 mins on the gas. I'm sure other CO2 users
will back me up when I say that with humidity, flies can be gassed for much
longer, and that male sterility becomes a problem after 30 mins or so, long
before death is a factor.
Thanks again to all who wrote. I have replied separately to many,
apologies if I missed you - there were a lot of you!