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CO2 regulation

OK, now I am mad ;^)>

Actually, some folks have accused me of being mad for a long time.....

Here is just about all I know about managing compressed CO2 (sorry for the 
long post):

In an un-managed configuration, a CO2 tank after a year to ten years of 
faithful service will run out of gas.  When this happens, the regulator 
which has been providing a steady outlet pressure of say 10 or 20 or 30 psi 
(depending on how you have it set up) will suddenly fail and dump the 2-400 
psi gas straight into the appliance (reactor or whatever) that you are feeding.

The purpose of this little diatribe is to tell you all I know about 
avoiding this problem.

There are three basic ways to manage your CO2 system and avoid end-of-tank 
failure.  These are anticipation of end-of-tank, relieving the pressure 
pulse, or avoidance of the pressure pulse.  Anticipation means that you 
track your CO2 usage or pressure and take steps to avoid the end-of-tank 
condition entirely.  Relief of the pressure involves a special device 
designed to relieve the excess pressure during the pulse so that the rise 
at the appliance is small.  Avoidance refers to using a regulator and 
associated control parts so that the pulse does not occur until the tank is 
totally exhausted, at which time the flow will stop.


Anticipation is the simplest and cheapest solution.  One way to anticipate 
end-of-tank is to weigh the tank regularly.  You can expect it to deliver 
about the announced volume (in pounds of CO2).  If you plot weight versus 
tank age in days it should be easy to guess about when you will have 
reduced its weight by the 20 pounds of CO2 you bought.  If you decide to 
replace it based on low weight, it would be interesting to empty it 
completely by opening the shutoff valve with the regulator removed to see 
(and weigh) how much gas actually remained.

Another way to anticipate end-of-tank is to closely monitor the 
high-pressure gauge on your regulator (if it has one).  The pressure will 
START to drop weeks before an end-of-tank dump occurs.

Once an end-of-tank condition is anticipated, you can simply turn the gas 
off at night until you have a chance to refill your tank.

Sadly, anticipation is unforgiving of inattention and if you don't notice 
the end-of-tank a disaster can occur.  If your failure occurs during the 
day when your plants are metabolically active and producing lots of oxygen 
then no problem will ensue.  Or if your tank is open-top, so that there is 
lots of atmospheric oxygen in contact with the water surface you are 
reasonably safe.  But if your top has a tight fitting cover and the dump 
occurs when lights are out and your tank is heavily populated with fish, 
the CO2 in the gap between the water surface and the cover may physically 
drive off all oxygen, at which point your fish can suffocate.


Relief is based on the idea that the excess gas that would be delivered 
during an end-of-tank dump is simply vented into the atmosphere 
somewhere.  The most convenient place to vent is usually right near your 
tank, but you can also plumb a plastic vent line to a more convenient 
location if desired.  A relief system should also be tested on a routine 
basis to make sure that it is continuing to provide the protection it was 
designed to give.

Pressure relief devices come in two kinds:  manufactured valves and 
water-head homebrew devices.  Many valve companies make pressure relief 
valves that will trigger at about 20 psi which should suit most users.  If 
you have a higher operating pressure, make sure you select a valve that 
vents at a pressure just a little higher than your norm.  One manufacturer 
of pressure relief valves is Rexarc, you can contact their sales department 
at (937) 839-4604 (info at rexarc_com).

You can also make a pressure relief device at home if you tap your system 
at a point where the pressure is supposed to be low (like right before the 
appliance, after any pressure-reducing check valve).  At this point the 
pressure should be only a few feet of water (2 feet of water is about 
1psi).  Simply take a 6 foot piece of 3" plastic sewer pipe, put a plastic 
cap on one end, set it upright open end up and fill with water to about a 
foot from the top.  Run a long piece of plastic tube or airhose down to the 
bottom of the sewer pipe and connect the other end to your CO2 line.  As 
long as the pressure is less than the five feet of water in the sewer pipe, 
the pipe will do nothing.  When the pressure rises to more than five feet 
of water, the excess gas will vent out thru the airhose, down to the bottom 
of the sewer pipe, and bubbling out the top.  You can enhance this design 
by also capping the top and running a CO2 vent line made of plastic water 
pipe out to a more convenient location if you wish.  Ensure the low end of 
the hose from the CO2 system stays at the bottom of the sewer pipe by 
weighing it down.  Or use a rigid plastic pipe to convey the CO2 down to 
the bottom of the sewer pipe.


A third strategy for managing end-of-tank failure is avoidance, referring 
to selecting system components so that failure simply does not occur.  This 
can be an expensive strategy but should be considered if the stakes are 
high (such as many tanks or a very expensive setup).  Avoidance strategies 
are like relief strategies in that they involving using different hardware.

One place to attack avoidance is at the regulator.  There are two stage 
regulators (where one regulator feeds another, do not confuse with two 
gauge regulators), down-stream regulators (that regulate based on output 
pressure, not input pressure), and better grade regulators (that undergo 
much less pressure rise as the input pressure falls).  All these strategies 
increase the cost of the system.  Whether they are worth it depends on many 
too many factors for one person to attempt to decide for 
another.  Generally speaking. if you are handy with tools and savvy enough 
to do your own CO2 system design, you should expect avoidance to add an 
extra $100-300 to the cost of your CO2 system.

I had considered the idea of adding a "avoidance" type system to my CO2 
system line.  I researched the issues carefully.  Although I could buy a 
better grade regulator for only about $30 more than a cheaper one, the 
company that made it interprets ISO rules to prohibit making special 
versions of their products (even if it only means omitting parts that are 
normally added at the end).  So I would have had to remove the original and 
build new output plumbing of the regulator myself.  With the labor costs, 
warranty, and liability issues, this just was not practical.  So I dropped 
the idea.  If you want to pursue this idea on your own, call Praxair and 
ask them about a Victor SR253-B320-06 and get out your tool box.

Dave Gomberg, San Francisco            mailto:gomberg at wcf_com
NEW Planted Aquaria Magazine:        http://www.wcf.com/pam