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Dave G. wrote in justification of criticism of low pressure systems:
"Here is the sad part.   If your input pressure is 5psi (go below this
and you risk serious instability problems) and your output is 1.5 psi
(36" of water) and your temperature is 70F, then the Cv you need is
.000007 for one liter per day.   If you want one 2mm bubble of CO2 per
second, that is about 4mm3/sec or 15cm3/hour or .4L/day or a Cv of about
.000003 That is a long way from .004, or the Hoke value of .0008 which
still doesn't hack it."

Here's what's wrong with what Dave is saying, even assuming his calcs
are 100% correct:  There is no reason on earth to run your input
pressure to the metering valve at 5 psi.  I run my Nupro input pressure
at 40 psi and as I said in my last note, Dave could set his watch by my
bubble rate!  <G> A 10# CO2 bottle has lasted over 1 year on my 40
gallon setup.  

BTW, on all of my trips to Dave's house his CO2 levels were over 100 ppm
in his 55 gallon tank!  On the first visit the level was over 200 ppm!
I'm not entirely sure if this is intentional on Dave's part (to a
certain extent I think it is), but it leaves me a little worried that
using just a regulator and Eheim disk without any metering valve is
difficult to control.

I also use the high pressure system that Dave recommends in one of my
tanks (but with a metering valve) and I am quite happy with that setup
as well.  But there is nothing wrong or particularly difficult about
making a low pressure system work quite well either.  Just don't follow
Dave's straw man advice re the input pressure to the metering valve.
It's a problem you don't need to solve.

Steve Dixon in San Francisco