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Re: more high low pressure dabate

Okay folks lts really look at the facts here.  The big end result is that we 
looking a set amount of flow of CO2 into the tank.  This amount of CO2 can
vary considerably from one individual tank size, density of plants absorbing
the CO2, light being applied, and even fish load.  Therefore instead of simply
saying that one needs an 0.4 liters of CO2 per day is a real vague statement.
Individual daily need here could be anywhere from probably 0.1 liters to a 
full liter.  The only way of really telling our exact needs is the measure 
what we have.

If we could guestimate the daily need say we put this need at 0.2 liters of 
per day.  Than we need to determine what type of equipment we need to 
determine this.
So let us pull out the formula for flow which is:
	Flow = ([inlet pressure -- outlet pressure)/ (an equasion with 
density and temperature 
variables)) Cv.
	Breaking this down further we can determine that the Cv required will 
be an indirect
proportion to the pressure difference between inlet and outlet pressure.  Or 
if the Cv
is small the pressure difference needs to larger than if the the pressure is 
	Next let us look at determines the pressure difference.  Outlet 
pressure is basically the
total of how much resistance to flow is in the system.  This is determined by 
the height
of the water column (a 14 inch high tank will have 1/3 the pressure as a 42 
inch high
tank)  the barometric pressure, the length and diameter of the tubing, and 
the diffuse on the end if any.  Most of these factors we can keep vary 
constant with
the exception of barometric pressure.  Therefore lets say that this totals to 
be 1.2 psi
of total outlet pressure with a variance of .05 psi in a low pressure system 
and in 
a high pressure system it is 20psi with a similar variance of .05 psi.

Moving onward to inlet pressure we need a value slightly larger than the 
pressure since pressure differential is inlet pressure minus outlet pressure. 
simple terms the outlet pressure is what we have set the regulator to.  So if 
a differential of .2 psi.  For a low pressure system we need to set our 
at 1.4 and for a high pressure system at 20.2 psi.  Now we get the deciding 
to me and that is most regulators are rated as stable within X percentage of 
setting.  If this is within 1% than let us look at the variable pressure 
we get.

In a low pressure system with the regulator set at 1.4 psi the inlet pressure 
will range
from 1.386 to 1.414 while the outlet outlet pressure will range from 1.15 to 
The final pressure difference  for a low pressure system is  0.136 to 0.264 
or about
32% swing from the ideal 0.2 psi.

In a high pressure system with the regulator set at 20.2 psi the actual  
will range from  19.98 to 20.402 psi.  With the outlet pressure of the system 
now ranging
from 19.95 to 20.05 we can recalculate the worst case of the pressure 
and find the final outlet pressure will be between -0.07 and 0.452 psi.  This 
creates a 
swing of about  226% plus or minus.

To me this make the lower pressure system with a small orifice valve 
extremely more
stable.  Yet neither system in this range is really acceptable.  The smaller 
the orifice
of the valve the larger the pressure difference you can work with therefore 
the system.

However, in reality for a free flowing unmonitored system neither method in 
itself is really acceptable.  The Cv of the valve being reduced would allow a 
larger differential
pressure making things more stable but still there would be considerable 
As I see it the only real solution is the constant monitoring of the pH 
allowing the
automation of the system to assure a constant level of CO2.