(I've copied this from another forum site so I can reference it more easily on this forum).
I've written this as a "primer" to Carbon Dioxide (CO2) management for kegs of home-brew. Its for those starting out or those moving up from small CO2 "bulbs" or other small cylinder arrangements (like "sodastream" cylinders). CO2 management can be considered in three sections: The CO2 bottle, the "regulator" and the (optional) output "manifold".
First you have your CO2 bottle which is often the 6.35Kg "pub" bottle. 6.35Kg is covenient, but don't be fazed by size: With CO2 the management systems are identical as long as you have the correct attachments (you can use "sodastream" type bottles if you don't wont big "pub" bottles). Handling big cylinders is no different from small cylinders, but big cylinders have the potential to do more damage if you do anything daft! Some brewers use fire extinguishes but these might have to be modified or used upside-down ... all a subject for another thread.
Next you have your "regulator" which drops the bottle pressure from about 900psi (more later) to what is often about 60psi (4bar) or less. The regulator is commonly variable so you can adjust the output pressure from, say, the 60psi down to 7 or 8psi (1/2bar). Down to 7psi if you're lucky - the common (cheap) regulators are far too imprecise to get lower pressures and you can be happy with 10psi (cue howls of protest from people who think they can get less). The "cheap" regulators are usually what is known as "single stage"; you can also get much more accuracy with "dual stage" regulators which internally have a fixed output passed (also internally) to another variable regulator mechanism (these are common in "aquarium" circles but much more expensive).
Thirdly (optional if you only attach one keg at a time) you have your "manifold" which splits the output to two or more kegs. These come in three forms:
1) Simple: Just a tube with one input (from the regulator) and multiple outlets (to the kegs). The outlets commonly have isolation taps and "check valves" so the gas can come out but it and anything else can't go back in. All outlets have the same pressure.
2) Complex: The manifold also has smaller (secondary) regulators allowing each output (or group of outputs) to have their own set pressure lower than the main (primary) regulator. These are less common.
3) Compound: Like "simple" but the input ("primary") regulator is possibly fixed pressure (2-5bar). Each output is then terminated with its own small "secondary" regulator. This is the method I favour as you have complete control over the output for that line (some "secondary" regulators - e.g. LPG regulators - can work at fractions of a psi). It can also be an expensive and complicated method. The use of "secondary" regulators effectively provides a "dual stage" set up mentioned above. IMPORTANT: "Secondary" regulators can't handle the pressure from a CO2 cylinder, hence the "primary" regulator.
Main (attached directly to the bottle) regulators commonly (not all) have two pressure guages: One displays the low pressure side and allows you to set a variable regulator and the other displays the high pressure side (the pressure in the bottle). The mistake is to think the high pressure guage tells you how full the bottle is; it doesn't! Carbon Dioxide has the useful property of being liquid in the cylinder. Pressure in the cylinder keeps the CO2 from boiling into a gas, that pressure being about 900psi (it varies a bit depending on ambient temperature). So as long as there is liquid in the bottle the pressure stays about 900psi. If there is no liquid the cylinder is empty. If the guage is reading 7-800psi or less get a refill quick, very quick, because you are running on fumes!
One other gotcha is some very cheap "Chinese" cylinder regulator imports (usually for aquariums). These "regulate" the flow with a needle valve. They do not regulate the pressure. Connected to a keg at static pressure they will quickly try to pressurise the keg to cylinder pressure (900psi). Boom!
A forum to discuss the various ways of getting beer into your glass.
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