Compressed gas regulators control the flow of gas from high pressure sources (e.g., compressed gas cylinders) to lower pressure applications. The regulators must: (a) be compatible with the gas being used, (b) accommodate the maximum cylinder pressure, and (c) have an appropriate output pressure range for the intended application.
Given these requirements and the myriad options available for regulators, choosing an appropriate one can be daunting. However, basic information provided here (not comprehensive) can assist with selecting the appropriate regulator. It is always prudent to consult the manufacturer to determine what regulator is needed for the gas being used.
Single-Stage vs. Double-Stage
Single stage and double stage regulators reduce the high variable pressure from the gas cylinder to a constant adjustable output pressure.
Single stage regulators reduce cylinder pressure to the desired output in a single step. The main advantage of these regulators is that they are inexpensive. The disadvantage is that the output pressure is not entirely stable and will significantly increase as the cylinder pressure decreases. This may necessitate manual adjustments to compensate. According to Matheson, single stage regulators can be a reasonable choice if gases are used for short periods (so the cylinder pressure does not change much) or manual pressure re-adjustment is practical over longer usage times.
The double stage regulator, as the name implies, is essentially two single-stage regulators plumbed in series in a single package. The first stage is nonadjustable; it reduces the cylinder pressure to an intermediate pressure, commonly around 200-300psi, which feeds the second stage. The second stage is adjustable and allows the user to further drop the pressure to the desired output. Double stage regulators are more expensive than their single stage counterparts, but the two-step reduction in pressure offers a stability and consistency in output pressure that is much greater. This stability often makes them the preferred choice, especially if the gas is to be used for long periods of time and/or pressure changes are unwanted.
For information on how to safely attach a regulator, review the CDC’s “Attaching the Regulator” video.
CGA Fittings
The Compressed Gas Association (CGA) has standardized a range of numbered connectors which are used to attach regulators to cylinders. Male and female fittings will only connect if they have the same CGA number, which minimizes the likelihood of using an incompatible regulator with gases that have special hazards e.g., corrosive, toxic, flammable, or oxidizing.
Certain fittings are also specific to extra-high purity gases, or extra-high pressures (>3000 psi). Before purchasing a regulator, be sure to check with your gas supplier or physically look at the cylinder to determine what CGA fitting is needed on the regulator.
There are a number of resources online which list the various CGA fittings and their applications, with schematics, e.g., CONCOA Precision Gas Controls CGA Fitting Reference. However, there is no substitute for getting information directly from the gas vendor and the regulator supplier to ensure full compatibility between regulator and gas cylinder.
NOTE: The specificity of CGA fittings exists for a reason. NEVER use an adapter to connect a regulator to a cylinder with a different CGA fitting. Using regulators with incompatible gases or with incompatibly high cylinder pressure may result in catastrophic failure and extreme danger. For other precautions on CGA fittings, see Fittings.
Cross-Purge Assembly
A cross-purge assembly is a regulator accessory that can be useful in safely changing over gas cylinders and extending the life of the system. These are most often employed in systems where corrosive or toxic gases are used. A cross purge assembly consists of three valves: (a) one is attached to the regulator, (b) the second is hooked up to a nitrogen line, and (c) the third is used to vent. During gas cylinder change-out, nitrogen gas floods the assembly and then is vented out, thus removing toxic or corrosive gases from the CGA connection. Once a new cylinder is attached, this process is repeated (before the cylinder valve is opened) to remove any air or moisture that entered the system during the changeover. This can help extend the life of systems using corrosive gases, as it prevents acidic or caustic compounds from forming inside the system. Cross-purge assemblies also minimize potential personnel exposure to harmful gases during cylinder changeover. If in doubt about whether a cross purge assembly is needed, consult with the manufacturer.
Cross-purge assemblies are vital in ultra-high-purity gas systems (e.g., as used for semiconductor synthesis) to eliminate contamination. Some high-purity gas systems may incorporate a vacuum cycle into the cross-purge, rather than just purging with inert gas.