Here's an article that explains the difference between a single stage and dual stage regulator.
Source:
http://www.alspecialtygases.com/Prd_...egulators.aspx
Single-Stage versus Two-Stage
Single-stage pressure regulators reduce cylinder pressure to delivery or outlet pressure in one step. Two-stage pressure regulators reduce cylinder pressure in two steps. Since the performance of each is influenced by mechanical characteristics, the choice of regulator depends on the requirements of the application.
The two most important variables to be considered are droop and supply pressure effect. Droop is the difference in delivery pressure between zero flow conditions and the regulator’s maximum flow capacity. Supply pressure effect is the variation in delivery pressure as supply pressure decreases while the cylinder empties.
Single-stage and two-stage regulators have different droop characteristics and respond differently to changing supply pressure. The single-stage regulator shows little droop with varying flowrates, but a relatively large supply pressure effect. Conversely, the two-stage regulator shows a considerable droop, but only small supply pressure effects. Generally, a single-stage regulator is recommended where inlet pressure does not vary greatly, or where periodic readjustment of delivery pressure does not present a problem. A two-stage regulator, however, provides constant delivery pressure with no need for periodic readjustment.
