What Is The Difference Between Single Stage and Dual Stage Regulators
Gas pressure regulators are used to reduce the pressure of gas supplied from a high-pressure cylinder of gas to a workable level that can be safely used for operating equipment and instruments. There are two basic types of gas pressure regulators: single-stage and two-stage. Single-stage pressure regulators reduce the cylinder pressure to the delivery or outlet pressure in one step.
Two-stage pressure regulators reduce the cylinder pressure to a working level in two steps. Since the performance of each is influenced by mechanical characteristics, the choice of gas regulator depends on the type of application for which it is intended.
The two most important parameters to be considered are droop and supply pressure effect.
Droop is the difference in delivery pressure between zero flow conditions and the gas regulator’s maximum flow capacity. Supply pressure effect is the variation in delivery pressure as supply pressure decreases while the cylinder empties. For most regulators, a decrease in inlet pressure causes the delivery pressure to increase.
The effect of these differences on performance can be illustrated with some examples. For instance, when a centralized gas delivery system is supplying a number of different chromatographs, flow rates are apt to be fairly constant. Supply pressure variations, however, may be abrupt especially when automatic changeover manifolds are used. In this scenario, a two-stage regulator with a narrow accuracy envelope (supply pressure effect) and a relatively steep droop should be used to avoid a baseline shift on the chromatographs.
Single-stage and two-stage gas regulators have different droop characteristics and respond differently to changing supply pressure. The single-stage regulator shows little droop with varying flow rates, but a relatively large supply pressure effect. Conversely, the two-stage regulator shows a steeper slope in droop but only small supply pressure effects.
On the other hand, if gas is being used for a short duration instrument calibration, a single-stage gas regulator with a wide accuracy envelope (supply pressure effect) but a comparatively flat droop should be chosen. This will eliminate the need to allow the gas to flow at a constant rate before the calibration can be done.
High Purity Gas Regulators
The ideal construction for high-purity gas service is a gas regulator that has a stainless steel diaphragm. Such regulators are non-contaminating and assure satisfactory use for all applications of noncorrosive and mildly corrosive gases.
Regulators for corrosive gases must be selected from those recommended with each gas listing. A gas regulator equipped with a stainless steel diaphragm has several advantages over the elastomeric type. It does not outgas organic materials and it also prevents the diffusion of atmospheric oxygen into the carrier gas. Both Buna-N and Neoprene diaphragms are permeable to oxygen. The chemical potential of oxygen between the carrier gas and the atmosphere provides sufficient driving force for oxygen to intrude the carrier gas through a permeable diaphragm.