May I correct the last statement? It is wrong. Instead of using the word "dual", do the words "two stage" and "two gauge" make the terminology easier?
Most regulators that we use have two gauges whether they are single stage or two stage models. The gauges are the high pressure gauge and the low pressure gauges. Sometimes the low pressure gauge is called the working pressure gauge. On most regulators this is adjustable while the high pressure gauge isn't.
The high pressure gauge measures the pressure in the CO2 cylinder/CO2 tank. It usually reads from 0 to 3000 or 0 to 4000 psi. The other gauge measures the output pressure. This is the low pressure gauge. There are several different pressure ratings that we can use. The most common low pressure gauges on the regulators that we use are:
0 to 30 psi
0 to 60 psi
0 to 100 psi
and 0 to 200 psi
Maybe the following will help some. It is easy to tell the difference between them. Both single and two stage regulators usually have a hump on the front. The single stage regulators usually have a flat back side. Two stage regulators usually have a hump on the back side as well. This is where the other stage resides. I am using the term "usually" because there can be some shapes of regulators that are not the norm. They can look a bit odd. But, most of the time if they are flat on the back with a hump on the front, they are single stage. If they have a hump on both sides, they are two stage regulators.
In the diagrams below, you can see the flat back side of the single stage model and the hump on the backside of the two stage model.
From: http://www.barrreport.com/showthread.php/6470-Dual-Stage-Regulators
Some people want to understand the differences between single and two stage regulators a little more as well as how they work or don’t work in some cases. "End of tank dump" is explained even though that phrase is not mentioned. The following comes from a Matheson catalog. It is well written and easy to understand.
TWO STAGE regulators reduce the source pressure down to the desired delivery pressure in two steps. Each stage consists of a spring, diaphragm, and control valve. The first stage reduces the inlet pressure to about three times the maximum working pressure. The final pressure reduction occurs in the second stage. The advantage of a dual stage regulator is its ability to deliver a constant pressure, even with a decrease in inlet pressure. For example, as a cylinder of gas is depleted, the cylinder pressure drops. Under these conditions, single stage regulators exhibit a “decaying inlet characteristic”; where the delivery pressure increases as a result of the decrease in inlet pressure. In a two stage regulator, the second stage compensates for this increase, providing a constant delivery pressure regardless of inlet pressure conditions. The dual stage regulator is recommended for applications where a continuous supply of gas is required; such as the gas supplied to analytical instruments where constant delivery pressure is critical.
SINGLE STAGE regulators perform the same function as the two stage regulator using a single step reduction of source to outlet pressure. For this reason, the outlet pressure cannot be as accurately controlled as the source pressure decays. We highly recommend single stage regulators only be used in circumstances where the operator can monitor and adjust the regulator as needed or where the regulator is supplied a nearly constant source pressure.
Regulator Design & Construction Features
TWO-STAGE GAS REGULATOR
SINGLE-STAGE GAS REGULATOR
