A regulator has an input and an output. Liquids or gases enter the input side. The liquids or gases before the regulator fluctuates widely in pressure. For example, when a pump first turns on, the pressure is great, but then drops off. Regulators have complex internal mechanisms that sense the pressure fluctuations. When the liquid or gas exists the regulator at the outlet, it does so at a constant pressure despite wide fluctuations at the inlet. A regulator's output pressure is set usually set by an adjustment knob on top. For instance, a water supply can fluctuate between 40 to 60 psi at the inlet. The supply to the house is kept at a constant 45 psi at the outlet.
The pressure variations in a system can be so great, one regulator alone cannot do the job. if one regulator alone is used, it may "blow out" when the pressure peaks. Therefore, the first regulator is used to bring the pressure fluctuation down to a manageable level. Suppose the pressure at the inlet varies from 85 to 200 psi. The required final pressure at the end of the system is 75 psi. The first stage regulator controls the pressure from 80 to 120 psi at its output.
The second stage regulator is connected back to back with the first stage. This means the first stage output is the second stage input. The second stage regulator is a fine-tuning device. It takes the reduced pressure fluctuations at its inlet from the first stage regulator, and produces a very stable pressure at its outlet. In the first stage example, the output was 80 to 120 psi. The second stage takes the first stage's output of 80 to 120 psi into its input, and regulates the final pressure at 75 psi at its outlet.
Thousands of uses exist for two stage regulation systems, but a very common uses is liquid propane gas lines from the tank to the house to the appliance. In gas systems, technicians at "Propane 101" state the first stage regulator is mounted in the gas line near the tank, and the second stage regulator is mounted near the house, closer to the appliances. This is done because the gas pipeline running from the tank to the house may be very long, and the pipe itself may induce pressure differences. The first stage regulates the pressure out of the tank, and the second stage regulates the pressure coming out of the pipe. Out of the second stage, the gas is fed into the appliances.