The pressure of the supercritical fluid is controlled by the restrictor.
Restrictors can be broadly classified into two types: fixed and variable.
Fixed (diameter) restrictors are typically made of fused silica or metal tubing.
They are inexpensive and easy to replace, but are subject to plugging problems.
A common cause of plugging is water freezing at the restrictor tip because of the rapid expansion of the released supercritical fluid.
Plugging can also happen when the matrix has high concentrations of extractable materials such as elemental sulfur, bulk hydrocarbons, or fats.
Variable restrictors have an orifice or nozzle that can be adjusted electronically.
They are free from plugging, and a constant flow rate can be maintained.
Although variable restrictors are more expensive, they are necessary for real-world applications.