ASTM D-3035 (DR-PR) Based on Controlled Outside Diameter
This specification covers polyethylene pipe made in standard thermoplastic pipe dimension ratios and pressure rated for water. Pipe manufactured under this specification is based on a controlled outside diameter. For a given nominal size, the outside diameter will be constant regardless of the pressure rating.
Pipe manufactured to this specification is described using a prefix-diameter ratio. The prefix for ASTM D-3035 is DR. It is still common to refer to ASTM D-3035 pipe as SDR as that was how the ASTM used to describe it. They removed the "S" in hopes of clearing up the confusion between it and ASTM D-2737 CTS tubing. It did not. Some also call pipe made to this standard ductile iron pipe size or D.I.P.S.
The most common ratios under this specification are DR-21, DR-17, DR-13.5, DR-11, DR-9, and DR-7. The lower the DR number, the thicker the wall and the higher the pressure rating. The definition of DR (Dimension Ratio) is the specific ratio of the average specified outside diameter to the minimum specified wall thickness.