The terms "DR(dimension ratio)" and "SDR(standard dimension ratio)" are widely used in the PVC pipe industry. Both terms refer to the same ratio, which is a parameter that is obtained by dividing the average outside diameter of the pipe by the minimum pipe wall thickness.
DR and SDR were developed out of convenience rather than out of necessity. They have been established to simplify standardization in the specification of plastic pipe on an international basis. Since these define a constant ratio between outer diameter and wall thickness, they provide a simple means of specifying product dimensions to maintain constant mechanical properties regardless of pipe size. In other words, for a given DR or SDR, pressure capacity and pipe stiffness remain constant regardless of pipe size.
Even though the terms DR and SDR are synonymous, one minor difference between them is that SDR refers only to a particular series of numbers, i.e., 51, 41, 32.5, 26, 21, etc. This series of "preferred numbers" is based on a geometric progression, and was developed by a French engineer called Charles Renard. These numbers are often called "Renard's Numbers."
The term DR became widely used in 1975, with the publication of AWWA C900, which governs production of small diameter PVC pressure pipe. AWWA allowed the desired pressure capacity to dictate wall thickness. Since the OD/t values generated did not happen to fall on any of Renard's Numbers, AWWA removed the "standard" designation from the SDR term.
It is interesting to note that the most widely used product for small diameter sanitary sewer in the U.S., ASTM D3034, SDR 35, provides an apparent contradiction in terms. While 35 is not a Renard Number, it is still referred as a standard dimension ratio. In fact, all OD/t ratios in D3034 are listed as SDRs whether they are included in Renard's "preferred numbers" or not. This was probably for convenience's sake. D3034 was written in 1972, prior to the popularization of the DR term. Accordingly, ASTM may have allowed all OD/t ratios to be called SDRs.
The bottom line is simple: the two terms are interchangeable. SDR=DR=OD/t.