ASTM standard D2241 requires that PVC pipe be marked with the manufacturer's name or trademark. The pipe must also be marked with the nominal pipe size, the exterior diameter and the Standard Dimension Ratio (SDR) number with its corresponding pressure rating.
The SDR number is derived from the working pressure the pipe can be used under. SDR 21 PVC pipe is rated to withstand working pressures of up to 200 psi and is available in nominal sizes from 3/4 inch to 8 inches. SDR 26 PVC pipe can be used with working pressures of up to 160 psi and is available in sizes from 1 inch to 24 inches. SDR 41 is rated up to 100 psi working pressure and is available in 18-, 20- or 24-inch pipe sizes. The pressure rating is tested at 73 degrees. If the pipe is to be used at higher temperatures, the pressure rating must be multiplied by a de-factor to determine how much the working pressure should be lowered. For instance, the working pressure for a SDR 21 pipe to be used at 140 degrees is found by multiplying 200 by the de-factor of .22. The working pressure the pipe may be used at is 156 psi.
Pressure-rated PVC pipe must be between 3/4 inch and 24 inches nominal size. Each pipe size must have a specific exterior diameter, a minimum wall thickness and an average interior diameter. It must also meet weight per foot requirements. These requirements vary for each pipe size depending on the SDR number the pipe is rated for.