For practical purposes:
- Type I ultrapure water usually means water that has 18 Megohm-cm or greater specific resistance, with other attributes such as bacterial count, TOC (total organic carbon), pyrogen and/or endotoxin and/or R-Nase and/or D-Nase levels, and specific ionic contaminant levels usually specified by the end user. It is essentially a given that this water meets the other criteria specified by CAP, ASTM and NCCLS.
- Type II water usually means water that has 1-2 Megohm-cm or greater specific resistance, with other attributes such as bacterial count, usually specified by the end user. It is essentially a given that this water meets the other criteria specified by CAP, ASTM and NCCLS.
- Type III water, per the CAP and NCCLS, means water that has 0.1 Megohm-cm or greater specific resistance, while ASTM defines it as having 4 Megohm-cm or greater specific resistance!
- Type IV water is only defined by ASTM, as having 0.2 Megohm-cm or greater specific resistance.