What is di water (deionised water)?
Deionised water has had its dissolved mineral ions removed by ion-exchange resin, leaving conductivity typically below 10 µS/cm. Used as a final rinse it evaporates without leaving spots, which is why cosmetic parts, plated surfaces and precision cleanliness work all specify it.
Also called: demineralised water, DM water
- Typical mains water
- 300–800 µS/cm
- Target DI rinse
- Below 10 µS/cm
Tap water carries calcium, magnesium, chloride and silica. When a rinsed part dries, the water leaves but those solids stay, appearing as white spots, streaks or a haze. On a machined face nobody cares; on a visible casting or before painting, it is a reject.
Deionisation strips those ions out. The conventional measure is conductivity: mains water in Indian industrial areas commonly runs 300–800 µS/cm, while a good DI final rinse holds below 10 µS/cm. Monitoring conductivity on the rinse tank is a direct, continuous quality signal.
DI water is also aggressive toward bare steel — it is hungry for ions — so a ferrous part leaving a DI rinse will flash-rust unusually quickly. Those lines need an inhibited final rinse or immediate drying.
