639.5 g/kg = 642.634 PSU
| g/kg | PSU |
|---|---|
| 624.5 g/kg | 627.56 |
| 629.5 g/kg | 632.585 |
| 634.5 g/kg | 637.609 |
| 639.5 | 642.634 |
| 644.5 g/kg | 647.658 |
| 649.5 g/kg | 652.683 |
| 654.5 g/kg | 657.707 |
639.5 g/kg = 642.634 PSU
1 g/kg ≈ 1.0049 PSU · 1 mg/L ≈ 0.001 PSU · 1 ppm ≈ 0.001 PSU (dilute aqueous approximation)