640.5 g/kg = 643.638 PSU
| g/kg | PSU |
|---|---|
| 625.5 g/kg | 628.565 |
| 630.5 g/kg | 633.589 |
| 635.5 g/kg | 638.614 |
| 640.5 | 643.638 |
| 645.5 g/kg | 648.663 |
| 650.5 g/kg | 653.687 |
| 655.5 g/kg | 658.712 |
640.5 g/kg = 643.638 PSU
1 g/kg ≈ 1.0049 PSU · 1 mg/L ≈ 0.001 PSU · 1 ppm ≈ 0.001 PSU (dilute aqueous approximation)