| Description | Quantity |
| Scale intolerable | <4 |
| Heavy scale | 4 - 5 |
| Light scale | 5 - 6 |
| Little scale or corrosion | 6 - 7 |
| Corrosion significant | 7 - 7.5 |
| Heavy corrosion | 7.5 - 9 |
| Corrosion intolerable | >9 |