Hard water's TDS (Total Dissolved Solids) level isn't fixed; it varies depending on the specific water source and the minerals present. While TDS measures all dissolved solids, water hardness specifically refers to the concentration of calcium and magnesium ions. A high TDS reading often correlates with hard water, but it doesn't directly equate to a specific hardness level.
Understanding the Relationship Between TDS and Hardness
- TDS is broader: TDS encompasses all dissolved minerals and salts, including calcium and magnesium (which cause hardness), but also sodium, potassium, and other ions.
- Hardness is specific: Water hardness focuses solely on the concentration of calcium and magnesium. These are usually expressed in grains per gallon (gpg) or milligrams per liter (mg/L).
- Correlation, not equivalence: Higher TDS generally indicates higher mineral content, which can mean higher hardness. However, water with high TDS might have high levels of other dissolved substances and not be particularly hard. Conversely, water with moderate TDS might still be quite hard if calcium and magnesium are the dominant dissolved minerals.
Examples of TDS and Hardness Levels
The table below from YSI shows a general relationship:
| TDS (ppm) | Conductivity | Hardness Description |
|---|---|---|
| 0-70 | 0-140 | Very Soft |
| 150-250 | 300-500 | Slightly Hard |
| 250-320 | 500-640 | Moderately Hard |
| 320-420 | 640-840 | Hard |
Note: This is a general guideline. The exact TDS for a given hardness level can vary significantly.
Determining Hardness
To accurately determine water hardness, a specific hardness test is necessary, not just a TDS measurement. There are many at-home water hardness testing kits available.
