Water hardness is measured using a variety of units, most commonly milligrams per litre (mg/L) or parts per million (ppm) as calcium carbonate, alongside specific "degrees of hardness" such as Clark, French, and German degrees. Understanding these units is crucial for assessing water quality and its impact on household and industrial applications.
Water hardness refers to the concentration of dissolved minerals, primarily calcium and magnesium ions, in water. While these minerals are generally not harmful to health, high levels can lead to issues like scale buildup in pipes and appliances, reduced soap lathering, and altered taste.
Common Units for Measuring Water Hardness
The most widely accepted methods for expressing water hardness relate to the equivalent quantity of calcium carbonate (CaCO3), as it is a common measure for the scale-forming potential of water.
Milligrams per Litre (mg/L) and Parts per Million (ppm)
- Milligrams per Litre (mg/L): This unit expresses the weight of calcium carbonate (or equivalent hardness minerals) present in one litre of water. It's a straightforward and commonly used metric.
- Parts per Million (ppm): This unit indicates the number of parts of hardness minerals per million parts of water by weight. For practical purposes in water analysis, 1 mg/L is approximately equivalent to 1 ppm. This makes them interchangeable for most applications.
Both mg/L and ppm, when specified as CaCO3, provide a direct measure of the concentration of hardness-causing minerals.
Degrees of Hardness
Beyond mg/L and ppm, several older, regionally specific "degrees" are still encountered, each with its own definition based on different historical standards.
Clark Degrees (°e or °Clark)
Also known as English degrees, this unit defines one degree of hardness as the equivalent of one grain of calcium carbonate per imperial gallon of water.
- One grain is approximately 64.8 milligrams.
- One imperial gallon is approximately 4.546 litres.
- Therefore, 1 °Clark is roughly equivalent to 14.25 mg/L CaCO3.
French Degrees (°fH or °f)
French degrees define hardness as 10 milligrams of calcium carbonate per litre of water.
- This means 1 °fH is exactly equivalent to 10 mg/L CaCO3.
German Degrees (°dH or °GH)
German degrees, also known as Deutsche Härte, base their definition on calcium oxide (CaO). One German degree of hardness is defined as 10 milligrams of calcium oxide per litre of water.
- Given the molecular weight difference between CaO and CaCO3, 1 °dH is equivalent to approximately 17.86 mg/L CaCO3.
Hardness Unit Conversion Table
The following table provides a quick reference for converting between the most common units of water hardness:
| Unit | Equivalent in mg/L CaCO3 | Basis |
|---|---|---|
| mg/L CaCO3 | 1 | 1 milligram of CaCO3 per litre of water |
| ppm CaCO3 | 1 | 1 part of CaCO3 per million parts of water (by weight) |
| 1 °Clark | 14.25 | 1 grain (64.8 mg) CaCO3 per imperial gallon (4.546 L) |
| 1 °fH | 10 | 10 mg CaCO3 per litre of water |
| 1 °dH | 17.86 | 10 mg CaO per litre of water (converted to CaCO3 equiv.) |
Why Understanding Hardness Units Matters
Accurate measurement and understanding of water hardness units are essential for various reasons:
- Appliance Longevity: High hardness levels lead to scale buildup in kettles, water heaters, and dishwashers, reducing efficiency and lifespan.
- Detergent Efficiency: Hard water reacts with soap, forming scum and reducing lathering, requiring more detergent for effective cleaning.
- Water Treatment: Knowing the exact hardness helps in selecting and setting up appropriate water treatment systems, such as water softeners.
- Industrial Applications: Many industries require specific water hardness levels for their processes to prevent equipment damage or ensure product quality.
- Regulatory Compliance: Some regions or industries have regulations regarding acceptable water hardness levels.
By understanding these different units and their conversions, individuals and industries can effectively manage water quality and mitigate the potential negative impacts of hard water.
