Measuring Water Hardness

Hard water is water that has high mineral content (as opposed to "soft water"). Calcium is the most common mineral associated with water hardness. While hardness is not generally a health hazard it can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment. In domestic settings, hard water is often indicated by a lack of suds formation when soap is agitated in water, and by the formation of limescale in kettles and water heaters. Wherever water hardness is a concern, water softening is commonly used to reduce hard water's adverse effects.

hardness map

Calcium hardness in pools and spas can often present problems for the pool owner. Low calcium hardness can result in corrosion of metal equipment or erosion of plaster surfaces and tile grouting. This can lead to the need for resurfacing. High calcium hardness results in scale formation on the pool surfaces as well as scaling in the pipes, plumbing and filter. In extreme cases the water becomes dull and cloudy with the calcium precipitating out into the water rather than onto a surface. High calcium levels will also impact swimmers, causing irritated eyes in particular.

Although water hardness usually measures only the total concentrations of calcium and magnesium, iron can also be present at elevated levels. The presence of iron characteristically creates a brownish (rust-like) color to the calcification, instead of white (the color of most of the other compounds).

Types of Water Hardness

Temporary Hardness

Temporary hardness is a type of water hardness caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). However, unlike the permanent hardness caused by sulfate and chloride compounds, this "temporary" hardness can be reduced by the addition of lime (calcium hydroxide) through the softening process of lime softening.

Permanent hardness

Permanent hardness is hardness (mineral content) that cannot be removed. Ions causing permanent hardness of water can be removed using a water softener, or ion exchange resin.

Total Permanent Hardness = Calcium Hardness + Magnesium Hardness

The calcium and magnesium hardness is the concentration of calcium and magnesium ions expressed as equivalent of calcium carbonate.

Hardness can be quantified through instrumental analysis, an individual test kit, or through the use of test strips specifically formulated for making these measurements.


How to Measure Water hardness


A colorimeter passes a white light beam through an optical filter which transmits only one particular color or band of wavelengths of light to the photodetector, where it is measured. The difference in the amount of colored light transmitted by a colorless sample (blank), and the amount of colored light transmitted by a colored sample, is a measurement of the amount of colored light absorbed by the sample. In most colorimetric tests, including hardness, the amount of colored light absorbed is directly proportional to the concentration and is reported by the meter.

Test Strips

Test strips are typically produced from plastic, with chemically impregnated pads on the end. These pads are designed to react with specific ions and produce a specific color change. Once a test strip is reacted and a color is developed, the strip is then compared to a printed color chart. The color chart is specifically designed to represent color reactions at various concentrations. Matching the strip to the closest color match produces a concentration reading.