What Is a Hard Water Test?

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  • Written By: Sandi Johnson
  • Edited By: John Allen
  • Last Modified Date: 17 October 2019
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Hard water is typically tap water with a high concentration of dissolved minerals. Most often, these minerals express as a large amount of calcium (Ca2) and magnesium (Mg2) ions in a relatively small water sample. A hard water test is used to determine the presence of Ca2 and Mg2 in a particular water sample, as well as the estimated mass per volume concentration of these minerals. The rate of concentration, along with other factors such as pH and temperature, determine the water's hardness. Different tests require different methods for collecting samples to be tested and use different mechanisms to measure mineral levels.

Simple hard water test kits for homeowners typically feature nothing more than a test strip. Placed under running water for a few seconds, the strip changes color and is then compared to a color chart to gauge approximate hardness. More complicated tests with more detailed results typically require a special kit from a testing laboratory or other source. Such tests take a sample of water to be analyzed by a laboratory with industrial instruments specifically designed to analyze mineral content. For residential applications, the results of a hard water test typically only indicates whether a water softener solution is needed.


Common hard water test kits for residential water supplies differ from industrial instrument analysis tests used in commercial settings. Commercial operations that use boilers, water treatment plants that supply municipal water, and other industrial settings typically need more accurate hard water test options. Since hard water creates a build up of minerals called scale, industrial applications involving large volumes of water can experience a higher rate of breakdowns and poor equipment performance due to hard water.

Accordingly, industrial tests use different, more complex indices to predict problems with deposits. Using complex laboratory equipment, including sensors, filtration systems, and evaporators, such hard water test systems produce results based on one or more industrial hard water scales. Municipal water treatment facilities, for example, often use the Ryznar Stability Index to test how thick scale is likely to become, based on water pH, water temperature, as well as calcium and magnesium concentrations.

No matter which type of hard water test is used, seldom do either consumer or commercial tests provide specific information about the exact type or amounts of specific minerals present in a sample. Instead, hard water tests measure total calcium and magnesium levels in parts per million (ppm), grains per gallon (gpg), or milligrams per liter (mg/L.) Results of 61 mg/L or higher indicate moderately hard water, with results of 181 mg/L indicating very hard water.


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