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2. Grain per gallon - Wikipedia

   en.wikipedia.org/wiki/Grain_per_gallon

   The grain per gallon (gpg) is a unit of water hardness defined as 1 grain (64.8 milligrams) of calcium carbonate dissolved in 1 US gallon of water (3.785412 L). It translates into 1 part in about 58,000 parts of water or 17.1 parts per million (ppm). Also called Clark degree (in terms of an imperial gallon).

3. Hard water - Wikipedia

   en.wikipedia.org/wiki/Hard_water

   Recommendations have been made for the minimum and maximum levels of calcium (40–80 ppm) and magnesium (20–30 ppm) in drinking water, and a total hardness expressed as the sum of the calcium and magnesium concentrations of 2–4 mmol/L. [14] Other studies have shown weak correlations between cardiovascular health and water hardness.

4. Carbonate hardness - Wikipedia

   en.wikipedia.org/wiki/Carbonate_hardness

   Carbonate hardness, is a measure of the water hardness caused by the presence of carbonate (CO 2− 3) and bicarbonate (HCO − 3) anions. Carbonate hardness is usually expressed either in degrees KH (from the German "Karbonathärte"), or in parts per million calcium carbonate ( ppm CaCO 3 or grams CaCO 3 per litre|mg/L).

5. Hardness scales - Wikipedia

   en.wikipedia.org/wiki/Hardness_scales

   The Meyer hardness test; The Knoop hardness test; Other hardness scales. Hardness scales may also refer to: Methods of measuring the deposit formation by hard water.

6. Drinking water quality standards - Wikipedia

   en.wikipedia.org/wiki/Drinking_water_quality...

   * means action level; not a concentration standard. A public water system exceeding the action level must implement "treatment techniques" which are enforceable procedures. [14] ** TT (treatment technique). The public water system must certify that the combination of dose and monomer level does not exceed: acrylamide = 0.05% dosed at 1 mg/L (or ...

7. dGH - Wikipedia

   en.wikipedia.org/wiki/DGH

   In water testing hardness is often measured in parts per million (ppm), where one part per million is defined as one milligram of calcium carbonate (CaCO 3) per litre of water. Consequently, 1 dGH corresponds to 10 ppm CaO but 17.848 ppm CaCO 3 which has a molar mass of 100.09 g/mol.