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Water Treatment Conversion Factors - TDS

Total Dissolved Solids

One of the important measures of water quality is the amount of dissolved minerals in the water which is called Total Dissolved Solids(TDS). TDS is typically measured by devices which quantify how the water sample conducts an electrical current. All the measurements with the exception of the GPG figures are electrical measurements. The different units are used to make it convenient to express the Total Dissolved Solids(TDS) values.

The most important thing to remember is that the all the units measure the same thing and are all done by electrical current conduction. The measuring instrument?s internal electronics make the conversions to the unit of measurement shown on the dial.

For Instrumentation to measure TDS please see: Instrumentation - Myron-L Ultrameter.

Chart Definitions

Conductivity is measured in MS/cm2 which is Micro-Siemens per Square Centimeter. This measurement is used more commonly for Untreated City Water and often in Cooling Tower and Boiler Water Conductivity.

Resistivity is measured in Meg-Ohm/cm2 which is Million Ohms per Square Centimeter. This measurement is used commonly in Treated Deionized or RO/DI Water Systems for High Purity Applications. The reason that Resistivity is used is that Conductivity at High Purity results in low readings which are difficult to discern.

TDS. measured in Mg/L, translates to dissolved Milligrams per Liter of solution. To make sense out of all the different measurement units, we try to convert them all to this unit.

TDS, measured in GPG, translates to dissolved Grains per Gallon of solution. This is the old avoirdupois system in Grains, ounces pounds and tons. It is commonly used to measure water hardness and resin capacities in Grains per Cubic Foot(FT3) of Resin.

For Instrumentation to measure TDS please see: Instrumentation - Myron-L Ultrameter.

Other Conversion Factors for Water Treatment

  • One (1) Cubic Foot equals 7.48 US Gallons.
  • One (1) Cubic Foot equals 28.32 Liters.
  • One (1) US Gallon 3.785 Liters.
  • One (1) Liter equals 2.2 Pounds.
  • One (1) Liter equals 61.03 Cubic Inches.
  • One (1) Cubic Foot equals 1,728 Cubic Inches..
  • One (1) US Gallon equals 8.33 Pounds.
  • One (1) Grain per Gallon equals 17.1 PPM.
  • One (1) PSI equals 2.307 feet of head for water.

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Total Disolved Solids Table