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TDS Meters, Conductivity and Conversion Factors

Though there is a close relationship between TDS and Electrical Conductivity, they are not the same thing.  Total Dissolved Solids (TDS) and Electrical Conductivity (EC) are two separate parameters.

TDS, in layman's terms, is the combined total of solids dissolved in water.  EC is the ability of something to conduct electricity (in this case, water's ability to conduct electricity).

The only true method of measuring TDS is to weigh residue found in water after the water has evaporated.  You know those spots you see on a glass after you wash it and let it air dry?  That's TDS!  That residue has mass, and it's possible to weigh it, but if you're not in a lab, it can be tricky thing to do.  Therefore, we can estimate TDS levels based on the conductivity of the water since the hydrogen and oxygen molecules of the H2O carry almost no electrical charge.  The EC of most other metals, minerals and salts will carry a charge.  A A TDS meter measures that EC level and then converts it to a TDS measurement.  Since different metals, minerals and salts will be more or less conductive than others, there are different conversion factors that can be used.

ppm (parts per million) is the most commonly used scale to measure TDS (Total Dissolved Solids).

µS (micro-Siemens) is the most commonly used scale to measure EC (Electrical Conductivity).



TDS and Conversion Factors

EC: There is no conversion for electrical conductivity.  (NOTE: The three EC modes in the COM-100 differ only in their ATC programs.  The standard EC mode is KCl.)

TDS - NaCl: 0.47 to 0.50
TDS - 442: 0.65 to 0.85
TDS - KCl: 0.50 to 0.57

(NOTE: Most HM Digital meters use the NaCl factor.  The COM-100 has the above three modes, which are user-selected.  When converting EC to TDS, the COM-100 uses the non-linear scales, as they would occur in nature, thereby giving you more accurate readings than meters that use linear scales.)


Converting between different scales

PPM à µS: The conversion factor of the TDS meter must be known.  Once known, the conversion factor should be multiplied by the TDS level.  (NOTE: For the COM-100, simply change the mode on the meter.  There is no math required.)
PPM à PPT: Divide by 1000 (1000 ppm = 1 ppt)
µS à mS: Divide by 1000 (1000 µS = 1 mS)



What should the TDS of my water be?

à A TDS level is specific for each application and particular usage.  If you are using a meter to test the water pertaining to a particular device, object or operation, contact the manufacturer of that object.  For example, if you are using the meter to test the efficacy of a water filtration system, contact the manufacturer of that system for preferred TDS levels.  If you are testing the water for a pool, plants, fish, etc. contact a specialist for your specific application.

What is the difference between µS and µS/cm?

à There is no difference between µS and µS/cm.  µS is a simple abbreviation and is used to save space.

What is the difference between ppm and mg/L?

à ppm is an expression of quantity, and an abbreviation for “parts per million.” Mg/L (milligrams per liter) is an expression of weight.  Both are used as scales for TDS, but ppm is considerably more popular.  There is no conversion between the two.  (226 ppm = 226 mg/L)

What is the difference between a parameter and a scale?

à A parameter is the characteristic being measured.  A scale is a particular range applied to the measurement of that parameter.  For example, temperature is a parameter.  Fahrenheit or Celsius is a scale.

Is “EC” a parameter or a scale?

à “EC” is a parameter.  It stands for Electrical Conductivity.  There are a number of scales used in EC, most commonly micro-Siemens (µS) or milli-Siemens (mS).  For example, if a particular application calls for water with “2.0 EC,” this is an incorrect determination.  Most likely, the application is calling for an EC level of 2.0 mS.  2.0 mS = 2000 µS.

What is NaCl and KCl?

à These are abbreviations for different conversion factors based on salt content.  NaCl is sodium chloride and KCl is potassium chloride. 


For more information, read an article about TDS and EC Meters for Hydroponics Explained, from the June 2010 issue of Maximum Yield Magazine.  The article was written by Rob Samborn, Director of Sales & Marketing for HM Digital.

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