How to Read an EC/TDS Meter

An EC/TDS meter tests the electrical conductivity and total dissolved solids of samples of water or other liquids. Hydroponic gardeners use these meters to determine whether they are using the proper balance of nutrients and buffers. Homeowners who depend on well water use these meters to make sure their water does not have an overly high concentration of minerals or other impurities. The meter gives a reading on its screen after you insert its probes or electrodes into a sample of water. You can rely on the TDS reading from the screen or compute it yourself from the EC reading.

Plastic cup
Glass or plastic stirring rod or plastic spoon
Calculator

Instructions

- 1
  Calibrate your EC/TDS meter according to manufacturer recommendations.
- 2
  Fill a plastic cup with water or hydroponic gardening solution. Stir or shake the water with a stirring rod or plastic spoon. Place the probes of your meter into the cup so they are immersed in the water but do not touch the sides or bottom of the cup.
- 3
  Turn the meter on and set the dial or selector to the setting for an EC reading. Note the result and note whether the reading is in millisiemens (mS) or microsiemens (small "u" with a tail followed by a capital S).
- 4
  Switch the dial or selector to TDS if you want to use the conversion scale that the meter uses to compute the total dissolved solids in the solution. Note the result, which is expressed in ppm (parts per million). Continue to Step 5 if you want to convert the EC reading to a different scale.
- 5
  Multiply the EC reading you obtained by 700 if it is expressed in millisiemens or by 0.7 if expressed in microsiemens to compute the TDS reading according to the "442" scale that is used for measuring fresh water. For example, if your meter read 4,000 microsiemens, you would multiply 4,000 by 0.7 for a result of 2,800 ppm, according to the 442 scale. For 4.0 millisiemens, you would multiply 4.0 by 700 to obtain the same, correct result of 2,800 ppm.
- 6
  Multiply the EC reading you obtained by 500 if it is expressed in millisiemens or by 0.5 if expressed in microsiemens to compute the TDS reading according to the "NaCl" scale used for measuring salty water and chemical solutions. For example, if your meter read 4,000 microsiemens, you would multiply 4,000 by 0.5 for a result of 2,000 ppm according to the NaCl scale. For 4.0 millisiemens, you would multiply 4.0 by 500 to obtain the same, correct result of 2,000 ppm.
- 7
  Multiply the EC reading you obtained by 535 if it is expressed in millisiemens or by 0.535 if expressed in microsiemens to compute the TDS reading according to the "KCl" scale used in scientific measurement. For example, if your meter read 4,000 microsiemens, you would multiply 4,000 by 0.535 for a result of 2,140 ppm according to the KCI scale. For 4.0 millisiemens, you would multiply 4.0 by 535 to obtain the same, correct result of 2,140 ppm.
- 8
  Take readings of additional samples before you make any decisions to correct chemical imbalance in your hydroponic solutions or to adjust the hardness of your well water. Check the average of three to five readings against the recommendations for your plants or solutions or the level of hardness in ppm (according to the 442 scale) that you want to achieve for your drinking water.

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