How to Convert TDS to UMHOS

The total number of dissolved minerals that are in a solution is called the TDS.
Laboratory analysis is a starting point when determining the relationship between TDS and conductivity in a solution.Laboratory analysis is a starting point when determining the relationship between TDS and conductivity in a solution.
Common dissolved minerals in water include carbonates, sulfates, chlorides, calcium, sodium and magnesium. Total dissolved solids is best determined by laboratory analysis that involves evaporating all of the water and weighing the amount of solids that are left. Micromhos (μmhos) is a measure of the electrical conductivity of a solution. There is a relationship between TDS and micromhos. As the number of dissolved solids increase, the electrical conductivity increases. However, the amount of conductivity increase varies depending on which minerals are present in the solution. Thus, laboratory tests for TDS are needed to make the correct conversion from TDS to micromhos.

Step 1

Calibrate the conductivity meter with calibration solution. Use a calibration solution that has a conductivity near that of the water you are sampling. Adjust the meter to the appropriate reading using the calibration screw on the meter. Then rinse the probe or sample cup with distilled water. Remove any excess water.

Step 2

Collect your sample in water bottles that are new or have been thoroughly cleaned and rinsed with distilled or deionized water. Laboratories that analyze water samples will provide you with sample bottles. Check the conductivity of the solution. Rinse the probe with sample water before testing its conductivity. Then, take the sample to the laboratory for analysis.

Step 3

Determining the conversion factor from TDS to conductivity can be done on a calculator..

Take the result of the TDS analysis and calculate a conversion factor. Divide the value for TDS (in parts per million, ppm) by the conductivity value in micromhos or microsiemens. For instance, if the TDS analysis shows that your water sample has 250 ppm of total dissolved solids and the water conductivity is 600 micromhos, then the ratio with be 250 / 600 = 0.42. This conversion factor will work for a range of conductivity measurements for your particular type of water.

Things You Will Need

  • Clean sample bottle
  • Conductivity meter
  • Conductivity calibration standard
  • Screwdriver

Tips

  • 1. Conversion factors may be similar for waters within a given region, but you must know the TDS values first before making a general conclusion as to what value to choose.
  • 2. If you need to convert from micromhos to microsiemens, the standard SI unit for conductivity, the values are equivalent. For example, 400 micromhos is 400 microsiemens.
  • 3. If you are testing hydroponics water which has fertilizers, then send a sample of the water to a laboratory for analysis. The fertilzer composition will change the conversion factor that will apply to the tap water.

Warnings

  • 1. Analysis of one water source does not give the correct conversion value for another water source. General trends may be evident, but the conversion will not be exact.
  • 2. Many conductivity meters can give TDS measurements as well. These meters use a standard conversion factor for all measurements. The most common conversion is conductivity times 0.5 equals TDS, so if the value of conductivity is 1000 micromhos, the meter will read 500 ppm for TDS.

About the Author

As a scientist, Randy McLaughlin has been a professional technical writer since 1980. He has a Master of Science from Texas A&M University and a Ph.D. in plant pathology from the University of Wisconsin. McLaughlin covers diverse topics, including Costa Rica, technical guides, alternative healing and spiritual development.