TDS meter

After holding on for months, last week, I finally purchased a digital TDS (Total Dissolved Solids) meter.

TDS indicates the total dissolved inorganic and organic, charged compounds. The TDS meter measures electric conductivity (EC) in a solution, and the number is converted to ppm (parts per million) using the built-in conversion factor. It is important to realize that only dissolved compounds that carry an electrical charge will be quantified by a TDS meter.

The city water that I used to water my orchids is routinely measured at 50 ppm, which means 50 milligrams of dissolved, charged inorganic and organic solids in one liter of water. My city tap water is acceptable to water paphiopedilums (and is better than many other city waters; however, water quality with a TDS reading below 20 ppm, for an example, distilled water, which has a TDS reading close to zero, is better). Adding low-strength fertilizers to my city water increases the TDS reading to 100-200 ppm (aforementioned, molecules that carry no charge, including some compounds in certain fertilizers, will not be quantified by the TDS meter). A post in the Slippertalk forum states that it is fine to water paphs with water that has been added with fertilizers and read up to 500 ppm TDS.

As the TDS readings are dependent upon the predetermined EC to TDS conversion factors, the final converted TDS readings may vary from meters to meters due to the variation of the conversion factos used by different TDS meter manufacturers. If your meter can read EC, you can estimate the conversion factor used by taking a series of readings (the common conversion factors are 0.5 and 0.7; if you are interested to know more, search for this information on the internet or read the manufacturer's manual). I sometimes calibrate my TDS meter with the calibration salt solution that came with the meter (the calibration salt solution can be purchased separately from the meter) and routinely compared my tap water readings to those measured with distilled waters (from my laboratory); the distilled water has a reading of or close to zero.