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Solution Conversions
342 NaCl =
362 as KCl ; 478 as "442" ; 700 µS
1000 NaCl =
1060 as KCl ; 1450 as "442" ; 2000 µS
Any brand TDS meter can be calibrated to any brand of calibration solution.
The issues are 1) what the calibration solution composition is; and 2) what the value of that calibration solution is.
The calibration solution composition should match the conversion factor of the meter. HM Digital's TDS-3, for example, uses the NaCl conversion factor, so a NaCl calibration solution should be used. HM Digital's COM-100 uses one of three selectable conversion factors (NaCl, 442 or KCl), so any of these three types of calibration solutions can be used for the COM-100.
It should be noted that calibration compositions have equivalent values in alternate compositions. Better brands of calibration solutions print the equivalent values directly on the label or in the instructions. For example, 1060 ppm KCl = 1000 ppm NaCl. Therefore, if you have a 1060 ppm KCl calibration solution, but a meter with the NaCl conversion factor, such as the TDS-3, you can still calibrate the TDS-3 with the KCl calibration solution. In other words, if putting the TDS-3 into a 1060 ppm KCl solution, the meter should read 1000 ppm.
For EC meters, there is no conversion factor, so the points above are not relevant.
**Note - Some brands do not print the calibration solution composition on the label. Though NaCl is the most common, it is always recommended to contact the calibration solution manufacturer for verification of the composition.
All of HM Digital meters can be calibrated to any point within the range of the meter. For example, the TDS-4 has a range of 0-9990 ppm. Therefore, it can be calibrated to any point within that range. The COM-100 has varying ranges depending upon the conversion factor that is selected. On the TDS side, these ranges are from 0-5000 ppm to 0-8560 ppm. On the EC side, the COM-100's range is 0-9990 µS. Therefore, the COM-100 can be calibrated to any value within the selected range.
Certain brands of TDS/EC meters can only be calibrated to specific values that are per-programmed into the meter. HM Digital does not do this.
For best results, a meter should be calibrated as close as possible to what will be tested. For example, if the water you are testing is always around 150 ppm, then the meter should be calibrated as close to 150 ppm as possible.
If you are testing a range of values, the meter should be calibrated to the upper third of that range. For example, if you are testing RO water and tap water, and the RO water is approximately 0-10 ppm and the tap water is typically 400-500 ppm, the meter should be calibrated to approximately 350 ppm.