Total Dissolved Solids (TDS) is the best measurement of the nutrient concentration of a hydroponic solution. To estimate TDS, one can use a meter that measures the Electric Conductivity (EC) of a solution, and convert the number to TDS in parts per million (ppm). Many meters will do this conversion.
Total dissolved solids (TDS) is typically expressed in parts per million (ppm). It is a measurement of mass and determined by weighing, called a gravimetric analysis. A solution of nutrients dissolved in water at a strength of 700 ppm means that there are 700 milligrams if dissolved solids present for every liter of water. To accurately calculate total dissolved solids (TDS), one would evaporate a measured filtered sample to dryness, and weigh the residue. This type of measurement requires accurate liquid measurement, glassware, a drying oven, and a milligram balance. Example: 50 mL of the 700ppm solution would leave 35 mg of salt at the bottom of a crucible after drying.
Electrical Conductivity (EC) is expressed in siemens per centimeter (s/cm) or milliseimens per centimeter(ms/cm). It can be determined with an inexpensive hand held meter. Nutrient ions have an electrical charge, a whole number, usually a positive or negative 1, 2, or 3. EC is a measurement of all those charges in the solution that conduct electricity. The greater the quantity of nutrient ions in a solution, the more electricity that will be conducted by that solution. A material has a conductance of one siemens if one ampere of electric current can pass through it per volt of electric potential. It is the reciprocal of the ohm, the standard unit of electrical resistance. A siemens is also called a mho (ohm backwards).
For convenience, EC measurements often are converted to TDS units (ppm) by the meter.
The meter cannot directly measure TDS as described above, and instead uses a linear conversion factor to calculate it. Everyones nutrient mix is different, so no factor will be exact. The meter uses an approximate conversion factor, because the exact composition of the mix is not known. Conversion factors range from .50 to .72, *depending on the meter manufacturer, which do a good job of approximating a TDS calculation from the meters measurement of EC.
- All ppm pens actually measure the value based on EC and then convert the EC value to display the ppm value, having different conversion factors between differing manufacturers is why we have this problem communicating nutrient measurments between one another.
EC is measured in millisiemens per centimeter (ms/cm) or microsiemens per centimeter (us/cm).
One millisiemen = 1000 microsiemens.
EC and CF (Conductivity Factor) are easily converted between each other.
1 ms/cm = 10 CF
“The communication problem”…
So again, the problem is that different ppm pen manufacturers use different conversion factors to calculate the ppm they display. All ppm (TDS, Total Dissolved Solids) pens actually measure in EC or CF and run a conversion program to display the reading in ppm’s.
There are three conversion factors which various manufacturers use for displaying ppm’s…
USA 1 ms/cm (EC 1.0 or CF 10) = 500 ppm
European 1 ms/cm (EC 1.0 or CF 10) = 640 ppm
Australian 1 ms/cm (EC 1.0 or CF 10) = 700 ppm
For example,
Hanna, Milwaukee 1 ms/cm (EC 1.0 or CF 10) = 500 ppm
Eutech 1 ms/cm (EC 1.0 or CF 10) = 640 ppm
Truncheon 1 ms/cm (EC 1.0 or CF 10) = 700 ppm
Calculating the conversion factor
If your meter allows you to switch between EC and TDS units, your conversion factor can be easily determined by dividing one by the other.
Place the probe in the solution and read TDS in ppm. Change to EC on the meter and read EC in ms/cm.
Conversion factor = ppm / ec.
[Note: ms must be converted to us: One millisiemen = 1000 microsiemens (1.0 ms/cm = 1000.0 us/cm)
According to the chart below:
1.0 ms/cm = 500 ppm (USA Hanna)
1000 us/cm = 500 ppm
Conversion factor = ppm / (ms/cm * 1000)
.50 = 500ppm / (1000us/cm) ]
The answer is your meter’s convertion factor and should be a number between 0.50 and 0.72 To improve accuracy, take ec and ppm readings from your res daily for about ten days. Average the conversion factors. The more data points that you use, the closer you will be to finding your true conversion factor.
When reporting your PPM in a thread, please give the conversion factor your meter uses. For example: 550 PPM @0.7 or give the reading in EC, which should be the same meter to meter.
It may also be advisable to give the starting value of your water; there is a huge difference between RO and distilled water with a PPM of approximately 0 and hard tap water of PPM 300 @.5 (notice the conversion factor so others can work out the EC) or well water with a conductance of 2.1 ms/cm.
A note to Organic Growers:
An EC meter has fewer applications for a soil grower because many organic nutrients are not electrically charged or are inert. Things like Superthrive or Fish Emulsion, blood meal, rock phosphate or green sand cannot be measured with a meter reliably when they are applied or in runoff. Meters can only measure electrically charged salts in solution.
“The solution”…
When reporting your PPM in a thread please give the conversion factor your meter uses for example 550 PPM @.7 or give the reading in EC (the EC shoul d be the same meter to meter).
| EC | Hanna | Eutech | Truncheon | CF | |
| ms/cm | 0.5 ppm | 0.64 ppm | 0.70 ppm | 0 | |
| 0.1 | 50 ppm | 64 ppm | 70 ppm | 1 | |
| 0.2 | 100 ppm | 128 ppm | 140 ppm | 2 | |
| 0.3 | 150 ppm | 192 ppm | 210 ppm | 3 | |
| 0.4 | 200 ppm | 256 ppm | 280 ppm | 4 | |
| 0.5 | 250 ppm | 320 ppm | 350 ppm | 5 | |
| 0.6 | 300 ppm | 384 ppm | 420 ppm | 6 | |
| 0.7 | 350 ppm | 448 ppm | 490 ppm | 7 | |
| 0.8 | 400 ppm | 512 ppm | 560 ppm | 8 | |
| 0.9 | 450 ppm | 576 ppm | 630 ppm | 9 | |
| 1.0 | 500 ppm | 640 ppm | 700 ppm | 10 | |
| 1.1 | 550 ppm | 704 ppm | 770 ppm | 11 | |
| 1.2 | 600 ppm | 768 ppm | 840 ppm | 12 | |
| 1.3 | 650 ppm | 832 ppm | 910 ppm | 13 | |
| 1.4 | 700 ppm | 896 ppm | 980 ppm | 14 | |
| 1.5 | 750 ppm | 960 ppm | 1050 ppm | 15 | |
| 1.6 | 800 ppm | 1024 ppm | 1120 ppm | 16 | |
| 1.7 | 850 ppm | 1088 ppm | 1190 ppm | 17 | |
| 1.8 | 900 ppm | 1152 ppm | 1260 ppm | 18 | |
| 1.9 | 950 ppm | 1216 ppm | 1330 ppm | 19 | |
| 2.0 | 1000 ppm | 1280 ppm | 1400 ppm | 20 | |
| 2.1 | 1050 ppm | 1334 ppm | 1470 ppm | 21 | |
| 2.2 | 1100 ppm | 1408 ppm | 1540 ppm | 22 | |
| 2.3 | 1150 ppm | 1472 ppm | 1610 ppm | 23 | |
| 2.4 | 1200 ppm | 1536 ppm | 1680 ppm | 24 | |
| 2.5 | 1250 ppm | 1600 ppm | 1750 ppm | 25 | |
| 2.6 | 1300 ppm | 1664 ppm | 1820 ppm | 26 | |
| 2.7 | 1350 ppm | 1728 ppm | 1890 ppm | 27 | |
| 2.8 | 1400 ppm | 1792 ppm | 1960 ppm | 28 | |
| 2.9 | 1450 ppm | 1856 ppm | 2030 ppm | 29 | |
| 3.0 | 1500 ppm | 1920 ppm | 2100 ppm | 30 | |
| 3.1 | 1550 ppm | 1984 ppm | 2170 ppm | 31 | |
| 3.2 | 1600 ppm | 2048 ppm | 2240 ppm | 32 |