Water always contains some dissolved minerals, which commonly are referred to as total dissolved solids (TDS) or sometimes as total dissolved salts. Some of these minerals can be toxic if present at high enough concentrations.
In the laboratory, the standard method for measuring TDS is evaporating all the water from a 0.1-liter sample and weighing the residual minerals left in the vessel. However, collecting a sample and waiting for laboratory analysis can take time.
An electrical conductivity (EC) or TDS meter is a quick method to estimate TDS. Water conducts electricity, but the dissolved minerals (ions) in water are what actually conduct the electricity. Pure (distilled) water is a very poor conductor of electricity, so the more dissolved minerals in the water, the more conductive the water becomes.
The units for TDS usually are expressed as milligrams per liter (mg/l), which is the same as parts per million (ppm). Some meters show TDS as parts per thousand (ppt), which is equal to 1,000 ppm.
The EC is a proxy measurement to determine the TDS in water. Some of the confusion with using EC is that it can be expressed in different units. The units might be referenced as micromhos per centimeter (μmhos/cm) and millimhos per centimeter (mmhos/cm) or microSeimens per centimeter (μS/cm) and milliSeimens per centimeter (mS/cm). Thus, 1 mmho/cm = 1 mS/cm = 1,000 μmhos/cm = 1,000 μS/cm.
Most EC meters can change modes to read salinity in grams per liter, EC in μS and TDS in mg/l or ppt. The meter calculates an estimate of TDS by multiplying the EC reading with a conversion factor. In the chart below, note that the conversion factor changes as the EC readings increase.
To ensure an accurate reading, EC meters must be calibrated regularly. The accuracy of the EC meter should be checked with a calibration solution in the spring each year, prior to sampling. Calibrate again after adding new batteries or after the meter has been dropped on a hard surface.
We recommend you use a manufacturer-supplied calibration mixture and procedure, but you can make your own calibration solution using table salt (NaCl). Follow these instructions to create a known calibration solution.
Electrical Conductivity |
Approximate TDS |
Conversion Factor1
|
---|
μmhos/cm
|
mmhos/cm |
μS/cm
|
mS/cm |
mg/l or ppm |
---|
100 |
0.1 |
100 |
0.1 |
50 |
0.50 |
500 |
0.5 |
500 |
0.5 |
300 |
0.60 |
1,000 |
1.0 |
1,000 |
1.0 |
650 |
0.65 |
1,500 |
1.5 |
1,500 |
1.5 |
1,050 |
0.70 |
2,000 |
2.0 |
2,000 |
2.0 |
1,450 |
0.72 |
2,500 |
2.5 |
2,500 |
2.5 |
1,850 |
0.74 |
3,000 |
3.0 |
3,000 |
3.0 |
2,250 |
0.75 |
3,500 |
3.5 |
3,500 |
3.5 |
2,650 |
0.76 |
4,000 |
4.0 |
4,000 |
4.0 |
3,050 |
0.77 |
4,500 |
4.5 |
4,500 |
4.5 |
3,500 |
0.78 |
5,000 |
5.0 |
5,000 |
5.0 |
3,950 |
0.79 |
6,000 |
6.0 |
6,000 |
6.0 |
4,740 |
0.79 |
6,500 |
6.5 |
6,500 |
6.5 |
5,135 |
0.79 |
7,000 |
7.0 |
7,000 |
7.0 |
5,600 |
0.80 |
8,000 |
7.5 |
7,500 |
7.5 |
6,075 |
0.81 |
10,000 |
10.0 |
10,000 |
10.0 |
8,200 |
0.82 |
1 - Conversion factor is for natural waters