mS/cm (millisiemens per centimetre) is the unit used for electrical conductivity of nutrient solutions. The siemens is the unit of electrical conductance; dividing by the centimetre spacing of the probe gives conductivity. 1 mS/cm is numerically identical to 1 dS/m (decisiemens per metre), the unit preferred in soil science and irrigation papers.
In practice
mS/cm is the unit to use when comparing numbers across meters, nutrient charts and research, because it is a real measurement, while ppm depends on which conversion factor your meter uses. A grower whose pen reads "1.6" in EC mode is at 1.6 mS/cm; the same solution might read 800 on a 500-scale ppm meter or 1,120 on a 700-scale meter.
Some meters display EC without a unit, and some older ones read in mmho/cm, an old name for the same thing (1 mmho/cm = 1 mS/cm).
Units and formula
| mS/cm | µS/cm | dS/m | CF | ppm500 | ppm700 |
|---|---|---|---|---|---|
| 1.0 | 1,000 | 1.0 | 10 | 500 | 700 |
| 1.5 | 1,500 | 1.5 | 15 | 750 | 1,050 |
| 2.0 | 2,000 | 2.0 | 20 | 1,000 | 1,400 |
Don't confuse with
- µS/cm: a thousand times smaller unit, used for low-EC water.
- ppm: a calculated estimate, not a measured unit of conductivity.
