Short answer: multiply EC in mS/cm by 500 for the 500 scale, or by 700 for the 700 scale. An EC of 1.2 mS/cm is 600 ppm on a 500-scale meter and 840 ppm on a 700-scale meter (Bluelab).
Why there are two scales
PPM meters do not measure parts per million directly. They measure conductivity and convert it using a factor. Some makers base the factor on sodium chloride (500 scale, also labelled TDS or NaCl) and others on a different reference (700 scale, sometimes labelled 442 or KCl). A few meters use 640 or 650.
Conversion chart
| EC (mS/cm) | PPM, 500 scale | PPM, 700 scale |
|---|---|---|
| 0.5 | 250 | 350 |
| 0.8 | 400 | 560 |
| 1.0 | 500 | 700 |
| 1.2 | 600 | 840 |
| 1.5 | 750 | 1,050 |
| 1.8 | 900 | 1,260 |
| 2.0 | 1,000 | 1,400 |
| 2.5 | 1,250 | 1,750 |
| 3.0 | 1,500 | 2,100 |
Units to watch
EC is shown in mS/cm or µS/cm. 1 mS/cm equals 1,000 µS/cm. Some charts also use CF, which is EC × 10.
Which scale does your meter use?
- Check the manual or the meter’s settings; many let you switch scales.
- If the meter shows both EC and PPM, divide the PPM reading by the EC reading. The result is the factor.
- Measure a calibration standard of known EC and compare.
The simplest fix
Record and compare readings in EC. EC means the same thing on every meter, so published ranges such as Oklahoma State University Extension’s crop table can be used directly.
Related guide: EC and PPM for Kratky nutrient solution.
Sources
- Bluelab. What are the different conductivity scales?
- Dunn, B. and Singh, H. Oklahoma State University Extension. Electrical Conductivity and pH Guide for Hydroponics (HLA-6722).
