EC is the actual measurement. A ppm reading is EC multiplied by a conversion factor, and meters use different factors: on a 500-scale meter 1.0 EC reads 500 ppm, and on a 700-scale meter it reads 700. That's why one grower reports 1,250 ppm (500 scale) and another reports 1,750 ppm (700 scale) for the same 2.5 EC solution.
CropSalt at chart rate reads about 2.2 EC or a bit higher in RO water (1,100 ppm on the 500 scale, 1,540 on the 700 scale), and higher in tap water. Our ppm numbers use the 500 scale.
Conversion table
| EC (mS/cm) | 500 scale | 700 scale |
|---|---|---|
| 0.5 | 250 ppm | 350 ppm |
| 1.0 | 500 ppm | 700 ppm |
| 1.5 | 750 ppm | 1,050 ppm |
| 2.0 | 1,000 ppm | 1,400 ppm |
| 2.1 | 1,050 ppm | 1,470 ppm |
| 2.2 | 1,100 ppm | 1,540 ppm |
| 2.4 | 1,200 ppm | 1,680 ppm |
| 2.5 | 1,250 ppm | 1,750 ppm |
| 3.0 | 1,500 ppm | 2,100 ppm |
To go the other way, divide: 1,200 ppm on a 500-scale meter is 1,200 ÷ 500 = 2.4 EC. There's a full chart in our EC and ppm conversion guide.
Set your meter to EC if it can. If you send us ppm, tell us the scale.
What to expect at chart rate
The chart is in grams, not ppm: 4.6 g Veg A + 3.1 g Veg B per gallon in veg, 5.1 g Bloom A + 2.6 g Bloom B in bloom. The weights are fixed; the final reading moves with your water. Tap water already has some EC in it, so the same weights read higher than in RO.
- A reading a little under 2.2 in very clean RO can be fine. Check the water volume and the meter before adding powder to force the number.
- Don't change one part to chase a ppm display. Keep A and B at the chart ratio.
- The highlighted row on the bag is a lower strength for seedlings, not the full chart. Seedlings start there and work up.
- Tap water at about 250 ppm (0.5 EC, 500 scale) or lower can be run straight. Above that, blend in RO to bring it to about 250. See tap water, RO or a blend.
If the number doesn't match
- Compare in EC. If the readings don't fit one ppm scale, the meter's scale setting is the first suspect. If the difference disappears in EC, there's nothing to fix in the feed.
- Check the batch: the actual water volume (we've seen tanks hold more than the grower thought), the A and B weights, the mixing order and that the powder fully dissolved.
- Check the meter in a fresh EC calibration solution, and check its temperature compensation. A second meter on the same sample helps.
- If the feed really is weak, restore the complete A+B recipe. Don't add a guessed amount of one part.
When you compare feed with runoff, use the same meter and the same scale for both. The method is in how to test runoff pH and EC.
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