EC vs PPM: what a TDS meter actually measures
Conductivity. Every "TDS" or "ppm" meter is an EC meter: it passes a current through the water, measures how easily it flows, and reports that in millisiemens per centimeter — then, if it's a ppm meter, multiplies by a constant its maker chose and prints the result as if it were parts per million of dissolved solids. It isn't. Real ppm needs a lab; a meter can't tell calcium nitrate from table salt. As Bluelab puts it on its own support page: ppm cannot be measured by an EC meter. So the calculator turns whatever you typed back into EC first, and everything else follows from that.
The 500 scale was calibrated to sodium chloride solutions and is what most US meters (Hanna, Milwaukee, HM Digital, generic pens) use, often under the name "TDS". The 700 scale was calibrated to potassium chloride and is what Bluelab and most UK, European and Australian meters use. 640 is a compromise some European meters chose. None is more correct; they're different rulers for the same length.
Why two meters disagree by 40%
| EC, mS/cm | µS/cm | CF | ppm 500 | ppm 640 | ppm 700 |
|---|---|---|---|---|---|
| 0.4 | 400 | 4 | 200 | 256 | 280 |
| 0.6 | 600 | 6 | 300 | 384 | 420 |
| 0.8 | 800 | 8 | 400 | 512 | 560 |
| 1.0 | 1,000 | 10 | 500 | 640 | 700 |
| 1.2 | 1,200 | 12 | 600 | 768 | 840 |
| 1.4 | 1,400 | 14 | 700 | 896 | 980 |
| 1.6 | 1,600 | 16 | 800 | 1,024 | 1,120 |
| 1.8 | 1,800 | 18 | 900 | 1,152 | 1,260 |
| 2.0 | 2,000 | 20 | 1,000 | 1,280 | 1,400 |
| 2.2 | 2,200 | 22 | 1,100 | 1,408 | 1,540 |
| 2.4 | 2,400 | 24 | 1,200 | 1,536 | 1,680 |
| 2.6 | 2,600 | 26 | 1,300 | 1,664 | 1,820 |
| 3.0 | 3,000 | 30 | 1,500 | 1,920 | 2,100 |
Every row is one bucket of water. Read across, not down: the ppm columns differ by the ratio of the scales (700 ÷ 500 = 1.4), always.
Both are EC 1.8 mS/cm. The Bluelab multiplies by 700 and the Hanna by 500, so the Bluelab number is always 40% bigger and neither meter is wrong. This is the entire content of every "my two meters disagree" forum thread. It's also why a feeding chart that says "800 ppm" without naming its scale is telling you nothing: on a 500-scale meter that's 1.60 mS/cm, on a 700-scale meter 1.14 — the difference between early and late veg. Set your meter to EC and the problem disappears.
Which scale is my meter?
| Meter | Scale |
|---|---|
| Hanna (HI98318 and most pens) | 500 |
| Milwaukee | 500 |
| HM Digital (TDS-3, AP-1) | 500 |
| Apera | 500 (switchable on some) |
| Generic "TDS meter" pens | 500 — usually unstated |
| Bluelab (Truncheon, Guardian, pens) | 700 (switchable to 500) |
| Eutech | 700 |
| Any meter that shows EC | no scale — use it |
From the makers' manuals. The definitive test: a 1.0 mS/cm calibration solution (sold as "1000 µS", "1413 µS" is 1.413 mS) reads 500 on a 500-scale meter and 700 on a 700-scale one — 707 or 990 for the 1413 standard.
EC targets by stage — in every scale
The bands most feeding charts converge on, net of your starting water, printed in each dialect so a chart in one scale can be read on a meter in another:
| Stage | EC, mS/cm | ppm 500 | ppm 640 | ppm 700 |
|---|---|---|---|---|
| Seedlings & clones | 0.4–0.8 | 200–400 | 256–512 | 280–560 |
| Early veg | 0.8–1.2 | 400–600 | 512–768 | 560–840 |
| Late veg | 1.2–1.8 | 600–900 | 768–1,152 | 840–1,260 |
| Flower | 1.6–2.4 | 800–1,200 | 1,024–1,536 | 1,120–1,680 |
| Flush / final week | 0–0.6 | 0–300 | 0–384 | 0–420 |
| Lettuce & leafy greens | 0.8–1.2 | 400–600 | 512–768 | 560–840 |
| Tomato (greenhouse) | 2–3.5 | 1,000–1,750 | 1,280–2,240 | 1,400–2,450 |
Grower convention for the cannabis-style stages (it's what the charts print); lettuce from Cornell CEA; tomato from greenhouse practice, where EC is pushed deliberately to firm fruit. Above about 3 mS/cm most other crops are in salt stress. Your nutrient brand's own chart, read on the right scale, beats this table.
Subtract your starting water
Tap water isn't zero. Most municipal water reads 0.2–0.6 mS/cm before any nutrient goes in — calcium, magnesium, bicarbonate, a little sodium — and a meter counts all of it. If the chart says 1.4 and your tap is 0.4, the meter should read 1.8; mix to 1.4 on the meter and the plants get 1.0 of food. The calculator shows both numbers.
| Tap water EC | That is, on a 500 / 700 meter | Meter should read for a 1.4 target | Mixed to 1.4 on the meter, the plants get |
|---|---|---|---|
| 0.1 mS/cm | 50 / 70 ppm | 1.5 | 1.4 |
| 0.2 mS/cm | 100 / 140 ppm | 1.6 | 1.4 |
| 0.4 mS/cm | 200 / 280 ppm | 1.8 | 1.4 |
| 0.6 mS/cm | 300 / 420 ppm | 2.0 | 1.4 |
| 0.8 mS/cm | 400 / 560 ppm | 2.2 | 1.4 |
Hard water above 0.6 is worth an RO filter not for the EC but for what's in it: bicarbonate drives pH up all week and calcium competes with the calcium you're paying for. RO water reads 0.0–0.05.
Temperature: the 2% per degree nobody mentions
Conductivity rises about 2% for every degree Celsius, and every reading is defined at 25 °C. Meters with automatic temperature compensation (ATC) — most, including every Bluelab and Hanna — correct for it. The $10 pens often don't, and a cold reservoir in a winter basement then reads low by a fifth:
| Solution at | Uncompensated pen shows | Actual EC at 25 °C | Error |
|---|---|---|---|
| 10 °C (50 °F) | 1.5 | 2.14 | +43% |
| 15 °C (59 °F) | 1.5 | 1.88 | +25% |
| 20 °C (68 °F) | 1.5 | 1.67 | +11% |
| 25 °C (77 °F) | 1.5 | 1.50 | — |
| 30 °C (86 °F) | 1.5 | 1.36 | -9% |
| 35 °C (95 °F) | 1.5 | 1.25 | -17% |
Linear 2%/°C model, the one meters use. If your pen's manual mentions ATC, ignore this section; if it doesn't, the calculator's temperature field is for you — and so is a $30 meter that does.
Why is my reservoir's EC drifting?
Read the direction, not just the number. EC rising between top-offs means the plants are drinking water faster than they take up nutrient — normal in heat and under strong light — and the fix is topping off with plain water, not more feed; keep doing it and the salts concentrate until the roots can't drink at all. EC falling means they're taking nutrient faster than water: they're hungry, and the next top-off should carry feed. EC steady while pH climbs is usually bicarbonate in the tap water doing what bicarbonate does, and points at the starting-water section above. A reservoir that drifts the same way every week is telling you the mix is a little off in that direction; move the target a tenth, not the whole recipe.
Calibrate it, and read the manual once
Cheap pens drift. Calibrate monthly in a fresh standard solution (1.413 mS/cm is the common one — it reads 707 or 990 depending on scale, which is a handy way to learn which meter you own), rinse the probe in clean water after every dip, and store it wet if the manual says to. Then set it to EC if it lets you, save your scale here if it doesn't, and every nutrient page on this site will speak the same number your meter does.
Sources: Bluelab, What are the different conductivity scales? (support article — scale factors, NaCl/KCl bases, CF, "ppm cannot be measured by an EC meter"); Hanna Instruments and HM Digital meter manuals (500 scale, ATC); temperature coefficient of conductivity as specified in meter ATC documentation (~2%/°C referenced to 25 °C); Cornell CEA lettuce nutrient guidance; Resh, Hydroponic Food Production (tomato EC); stage bands as grower convention, stated as such.