PPM is not a direct count of fertilizer molecules
A TDS/PPM meter estimates dissolved solids from electrical conductivity using a conversion factor. The water’s true ionic strength is the EC reading (mS/cm or µS/cm).
Different manufacturers historically chose different factors. That is why your friend’s “900 PPM” can be your “640 PPM” while both meters are “working.”
Neither meter is counting nitrogen molecules. They are estimating conductivity and multiplying. Treat PPM as a dialect of EC — useful once you know which dialect you speak.
If a chart, bottle, or forum post only lists PPM with no scale, convert nothing until you ask or assume carefully and verify with plant response.
The common factors
500 scale (NaCl approximation): PPM ≈ EC (mS/cm) × 500. Common on many Hanna-style pens.
700 scale: PPM ≈ EC × 700. Often seen in Bluelab PPM contexts.
640 scale: PPM ≈ EC × 640. Appears on some controllers and regional meters.
Example: EC 1.6 mS/cm = 800 PPM (500) = 1024 PPM (640) = 1120 PPM (700).
Memorize one anchor: EC 1.0 mS/cm ≈ 500 / 640 / 700 PPM depending on scale. When someone says “run 1000 PPM,” ask which of those three they meant before you touch the bottle.
How charts go wrong
A forum post says “run lettuce at 600 PPM” with no scale. On 500 scale that is EC 1.2. On 700 scale “600 PPM” is only EC ~0.86 — a softer feed. Copying blindly underfeeds or overfeeds.
Commercial recipes that specify EC avoid this entirely. Prefer them.
Nutrient labels that print both PPM and EC still assume a factor. Cross-check: divide their PPM by 500 and by 700 and see which lands on their stated EC. That tells you which language the label spoke.
Controller setpoints inherited from a previous owner are another trap. If the display says PPM but the crop chart is EC, convert once and retape the lid with the EC target.
What to do in practice
1) Note your meter’s factor from the manual. 2) Convert every PPM chart to EC once. 3) Log EC in your reservoir notebook. 4) Use our PPM to EC converter when you only have PPM numbers.
If you own two meters that disagree on PPM, check whether they agree on EC before you throw either away.
Calibrate on the schedule the manufacturer states. A drifted EC pen that still “reads something” will quietly push every crop off target while you argue about 500 vs 700.
When mixing, set strength by EC after parts are in and stirred. PPM is fine for quick checks only if the scale never changes mid-project.
Worked examples you can reuse
Leafy target EC 1.0 mS/cm → 500 PPM (500 scale), 640 PPM (640), 700 PPM (700). Same water, three labels.
Fruiting veg often sits near EC 2.0–2.5. That is 1000–1250 PPM on 500 scale, or 1400–1750 PPM on 700 scale. Copying a “1200 PPM tomato” tip without the scale can land you a full point of EC off.
Kit schedules that say “800 PPM” for herbs: if your pen is 700 scale, that is only EC ~1.14; if it is 500 scale, EC 1.6. Measure and decide which crop you actually have in the tray.
FAQ-style gotchas
“My pens disagree.” Check EC mode or convert both PPM readings to EC. If EC matches, the pens agree — the scales do not.
“Should I buy a 500 or 700 meter?” Buy a meter that reads EC clearly. PPM mode is optional candy. Prefer logging mS/cm forever.
“Is 640 better?” It is just another factor. Consistency beats brand folklore. Pick one language for your notebook and stick to it.