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.”
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).
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.
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.