Tools & Microfiber2 min read

Why Two TDS Meters Can Disagree: Check the Conversion Factor

Understand how conductivity-to-TDS conversion factors can make two meters disagree, and align settings before judging water-treatment performance.

Updated · NANOLAB Knowledge

Many handheld TDS meters estimate dissolved solids from conductivity. The displayed value depends partly on a conversion factor or solution model. Two meters can therefore show different TDS values for the same sample without either being obviously broken.

Compare the underlying measurement

In a simplified illustrative calculation, a conductivity reading of 200 µS/cm multiplied by a factor of 0.50 produces an estimated 100 mg/L. A factor of 0.70 produces 140 mg/L from the same conductivity. The 40-unit difference comes from the conversion assumption.

Actual instrument behaviour depends on its settings and compensation method. Consult the manual rather than assuming every device uses the same factor. Do not adjust a factor merely to make a disappointing number look lower.

Align the comparison conditions

Use the same sample, sampling procedure and temperature-reference conditions. Check the meters with the appropriate standard and their documented verification procedure. A contaminated sample cup or probe can create another source of disagreement.

If possible, compare conductivity directly before comparing derived TDS. Record both the original electrical reading and the conversion model used. This makes a future change of instrument easier to interpret.

Set a defensible operating rule

Follow the water-treatment supplier's specified measurement and acceptance criterion. If it names a particular mode or factor, preserve that definition in the workshop instruction. A threshold copied from another meter's display may not mean the same thing.

For a NANOLAB wash process, the goal is a dependable water-quality check that supports the prescribed rinse and drying routine. A convenient display number is not a complete chemical analysis and does not guarantee that detergent, dust or water trapped in trim will leave no residue. Investigate those sources separately when marks recur.

Related guidance

Technical references: Hach: meter configuration and TDS conversion factors.

For product-specific application instructions, consult NANOLAB technical documentation or ask about your surface and intended use.