How to Choose a Conductivity / TDS Meter
2-electrode vs 4-electrode, cell constant & TDS factor
Conductivity (in µS/cm or mS/cm) is the fastest read of total ionic content in water — used for purity verification, brackish-water screening, dosing control, and a rough salinity proxy. The choice between a 2-electrode and a 4-electrode cell, the cell constant you specify, and how temperature compensation and the TDS factor are handled decide accuracy across your range.
1. What conductivity measures
Conductivity is the ability of water to carry current via dissolved ions; more ions means higher µS/cm. It is a fast, reagent-free indicator of purity (ultrapure loops near 0.055 µS/cm), salinity, and process load — but it reports total ionic content, not which ions, so pair it with specific-ion methods when identity matters.
2. 2-electrode vs 4-electrode cell
- 2-electrode. Simpler and cheaper; the same electrodes drive and sense. Polarization at high conductivity limits accuracy at the top of the range.
- 4-electrode. Separate drive and sense electrodes cancel polarization and cable effects, extending the usable range (often 0 to 200+ mS/cm) and improving high-conductivity accuracy. The better default for wide-range or process use.
3. Cell constant (K) selection
The cell constant K (distance/area) sets the range: K = 0.1 cm⁻¹ for pure/ultrapure water (low µS), K = 1.0 for general water, and K = 10 for high-conductivity brines and seawater. Match K to your typical conductivity or the reading sits at the noisy edge of the cell.
4. Temperature compensation
Conductivity rises ~2% per °C, so meters apply a temperature coefficient (usually linear 1.91–2.1%/°C, the '2% rule') referenced to 25°C. For best accuracy use a built-in or inserted RTD and the coefficient matching your solution; pure-water compensation differs from saline.
5. TDS and the conversion factor
TDS (total dissolved solids, mg/L) is estimated as TDS ≈ factor × conductivity. The factor depends on the ion mix (roughly 0.5 for NaCl-type, up to ~0.7–0.8 for mixed natural waters). Report the factor you used — TDS from conductivity is an estimate, not a gravimetric measurement.
6. Form factors
Benchtop for the lab, portable/IP67 for field surveys, and online 4-electrode probes (with RTD) for continuous loops and bypass streams. For conductive-liquid flow, a magmeter often carries the same 4–20 mA signal chain.
7. Buying checklist
- Cell type: 2-electrode (simple) vs 4-electrode (wide/accurate)
- Cell constant K matched to your range
- Temperature compensation (RTD) and coefficient
- TDS factor stated if you report TDS
Related buying guides
Frequently asked questions
2-electrode or 4-electrode cell?
Use a 2-electrode cell for simple, lower-range work; use a 4-electrode cell when you need wide range or high accuracy at high conductivity — the separate sense electrodes cancel polarization and cable effects.
How do I pick the cell constant K?
Match K to your typical conductivity: K=0.1 for ultrapure/low µS water, K=1.0 for general water, K=10 for brines and seawater. A mismatched K pushes the reading to the noisy edge of the cell.
Is TDS the same as conductivity?
No. Conductivity is the direct ionic-conductance measurement (µS/cm); TDS (mg/L) is estimated as factor × conductivity, where the factor depends on the ion mix. TDS from conductivity is an estimate, not a gravimetric value — state the factor you used.