Why EC Meters Drift
Mineral deposits on electrodes — Ca, Mg, and Fe from fertilisers accumulate on the plates, changing their effective surface area.
Temperature error — conductivity changes by approximately 2% per °C. Most meters are compensated for 25°C.
Electrode wear — physical degradation of the surface with prolonged use. Cheap graphite electrodes degrade faster than platinum ones.
Calibrating Correctly
Standard: use solutions with a known EC (1.413 or 2.76 mS/cm to NBS/NIST standards). Making your own standard by diluting fertilisers by hand is not appropriate.
Frequency: before every important solution preparation; at minimum weekly when used daily.
Procedure:
- Rinse the electrode with distilled water
- Submerge in the calibration standard
- Wait for the reading to stabilise (10–30 seconds)
- Calibrate if required
- Verify against a second standard if available
TDS vs EC
| Solution type | Conversion factor |
|---|---|
| NaCl | EC × 500 |
| KNO₃ or mixed fertilisers | EC × 700 |
| General use | EC × 640 |
Working in EC units is recommended; ppm should be used as a reference only.
Three Critical Mistakes
- Cleaning electrodes with cotton swabs or cloth — this damages the coating; use soaking in 0.1M HCl or citric acid instead
- Using a TDS pen without accounting for conversion factors
- Skipping verification after storage or after the meter has been dropped
Reliability Test
After calibrating at 1.413 mS/cm, verify against 2.76 mS/cm:
- ≤2% deviation: reliable instrument
- 2–5%: acceptable — calibrate more frequently
- >5%: clean or replace the sensor