ATC (automatic temperature compensation) uses a built-in temperature sensor to correct readings. It works differently for EC and pH, and growers often misunderstand the pH part.
In practice
EC: conductivity rises roughly 2% per °C. An uncompensated reading of a 1.40 mS/cm solution would read about 1.54 at 30 °C and 1.26 at 20 °C. With ATC, the meter converts readings to the 25 °C reference so numbers are comparable. This matters a lot for EC; without ATC, cold reservoirs look weak and warm ones look strong.
pH: ATC corrects for how the electrode's response (its slope, about 59 mV per pH unit at 25 °C) changes with temperature. It does not correct for the real pH change of the solution itself with temperature, which is small for nutrient solutions. So a compensated pH meter reports the true pH at the sample's current temperature.
Best practice: let probe and sample reach the same temperature, and calibrate near the temperature you measure at. Large gaps (cold reservoir, warm buffer) still cause errors and slow readings.
Units and formula
EC₂₅ ≈ EC_T ÷ (1 + 0.02 × (T − 25)), T in °C. Some meters let you set the coefficient (1.9–2.1% per °C).
Don't confuse with
- Calibration: corrects the electrode against standards; ATC corrects for temperature.
