Why Readings Drift
Electrode contamination — salts and deposits build up on the glass membrane, slowing ion diffusion.
Drying out — the glass membrane requires constant hydration. Leaving the electrode without its cap for even a few hours causes damage.
Temperature fluctuations — every degree of temperature change shifts the reading by approximately 0.002 pH units.
Correct Procedure
- Frequency: daily is ideal; at minimum once a week
- Number of calibration points: two-point calibration (buffers 4.0 and 7.0) is the standard for hydroponics
- Temperature: buffer solutions have their exact values at 25°C
- Order: rinse with distilled water → buffer 1 → confirm → rinse → buffer 2
Electrode Storage
Always store in a moist state — protective cap filled with 3M KCl solution.
Never store in distilled water — it leaches the substances the electrode needs to function correctly.
Three Costly Mistakes
- Infrequent calibration — "small deviations don't matter"
- Single-point calibration at buffer 7.0 only — does not correct the slope of the response curve
- Storing the electrode in distilled water or in a dry environment
Reliability Check
After two-point calibration, verify with a third buffer. Deviation ≤ 0.05 pH — the instrument is functioning correctly.