Multiparameter meter · Calibration procedure
Multi-point probe calibration
Calibrates the pH, conductivity and dissolved oxygen sensors of a multiparameter meter with traceable standards and checks slope, offset and accuracy.
PURPOSE
Why this is checked
Each sensor drifts with age and fouling. Calibrating at several points sets the slope and offset of each sensor so that field readings are accurate, and the recorded slopes show when a sensor needs cleaning or replacement.
Frequency
Typically at the start of each field day or deployment, with a post-use check at the end; full verification monthly, as defined in your SOP.
Learn how the Multiparameter meter worksYOU WILL NEED
Standards & tools
- pH buffers 4.01, 7.00 and 10.01 at 25 °C, traceable to certified reference materials
- Conductivity standard 1413 µS/cm at 25 °C (0.01 mol/L KCl), plus a second standard near the sample range if needed
- Water-saturated air (calibration cup with a damp sponge) for DO 100% saturation; zero-oxygen solution (sodium sulphite) if a zero point is required
- Calibrated barometer for local air pressure
BEFORE YOU START
Prerequisites
- Sensors clean and inspected; DO cap undamaged
- Standards and sensors at the same temperature
- Meter batteries charged
PROCEDURE
Step by step
- 1Rinse sensors with purified water and then with a small portion of pH 7.00 buffer.
- 2Calibrate pH at 7.00, then 4.01, then 10.01, waiting for a stable reading at each point. Record mV values and the slopes reported by the meter.
- 3Rinse, then calibrate conductivity with the 1413 µS/cm standard. Record the reading and cell constant.
- 4For DO, place the sensor in water-saturated air, enter the barometric pressure and calibrate at 100% saturation. Calibrate at zero if the SOP requires.
- 5Verify: re-read pH 7.00 buffer and the conductivity standard as samples (not in calibration mode).
- 6Record all values, standard lot numbers and expiry dates in the calibration log.
RECORD
Example observation table
| Sensor / standard | Nominal value | Reading | Result |
|---|---|---|---|
| pH 4.01 buffer | 4.01 | 177.3 mV | Slope 4–7 = 99.9% |
| pH 7.00 buffer | 7.00 | 0.5 mV | Offset +0.5 mV |
| pH 10.01 buffer | 10.01 | −175.8 mV | Slope 7–10 = 99.0% |
| Conductivity standard | 1413 µS/cm | 1408 µS/cm (verification) | −0.35% |
| DO, water-saturated air (25 °C, 101.3 kPa) | 100% saturation | 100.0% (after calibration) | Pass |
CALCULATION
How to calculate the result
Formula
pH slope (%) = [(mV₁ − mV₂) ÷ (pH₂ − pH₁)] ÷ 59.16 × 100 at 25 °C; Conductivity error (%) = (reading − nominal) ÷ nominal × 100
Worked example
- 1Slope 4–7: (177.3 − 0.5) ÷ (7.00 − 4.01) = 59.13 mV/pH → 59.13 ÷ 59.16 × 100 = 99.9%
- 2Slope 7–10: (0.5 − (−175.8)) ÷ (10.01 − 7.00) = 58.57 mV/pH → 99.0%
- 3Conductivity: (1408 − 1413) ÷ 1413 × 100 = −0.35%
ACCEPTANCE CRITERIA
Pass if
- pH slope typically 95–105% and offset within about ±30 mV (or the manufacturer's limits)
- pH buffer verification within ±0.05 to ±0.1 pH units, as defined in your SOP
- Conductivity verification within about ±1–2% of the standard value
- DO reading at 100% saturation within about ±2% saturation, and zero below about 0.2 mg/L if checked
IF IT FAILS
What to do
- Do not use the failing sensor for reported results.
- Clean the sensor, use fresh standards and recalibrate.
- Replace the DO cap, pH sensor or conductivity sensor if it still fails.
- If a post-use check fails, flag all readings since the last good calibration and assess them.
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.