Conductivity meter · Calibration procedure
Cell constant
Determines the actual cell constant of the conductivity cell by measuring a certified potassium chloride standard of known conductivity.
PURPOSE
Why this is checked
The meter converts conductance into conductivity using the cell constant. If the electrodes are coated or worn, the real cell constant changes and every result is wrong by the same percentage. USP <645> expects the cell constant to be known within ±2%.
Frequency
Typically every 6 months or yearly, and after cleaning or replacing the cell, as defined in your calibration SOP.
Learn how the Conductivity meter worksYOU WILL NEED
Standards & tools
- Certified conductivity standard (potassium chloride solution) traceable to NIST or equivalent, with a value suited to the cell, e.g. 146.9 µS/cm at 25 °C for a 0.1 cm⁻¹ cell (check your certificate)
BEFORE YOU START
Prerequisites
- Cell clean and rinsed with purified water, then with a portion of standard
- Standard and cell at 25.0 ± 0.1 °C in a water bath, or use the certificate value at the measured temperature
PROCEDURE
Step by step
- 1Rinse the cell three times with small portions of the standard and discard them.
- 2Fill a clean beaker with fresh standard and immerse the cell without trapping bubbles.
- 3Bring to 25.0 °C (or record the exact temperature).
- 4Read the conductance (or uncorrected reading) when stable. Repeat with a fresh portion of standard.
- 5Calculate the cell constant and enter it in the meter, or let the meter calculate it in calibration mode.
RECORD
Example observation table
| Replicate | Standard value at 25 °C (µS/cm) | Temperature (°C) | Measured conductance (µS) |
|---|---|---|---|
| 1 | 146.9 | 25.0 | 1462 |
| 2 | 146.9 | 25.0 | 1461 |
| 3 | 146.9 | 25.0 | 1463 |
CALCULATION
How to calculate the result
Cell constant K (cm⁻¹) = Conductivity of standard (µS/cm) ÷ Measured conductance (µS); % deviation = (K − K nominal) ÷ K nominal × 100
- Mean conductance = (1462 + 1461 + 1463) ÷ 3 = 1462 µS
- K = 146.9 ÷ 1462 = 0.10048 cm⁻¹
- Nominal K = 0.1000 cm⁻¹; deviation = (0.10048 − 0.1000) ÷ 0.1000 × 100 = +0.48%
ACCEPTANCE CRITERIA
Pass if
- Determined cell constant within ±2% of the nominal (or certified) cell constant, or the limit in your SOP.
- Replicate readings agree closely (for example within 0.5%).
IF IT FAILS
What to do
- Clean the cell and repeat with fresh standard.
- If the cell constant is still outside limits, replace the cell.
- Assess water results since the last passing calibration.
MORE
Other Conductivity meter calibrations
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.