Conductivity Calibration
AML Oceanographic conductivity cells are calibrated with 6 distinct conductivity readings at varying temperature and salinities. Results are compared to a Guildline salinometer, which is calibrated monthly.
Conductivity Calibration Equation
Conductivity ratio is calculated using the following equation:
RC = A+B*nct+C*nct2+D*nct3+(E+F*nct+G*nct2+H*nct3)*nc, where:
- RC is the conductivity ratio
- A, B, C, D, E, F G, and H are calibration coefficients
- nct is the conductivity sensor temperature compensation measurement
- nc is the conductivity sensor measurement.
The conductivity is calculated from the conductivity ratio using:
C = RC*42.914, where:
- RC is the conductivity ratio measured
- 42.914 is the conductivity of standard seawater
Many vendors of oceanographic instrumentation refer to accuracy and precision interchangeably. They are not interchangeable. In effect, accuracy refers to how well a sensor performs against a known third party standard. For example, a temperature sensor may be +/- 0.001 C, as compared to a Black Stack themistor module. Precision refers to the repeatability of the readings of a given sensor. A sensor is precise when it repeatedly provides the same reading, regardless of how accurate that reading is.
A good analogy is a dart board. The thrower of darts is accurate when he or she is able to reach the target, the bulls-eye. He or she is precise if, having thrown three darts, all three land in the same location, irrespective of whether or not that location is the bulls-eye.
There are a number of environmental conditions that can occur that will impact the results of a conductivity cell. They include:
- Mineral build up on electrodes caused by DC offsets
- Biofouling
- Chips in the glass
If your conductivity cell suffers from any of these three conditions, data may be suspect. Biofouling can be prevented by UV biofouling control; mineral build up and glass chips will require return to the factory for diagnostics and possible repair.
