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 semi-annually.

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, and
  • 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
What is the difference between precision and accuracy?

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.

Biofouling
What impacts a conductivity cell's performance?

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.

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