The Applied Microsystems Ltd. conductivity sensor is a unique four electrode cell described by U.S. patent 4275353.  The construction is modeled after a traditional 4 electrode conductivity cell.  The cells have been “broken” into pairs (representing as glass crosses) to reduce the overall cell length, thus increasing flush speeds.  The proximity effect around the openings of the C-Cell is less than 2 cm. 

The two Pyrex glass cross assemblies contain both current and potential electrode pairs. A low frequency square wave is passed between adjacent current electrodes. The potential measured is a linear function of the water's conductance dimensioned by the volume of water inside the glass crosses.  The glass crosses exhibit excellent dimensional stability over the operating temperature and pressure range of the sensor.

The switching and clocking waveforms are supplied by a synchronous amplifier circuit. The alternating cell current excitation is derived by chopping a fraction of the reference voltage used in the A/D converter circuit of the data logger. This ensures full ratiometric operation.

For seawater of 35 p.s.u., 15°C, and 0 dBar, the cell can be modeled as a 100 ohm resistor.

The Conductivity Ratio (R) is calculated from the following formula:

R = A + Bnr

The conductivity is calculated as shown in the following formula:

C = R * 42.921

A and B are calibration coefficients, which are determined at the factory.  nr is the raw value for the conductivity ratio, as measured by the instrument.

All Applied Microsystems CTDs can be calibrated with four different conductivity ranges, each of which results in varying accuracies, as specified below:

Conductivity Specifications by Range Selection

	Range (mS/cm)	Accuracy (mS/cm)	Precision (mS/cm)	Resolution (mS/cm)	Response (ms)
Oceanographic	0 to 70	+/- 0.01	+/- 0.005	+/- 0.001	25 ms, at 1 m/s flow
Dual range	0 to 70 and 0 to 7	+/- 0.01	+/- 0.005	+/- 0.001	25 ms, at 1 m/s flow
Freshwater	0 to 7	+/- 0.01	+/- 0.005	+/- 0.001	25 ms, at 1 m/s flow
Ultrafresh	0 to 1.5	+/- 0.01	+/- 0.005	+/- 0.001	25 ms, at 1 m/s flow

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.

For a detailed explanation of how we calibrate Applied Microsystems' conductivity sensors - including a listing of Frequently Asked Questions - please click here.

The glass used to provide dimensional stability in all conductivity cells makes the conductivity cell more fragile than some other sensors, such as the sound speed sensors.  However the split design of AML's conductivity sensors significantly reduces the size of the glass components.  The reduced size of the glass components improves the robustness of AML's conductivity sensor dramatically.

Applied Microsystems designs and manufactures sensors - conductivity, temperature and pressure - that have unusually fast response times.  Rapid response times are important in a wide variety of scenarios.  However, if you intend to do vertical profiling - where accurate mapping of the thermocline or picnocline is important - or if you plan to calculate salinity or density - where mismatches between sensor readings can generate spiking, then rapid sensor response times should be a critical decision factor in your choice of supplier.