User`s manual

Section 5: Configure Menu, Part III - Calibration Coefficients
48
Conductivity Calibration Coefficients
Enter g, h, i, j, Ctcor (or a, b, c, d, m) and Cpcor from the calibration sheet.
• Cpcor makes a correction for the highly consistent change in dimensions
of the conductivity cell under pressure. The default is the compressibility
coefficient for borosilicate glass (-9.57e-08). Some sensors fabricated
between 1992 and 1995 (serial numbers between 1100 and 1500) exhibit a
compression that is slightly less than pure borosilicate glass. For these
sensors, the (hermetic) epoxy jacket on the glass cell is unintentionally
strong, creating a composite pressure effect of borosilicate and epoxy.
For sensors tested to date, this composite pressure coefficient ranges from
-9.57e-08 to -6.90e-08, with the latter value producing a correction to
deep ocean salinity of 0.0057 PSU in 5000 dbars pressure (approximately
0.001 PSU per 1000 dbars).
Before modifying Cpcor, confirm that the sensor behaves differently from
pure borosilicate glass. Sea-Bird can test your cell and calculate Cpcor.
Alternatively, test the cell by comparing computed salinity to the salinity
of water samples from a range of depths, calculated using an AutoSal.
Enter values for slope (default = 1.0) and offset (default = 0.0) to make small
corrections for conductivity sensor drift between calibrations:
Corrected conductivity = (slope * computed conductivity) + offset
where
slope = true conductivity span / instrument conductivity span
offset = (true conductivity – instrument reading) * slope; measured at 0 S/m
Conductivity Slope and Offset Correction Example
At true conductivity = 0.0 S/m, instrument reading = -0.00007 S/m
At true conductivity = 3.5 S/m, instrument reading = 3.49965 S/m
Calculating the slope and offset:
Slope = (3.5 – 0.0) / (3.49965 - [- 0.00007]) = + 1.000080006
Offset = (0.0 - [-0.00007]) * 1.000080006 = + 0.000070006
The sensor usually drifts by changing span (slope of the calibration curve),
typically resulting in lower conductivity readings over time. Offset error
(error at 0 S/m) is usually due to electronics drift, and is typically less than
± 0.0001 S/m per year. Because offsets greater than ± 0.0002 S/m are a
symptom of sensor malfunction, Sea-Bird recommends that drift corrections
be made by assuming no offset error, unless there is strong evidence to the
contrary or a special need.
Wide Range Conductivity Sensors
A wide range conductivity sensor has been modified to provide conductivity
readings to 15 Siemens/meter by inserting a precision resistor in series with
the conductivity cell. Therefore, the equation used to fit the calibration data is
different from the standard equation. The sensor’s High Range Conductivity
Calibration sheet includes the equation as well as the cell constant and series
resistance to be entered in the program.
If the conductivity sensor serial number includes a w (an indication that it is a
wide range sensor):
1. After you enter the calibration coefficients and click OK, the Wide Range
Conductivity dialog box appears.
2. Enter the cell constant and series resistance (from the High Range
Conductivity Calibration sheet) in the dialog box, and click OK.
Note:
See Application Note 31 for
computation of slope and offset
correction coefficients from pre-
and post-cruise calibrations
supplied by Sea-Bird or from
salinity bottle samples taken at
sea during profiling.
Note:
Use coefficients g, h, i, j, Ctcor,
and Cpcor (if available on
calibration sheet) for most
accurate results; conductivity for
older sensors was calculated
based on a, b, c, d, m, and Cpcor.