Sea-Bird Electronics SBE 45 MicroTSG, User Manual

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APPLICATION NOTE NO. 31 Revised February 2010
Computing Temperature and Conductivity Slope and Offset Correction
Coefficients from Laboratory Calibrations and Salinity Bottle Samples
Conductivity Sensors
The conductivity sensor
slope and offset entries in the configuration (.con or .xmlcon) file in SEASOFT permit the user
to make corrections for sensor drift between calibrations. The correction formula is:
(corrected conductivity) = slope * (computed conductivity) + offset
where :
slope = (true conductivity span) / (instrument reading conductivity span)
offset = (true conductivity - instrument reading conductivity) * slope
measured at 0 S/m
For newly calibrated sensors, use slope = 1.0, offset = 0.0.
Sea-Bird conductivity sensors usually drift by changing span (the slope of the calibration curve), and changes are
typically toward lower conductivity readings with time. Any offset error in conductivity (error at 0 S/m) is usually due
to electronics drift, typically less than ±0.0001 S/m per year. Offsets greater than ±0.0002 S/m per year are symptomatic
of sensor malfunction.
Therefore, Sea-Bird recommends that conductivity drift corrections be made by assuming
no offset error, unless there is strong evidence to the contrary or a special need.
Example
true conductivity =3.5 S/m
instrument reading conductivity = 3.49965 S/m
slope = 3.5 / 3.49965 = 1.000100
Correcting for Conductivity Drift Based on Pre- and Post-Cruise Laboratory Calibrations
Suppose a conductivity sensor is calibrated (pre-cruise), then immediately used at sea, and then returned for post-cruise
calibration. The pre- and post-cruise calibration data can be used to generate a slope correction for data obtained
between the pre- and post-cruise calibrations.
If
α is the conductivity computed from the pre-cruise bath data (temperature and frequency) using post-cruise
calibration coefficients and β is the true conductivity in the pre-cruise bath , then:
postslope = (postslope is typically < 1.0)
Sea-Bird calculates and prints the value for postslope on the conductivity calibration sheet for all calibrations since
February 1995 (see
Appendix I: Example Conductivity Calibration Sheet )
.
Σ
( α
i
)( β
i
)
n
Σ
( α
i
)( α
i
)
i=1
n
i=1
Sea-Bird Electronics, Inc.
13431 NE 20
th
Street
Bellevue, WA 98005
USA
Phone: (425) 643-9866
Fax: (425) 643-9954
E-mail: seabird@seabird.com
Web: www.seabird.com
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