How accurate is salinity measured by my CTD? What factors impact accuracy?
One of the reasons that this is not a simple question is that there are several factors to take into consideration regarding the error margin for practical salinity measurements. Salinity itself is a derived measurement from temperature, conductivity, and pressure, so any errors in these sensors can propagate to salinity. For example, Oour initial accuracy specification for the SBE 3plus temperature sensor and SBE 4 conductivity sensor on an SBE 9plus CTD is approximately equivalent to an initial salinity accuracy of 0.003 PSU (note that conductivity units of mS/cm are roughly equivalent in terms of magnitude to PSU).
However, another issue to consider is that this accuracy is defined for a clean, well-mixed calibration bath. In the ocean, some of the biggest factors that impact salinity accuracy are 1) sensor drift from biofouling or surface oils for conductivity in particular and 2) dynamic errors that can occur on moving platforms, particularly when conditions are rapidly changing, which will be true for all sensors that measure salinity. Sea-Bird provides recommendations, design features such as a pumped flow path, and data processing routines to align and improve data for the salinity calculation to account for thermal transients and hysteresis, and to match sensor response times. Depending on the environment and the steepness of the gradient, and after careful data processing, this may continue to have an impact on salinity on the order of 0.002 PSU or more, for example. For more details, see Application Note 82.
Lastly, note that salinity in PSU is calculated according to the Practical Salinity Scale (PSS-78), which is defined as valid for salinity ranges from 2 – 42 PSU.
Many cables, mount kits, and spare parts can be ordered online.
To SBE 32
- 50121 SeaCAT/Sealogger/AFM to SBE 32 Bottle Position Mount Kit (document 67020)
- 50124 SBE 32 Cage Mount Kit — Ring Top; required to mount older CTD cage with ring top in CTD extension stand below full size SBE 32 Carousel (document 67186)
To SBE 55
- 50422 SBE 55 Universal CTD / Electronics Installation Kit (document 67179)
- 50012 O-ring kit for SBE 16 or 19 (document 67026)
- 50009 O-ring kit for SBE 19 (document 67076)
- 50092 Jackscrew kit for SBE 16, 17plus, 19, 21, 25, 26/26plus, 52-MP, 53, 54, AFM, or PDIM
- 50249 Small hardware kit for SBE 19 (document 67015)
- 60021 Battery end cap hardware & O-ring kit for SBE 16/16plus/16plus-IM/16plus V2/16plus-IM V2, 17plus, 19/19plus/19plus V2, 25, 26/26plus, 53, 54, 55, or AFM (document 67042)
- 50026 Magnetic switch kit for SBE 19 or 25 (document 67035)
- 50236 TC duct kit for SBE 19 (document 67088)
- 90087 CTD plumbing kit (document 67108)
- 50072 Support kit for SBE 16 or 19 (document 67017)
- 801542 AF24173 Anti-Foulant Device (pair, bagged, labeled for shipping)
- 50315 Anti-foul holder kit — installs on SBE 4 & SeaCATs (not SeaCATplus) to allow use of AF24173 in place of obsolete 24012 Anti-Foulant Devices. Order AF24173 separately. (see Application Note 70)
- 232012 Stainless-steel protective cage with small footprint, for deploying through ice (drawing 22149)