Notice: Revised Conductivity Pressure Correction Coefficient

Original communication: November 2, 2026

To: Users of Sea-Bird Scientific HydroCAT-EP V2s, SBE 16plus V2 SeaCATs, SBE 37 MicroCATs, SBE 52-Moored Profilers, SBE 19plus V2 SeaCATs, SBE 25plus Sealoggers, SBE 49 FastCATs, SBE 45s, Glider Payload CTDs, SBE 61s and SBE 41s

Subject: Notice of Revised Conductivity Pressure Correction Coefficient

Summary

Effective 1 November 2026, Sea-Bird Scientific will adopt a revised conductivity pressure correction coefficient (CPcor) of −1.17 × 10⁻⁷ for CTDs equipped with urethane-encapsulated conductivity cells. This replaces the previously used value of −9.57 × 10⁻⁸ for these instruments.

This update applies only to instruments with urethane-encapsulated conductivity cells.

Why is this value changing?

Accurate conductivity measurements require accounting for pressure-induced changes in conductivity-cell geometry. As pressure increases, the cell and the water volume it contains are compressed. Changes in cell geometry can introduce a pressure-dependent bias in the measured conductivity.

The CPcor term in Sea-Bird’s conductivity equation compensates for this pressure-dependent effect.

The original CPcor value of −9.57 × 10⁻⁸ was established using the SBE 4 conductivity cell, which has epoxy encapsulation. Because urethane is more compressible than epoxy, this coefficient does not adequately correct the pressure response of urethane-encapsulated conductivity cells.

The revised CPcor value was derived from field measurements collected during three research cruises in 2014, 2021, and 2023, using seven SBE 61 CTDs. CTD salinity measurements were compared with discrete bottle-sample reference values. Applying the revised CPcor value (−11.7 × 10⁻⁸) substantially improves agreement with the reference salinities (Figure 1). With the previous coefficient, the mean salinity difference (SBE 61 minus bottle reference) was −0.0037 ± 0.0016 (PSS-78), indicating a systematic fresh bias. The revised coefficient reduces the mean difference to −0.0004 ± 0.0015 (PSS-78), bringing it substantially closer to zero and within a ±0.002 PSS-78 accuracy (red horizontal lines) that is the target accuracy of the Deep Argo mission.

Figure 1: Difference in salinity (PSS- 78) between SBE-61 using CPcor_new (a) and CPcor_old (b) and bottle measurements collected between 2000-6000 dbar during the RV Tangaroa research cruises of 2014, 2021, and 2023. Results are color coded for individual SBE-61 CTDs. The horizontal lines show the Deep Argo target accuracies for salinity.

Confirming the CPcor value of your instrument:

#1) The revised CPcor value will be visible on the Calibration Certificates for applicable urethane-encapsulated conductivity cells calibrated on or after 1 November 2026.

#2) The new CPcor value will also be programmed into instrument firmware. Programmed coefficients can be viewed using the appropriate command (most often “DC”, but consult your instrument manual).

Guidance For Users

The impact of the revised CPcor value is negligible near the surface and becomes apparent at deeper depths. Table 1 provides an example of the differences in conductivity and salinity obtained using the old and revised CPcor values over a pressure range of 350 to 7000 dbar.

Table 1. Comparison of conductivity and salinity using the old and revised CPcor values at selected pressures. ΔC and ΔS represent the differences between values calculated using the two CPcor values.

As shown in Table 1, the effect of the revised CPcor remains minimal in the upper ocean but increases as pressure increases. Users wishing to revise their historical data are encouraged to contact Sea-Bird Scientific for guidance on the appropriate correction procedure.

If you have any questions or concerns, please contact techsupport@seabird.com.