Comparison of Pumped and Non-Pumped SBE-41CP CTD Sensors on the Slocum

Coastal Electric Glider

Clayton Jones1, Scott Glenn2, John Kerfoot2, Oscar Schofield2, Dave Aragon2, Chip Haldeman2, Dave Pingal11 Teledyne Webb Research Corporation, Falmouth, MA, USA

2 Institute of Marine & Coastal Sciences, Rutgers University, New Brunswick, NJ, USA

Slocum Coastal Electric Glider• Modular

• 3 meters in length

• Uses buoyancy to move through the water column in a saw-tooth pattern

• RF and satellite communications

• Non-pumped SBE-41CP CTD (@ 0.5Hz)• Wide variety of additional payloads

• Dissolved Oxygen• Inherent Optical Properties• Chlorophyll a• Colored Dissolved Organic Matter

CTD Types: SBE-41CP

Current: Non-Pumped Next Generation: pumped

Non-Pumped CTD: Cost/Mechanical Considerations

• Lower Power Consumption (~130mW)• Longer Duration Deployments• Requires Less Physical Space• More Space Available for Additional Sensors• Less Expensive

Pumped CTD: Cost/Mechanical Considerations

• Higher Power Consumption (~150mW @ 10mL sec-1)• Shorter Duration• Requires More Physical Space• Less Space Available for Additional Sensors• More Expensive (> 2x un-pumped)

http://www.seabird.com/pdf_documents/datasheets/GliderCTDPrelimSpec3.pdf

Correction of Raw CTD Profiles: Previous Work

• Fofonoff, N. P., S. P. Hayes, and R. C. Millard Jr., 1974: W.H.O.I./Brown CTD microprofiler: Methods of calibration and datahandling. Woods Hole Oceanographic Institution Tech. Rep. WHOI-74-89, 64 pp.

• Lueck, R. G., 1990: Thermal inertia of conductivity cells: Theory. J. Atmos. Oceanic Technol., 7, 741–755.

• Lueck, R. G. and J. L. Picklo, 1990: Thermal inertia of conductivity cells: Observations with a Sea-Bird cell. J. Atmos. Oceanic Technol., 7, 756–768.

• Morison, J., R. Andersen, N. Larson, E. D’Asaro, and T. Boyd, 1994: The correction for thermal-lag effects in Sea-Bird CTD data. J. Atmos. Oceanic Technol., 11, 1151–1164.

• Kerfoot, J.K., Glenn, S., Jones, C., 2006: Correction for Sensor Mis-match and Thermal Lag Effects in Non-Pumped Temperature Conductivity Sensors on the Slocum Coastal Electric Glider. Presented Poster at Ocean Sciences 2006.

• Johnson, G. C., Toole, J.M., Larson, N.G., 2007: Sensor Corrections for Sea-Bird SBE-41CP and SBE-41 CTDs. J. Atmos. Oceanic Technol., 24, 1117-1130.

RU22/RU23 Glider Deployments: September 2009

Non-pumped CTD: RU22 Segment: ru22_2009_264_1_60September 26, 2009 14:42 – September 26, 2009 17:42 GMT

Pumped CTD: RU23 Segment: ru23_2009_264_1_64September 26, 2009 23:22 – September 27, 2009 02:24 GMT

Selected segments were ~4km apart

Raw CTD SectionsNon-Pumped Pumped

Raw CTD Profiles and MeansNon-Pumped Pumped

Correction of Raw CTD Profiles: CT Sensor Mismatch

• Difficult since CT mismatch offsets are much lower than the nominal sampling frequency (0.5 Hz) of the CTD.

Temperature AlignmentComparison of ΔTemp and ΔCond on upcast and downcast – highly variable!

Correction of Raw CTD Profiles: Thermal Inertia

•Theory: Within the same water mass and in the absence of internal wave action, 2 consecutive (upcast and downcast) profiles should possess the same hydrographic (T-S) characteristics.•Select 2 consecutive profiles which do not exhibit internal wave activity. •Morison, et.al (1994):

TT(n) = -bT(n-1) + a[T(n) – T(n-1)]

a = 4fnατ/(1 + 4fnτ) and b = 1 – 2a/α

•Vary α and Τ to minimize the area of the polygon formed by joining the resulting salinity profiles.

Thermal Inertia Results: pumped CTD

Thermal Inertia Results: pumped CTD – Unsmoothed

Salinity Density N2

Thermal Inertia Results: pumped CTD - Smoothed

Salinity Density N2

Conclusions

• Glider vertical velocity and pitch are key factors in any attempt at correcting un-pumped CTD

• Vertical velocity and pitch vary significantly, depending on ballast accuracy and water column density.

• Pumped CTD provides constant flow which significantly improves success rate of correcting for thermal inertia