1. TT(n) = -bT(n-1) + a[T(n) – T(n-1)]
Comparison of Seabird Electronics© Pumped and Non-Pumped Conductivity-Temperature-Depth (CTD) Sensors on the Slocum Coastal Electric Glider
John Kerfoot1, Scott Glenn1, Oscar Schofield1, Clayton Jones2, Dave Pingal2, David Aragon1, Chip Haldeman1
1Coastal Ocean Observation Lab, Institute of Marine & Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, 2Teledyne Webb Research Corporation, Falmouth, MA 02536
Abstract
Datasets – September 2009
Flight Dynamics
Individual Profiles
We present a comparison of the responses of pumped and non-pumped SBE-41CP CTDs on the Slocum Coastal Electric Glider and investigate techniques for correcting short-term sensor (pressure, temperature and conductivity) mismatch. Misalignment of the C/T sensors on the pumped unit appears
Figure 6: CTD profiles with the mean up and downcast superimposed for the non-pumped (A) and pumped (B) CTD sensors. C/T sensor alignment is improved with the pumped system, along with a decrease of the thermal lag effect in the conductivity cell, most likely due to continuous flushing of the cell with new water.
The 2 lower panels display the corresponding flight characteristics of pitch and calculated velocities through the water for the non-pumped (C) and pumped (D) sensors. Of particular interest is the highly variable vehicle velocities.
Figure 1
2 gliders were deployed from the Rutgers University Marine Field Station (Tuckerton, NJ, USA, Figure 3). RU22 (blue track) was configured with the standard, non-pumped SBE-41CP CTD and the RU23 (red track) was configured with a first-generation pumped SBE-41CP CTD sensor. The gliders maintained a maximum separation of 4km, sampling the same stratified 2-layer water mass, separated by a strong thermocline.
A single profile was then chosen from the segment and the C/T sensors were aligned using both of the resulting time-shift constants. The RMS of the change in the resulting salinity and density profiles (Table 3) was used to quantify the alignment effect. The RMS values are lower for the Seabird values, as compared to the uncorrected values, and the Rutgers values are slightly lower than the Seabird values.
Salinity RMS
Density RMS
Uncorrected
0.0297
0.0264
Seabird
0.0274
0.0248
Rutgers
0.0240
0.0220
Table 3: Results of the 2 methods used to determine the C/T time-shifts for correcting short-term sensor mismatch in a single profile.
to be significantly decreased, resulting in more closely aligned downcasts and upcasts in the raw profile data. While the absolute C/T time shift values appear to vary, the response of the conductivity cell appears to lag the thermistor by ~ 0.4 seconds.
Finally, we apply a previously developed algorithm for correcting the thermal lag effect associated with the identical conductivity cells present on both systems.
Figure 3: Deployment glider tracks (ru22 – blue, ru23 – red).
C/T Sensor Alignment: Pumped CTD
Slocum Glider Specifications
CTD Comparisons
Results from the non-pumped CTD have been previously presented (Kerfoot et. al., 2006). The following analysis was performed on an individual segment using the pumped CTD carried by RU23.
AUV (Autonomous Underwater Vehicle)
Manufactured by Teledyne Webb Research Corporation – Falmouth, MA, USA
Horizontal movement using wings to convert vertical velocity
Coastal version
Depth range: 7 – 200m
Range: 30 – 60 days at 1km/h (500 – 1500 km)
Sampling frequency: ~0.5Hz
Deep version
Depth range: 100 – 1000m
Range 30 – 60 days at 1km/h (500 – 1500km)
Sampling frequency: ~0.25 Hz A) E)
Figure 9: Results (A, Temperature, B, Conductivity, C, Salinity and D, Density) of C/T alignment procedure using the Seabird recommended constants compared to the Rutgers constants for a single profile from the selected segment.
Alignment Protocols:
Pressure (P) recorded nearly instantaneous – no alignment
Calculate mean upcast and downcast for entire segment.
Shift temperature backward(-)/forward(+) to align with P.
Shift conductivity backward(-)/forward(+) to align with P.
Calculate RMS of Δ Salinity for entire profile (Table 2).
Calculate RMS of Δ Density for entire profile (Table 2).
Thermal Lag Correction B) F)
The Leuck and Picklo (1990) recursive filter as applied by Morrison et al (1994) to estimate the temperature inside the conductivity cell is:
TT(n) = -bT(n-1) + a[T(n) – T(n-1)] C) G)
Segment Means: Method #1
where TT is the temperature correction subtracted from the measured temperature T, n is the sample index, and the constants a and b are given by: A B C D D) H)
a = 4fnατ/(1 + 4fnτ) and b = 1 – 2a/α
where fn is the Nyquist frequency, or ½ the CTD sampling interval and where α and τ are the magnitude and time constant of the error.
Figure 4: 2-day (September 26 – 27, 2009) cross-sections of raw CTD parameters used in the analysis.
2 consecutive profiles were chosen (upcast followed by a downcast) for which to solve this equation; however, rather than solving for the entire profile, we elected to solve for the portion of the upcast lying above the thermocline (Figure 10):
As the glider is diving from the surface through an isothermal water column (above the thermocline), we assume that the measured salinity is accurate.
Solve the equation for the portion of the upcast that lies in this isothermal water column.
Non-Pumped SBE-41CP ([ and upcasts.
Banding and anomalous spiking/inversions evident in calculated salinity (Figure 4C).
Banding and anomalous spiking/inversions evident in calculated density (Figure 4D).
Likely causes:
Differences between C cell and thermistor response times.
Physical separation of C/T sensors.
Highly variable C cell flush times (Figure 6C).
Figure 7: Measured and derived hydrographic properties recorded by the pumped SBE-41CP CTD (RU23) and time-shifted following Seabird recommendations.
Seabird Electronics Recommendations:
Pressure recorded nearly instantaneously
Thermistor response lags pressure by 0.5 seconds.
Conductivity cell response lags pressure by 0.1 seconds.
Segment Means: Method #2
Figure 2: External (A,B) and internal (C,D) views of the non-pumped (A,C) and pumped (B,D) SBE-41CP CTD sensors.
Methodology:
Pressure recorded nearly instantaneously
Thermistor response lags pressure by 1.2 seconds:
Shift mean upcast and downcast in 0.1 second intervals.
Minimize the area between the shifted profiles (Figure 8A-inset).
Conductivity cell response lags pressure by 0.8 seconds:
Shift individual conductivity profiles in 0.1 second intervals.
Visual inspection to minimize inversions in the resulting salinity profiles (Figure 8B-inset). A) B) 20 40 60 80 C)
α = 0.025
τ = 37.5
3 Dry Compartments
The pumped system reduces errors associated with C/T sensor misalignment and physical separation as well as provides a uniform flush time through the conductivity cell.
Forward compartment:
Battery packs
Buoyancy engine
Pitch motor
Science bay (Figure 2):
CTD (Table 1)
Ancillary science sensors
Aft compartment:
Main board
Batteries
Iridium phone/Freewave© RF modem A) E)
Figure 10: A) Thermal inertia equation solved for portion of the upcast lying above the thermocline. B) Solution to the thermal inertia equation for the selected profiles. C) Upcast and downcast corrected using the derived values of α and τ. A B C D B) F)
Conclusions
The pumped CTD provides a more constant flow rate, resulting in a decrease in the C/T sensor misalignment.
Raw upcasts and downcasts gathered with the pumped CTD are more accurate.
The offset between the C/T sensor misalignment was found to be approximately 0.4 seconds, regardless of the method used.
More datasets must be gathered to refine both the C/T sensor alignment constants as well as the thermal lag corrections.
Non-Pumped SBE-41CP
Pumped SBE-41CP
Sampling Frequency
0.5 Hz
0.5Hz
Flow Rate
Dependent on vertical velocity
8 – 10 ml/s
Thermistor Response Time
0.5 seconds
Conductivity Cell Response Time
0.03 seconds + variable flush time
0.1 – 0.18 seconds
Power
~130 mW
~200 mW C) G)
Figure 8: Results of the alternate temperature (A) and conductivity (B) mean alignment procedures for the pumped SBE-41CP with resulting salinity (C) and density (D). D) H)
C/T Shift Results
Table 2
Downcast Salinity RMS
Upcast Salinity RMS
Downcast Density RMS
Uncorrected (0 sec T/0 sec C)
0.0162
0.150
0.0178
0.0170
Seabird (-0.5 sec T / -0.1 sec C)
0.0164
0.0149
0.0180
Rutgers (-1.2 sec T / -0.8 sec C)
0.0145
0.0167
Figure 5: Dive segment CTD cross-sections from RU22 (non-pumped): A) temperature, B) conductivity, C) salinity, D) density and RU23 (pumped): E) temperature, F) conductivity, G) salinity, H) density. Spiking in the salinity (5C) and density (5D) resulting from the misalignment of the C/T sensors. The pumped system reduces some of this misalignment, resulting in smoother salinity (5G) and density (5H) profiles.
Acknowledgements
Teledyne Webb Research Corporation for providing the pumped CTD unit, David Aragon and Chip Haldeman for deployment and recovery of the gliders, countless undergraduate and graduate students at the Institute of Marine & Coastal Sciences, Rutgers University.
Table 1: Mechanical specifications of the non-pumped and pumped SBE-41CP CTD sensors.