Sensor Stability Summarized by Breck Owens David Murphy, Sea-Bird Electronics AST-14 Wellington March 18-21
Physical Standards are the Foundation of Temperature Accuracy Standards grade platinum thermometer (SPRT) is calibrated against physical standards Each data point represents 1 2 days work and more than 1 measurements 2 µc jump in Ga melt is caused by NIST calibration of standard resistor Temperature (Deg.C.) Temperature (Deg.C.).15.13.11.99.97 SPRT Data at Triple Point of Water.95 1994 1999 25 21 29.765 29.7648 29.7646 29.7644 1 mdegree Full Scale SPRT Data at the Gallium Melt Point 1 mdegree Full Scale 29.7642 1994 1999 25 21
Estimating Temperature Stability SBE 38 Proxy: Calibration history of 411 SBE 38s Average error over - 1.5 32.5 degree range Shares thermistor and circuit with Argo CTD Argo CTD: 96 calibrations of Argo CTD 24 213 This one will never see the ocean Residual Error (C).1.8.6.4.2. -.2 -.4 -.6 -.8 -.1 Residual, (Degrees C).4.2. -.2 -.4 SBE 38 Drift (C) 2 4 6 8 1 12 14 Years After Manufacture +.2 -.2 SBE41cp 746-5 5 1 15 2 25 3 35 Temperature, Degrees C
Estimating Pressure Stability Past hurdles Electro-static discharge sensitivity Failed glass metal seals Plots below are for an initial 424 floats Most have very small offset after 72 profiles Mode at 4 decibars possibly induced by seal failure Pressure Offset at Profile 1 Pressure Offset at Profile 72 25 25 Number of Floats 2 15 1 5 Number of Floats 2 15 1 5-1 -8-6 -4-2 2 4 6 8 1-1 -8-6 -4-2 2 4 6 8 1 Pressure Offset Decibars Pressure Offset Decibars
CTD Pressure Sensor Screening Process Starting in 29 Accelerated testing for seal failure method developed in collaboration with Druck 5 pressure cycles 3 psia 15 hours at 4 psia and 4 C Twice a week pressure vented Offset measured against barometric pressure Returned to 4 psia Hand selected after infant drift Drift must be within +/-.25 decibar +.25 dbar -.25 dbar HOURS - 15
Reducing Pressure Span Error Over Operating Temperature Range Dana Swift reports errors of as much as 3 dbars at 2 dbars Provoked investigation with CTD 2795 Currently sensor span temperature error is corrected with data at -1 and 35 degrees C with data from Druck Residual, (% F S R).1 + 1 dbar.5. -.5-1 dbar SBE41 2795 -.1 5 1 15 2 25 Pressure (PSIA) 3 psia 26-Sep-11 -. 26-Sep-11 -.2 27-Sep-11 -.2 27-Sep-11 -.2 27-Sep-11.1 No Error at Room Temp ~1 dbar Error at 1 Degree
Reducing Pressure Span Error Over Operating Temperature Range Dana Swift reports errors of as much as 3 dbars at 2 dbars Provoked investigation with CTD 2795 Currently sensor span temperature error is corrected with data at -1 and 35 degrees C with data from Druck Residual, (% F S R).1 + 1 dbar.5. -.5-1 dbar SBE41 2795 -.1 5 1 15 2 25 Pressure (PSIA) 3 psia 26-Sep-11 -. 26-Sep-11 -.2 27-Sep-11 -.2 27-Sep-11 -.2 27-Sep-11.1 No Error at Room Temp ~1 dbar Error at 1 Degree
Proposed New Span (slope) correction Span Data Provided by Druck Sensor Span mv 4 38 36 34 32 3 28 26 24 22 2 y = -.3245x + 33.377-5 5 1 15 2 25 3 Sensor Temperature C Span Data Measured at Sea-Bird Seabird proposing to implement quadratic temperature correction. Sensor Span A/D counts 21 2995 299 2985 298 2975 297 2965 296 2955 5 1 15 2 25 3 35 Sensor Temperature C
Pressure Sensor Span Sensitivity Not Try multitemperature Point Pressure Span Correction Improves error for mid range temperatures Linear Residual, (% F S R).1.5. -.5 -.1-1 dbar SBE41 2795 5 1 15 2 25 3 Pressure (PSIA) 26-Sep-11. 26-Sep-11 -.3 27-Sep-11 -.2 27-Sep-11 -.1 27-Sep-11 -.1 ~1 dbar Error at 1 Degree
Salinity / Conductivity Performance in the Laboratory 97 calibrations 24 213 Error less than +/-.5 PSU Residual, (S /m ).1.5 +.5 PSU. -.5 PSU -.5 SBE41cp 746 equivalent -.1 Conductivity Range 7 S/m 1 2 3 4 5 6 7 Conductivity (Siemens/m)
Investigating Conductivity (Salinity) Stability Salinity performance measured against climatology PMEL reports 25% of their floats require salinity correction UW reports 5% require correction Salinity drifting fresh or negative typical of cell fouling Salinity drifting salty or positive not well understood Error at 2. degrees.323 PSU.263 S/m
Salinity Error Sources Error evaluated at potential temperature of 2. degrees, P and C are dependent variables Possible sources of salinity error Pressure error 6 dbar errpr =>.3 PSU error Temperature error 3 millideg error =>.3 PSU error Conductivity error
Conductivity as Source of Salinity Error Error sources Conductivity Cell Fouling Negative Drift Reduces cell radius Electrode degradation Cell damage Can show positive drift Typically magnitude of error is high Conductivity Sensor Circuit Frequency of oscillation dependent on circuit components (resistors and capacitors) Indeterminate direction of drift
Plausible Mechanism Presume that fouling dominates negative drift and will not discuss further Identifying conductivity circuitry as a likely source for the positive salinity drift does not explain why some investigators are more impacted. Strategies: Investigate difference in climatology and CTD for clues for plausible mechanism for positive drift Use next to the bottom bin for each profile of each float If bin depth greater than 9 decibars Next to bottom bin assures flushed and equilibrated cell
ARGO Salinity Error Profile 1 Compared to Profile 72 Histograms are scaled to show distribution of corrections, number of floats corrected +/-.2 ranges to 5 25 2 15 1 5 25 2 15 1 5 UW Profile 1 for 747 floats -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2.2.4.6.8.1.12.14.16.18.2 UW Profile 72 for 687 floats -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2.2.4.6.8.1.12.14.16.18.2 25 2 15 1 5 25 2 15 1 5 Scripps Profile 1 for 861 floats -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2.2.4.6.8.1.12.14.16.18.2 Scripps Profile 72 for 531 floats -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2.2.4.6.8.1.12.14.16.18.2
ARGO Salinity Error Profile 1 Compared to Profile 72 Histograms are scaled to show distribution of corrections, number of floats corrected +/-.2 ranges to 3 25 2 15 1 5 WHOI Profile 1 for 397 floats -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2.2.4.6.8.1.12.14.16.18.2 25 2 15 1 5 PMEL Profile 1 for 418 floats -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2.2.4.6.8.1.12.14.16.18.2 25 2 15 1 5 WHOI Profile 72 for 253 floats -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2.2.4.6.8.1.12.14.16.18.2 25 2 15 1 5 PMEL Profile 72 for 376 floats -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2.2.4.6.8.1.12.14.16.18.2
Positive Conductivity Drift Correlates with Low Latitude Deployments More likely, correlation is with mixed layer temperature 3% 25% 2% 15% 1% 5% % Percent of Positive Drifters by Latitude -8-6 -4-2 2 4 6 8 4 Floats, 1 Positive Drifter Number of Floats Number of Floats 2 15 1 5 25 2 15 1 5 All Floats by Latitude -8-6 -4-2 2 4 6 8 Latitude Positive Drifters by Latitude -8-6 -4-2 2 4 6 8 Latitude
Possible Mechanism More water vapor inside floats at warmer latitudes Components on CTD circuit board effected by higher humidity Desiccants hold less water at warmer temperatures Sud-Chemie, Inc 211 Similar problems with slow leaking O-ring seals for moored instrumentation (Weller).
Work in Progress Work in progress Humidity experiments to verify mechanism of positive conductivity drift Acquisition of environmental chamber to calibrate pressure sensor span sensitivity to temperature Qualification of Kistler 7 dbar pressure sensors Work planned Redesign of calibration bath to improve thermal noise Comparison of SBE Microcat post deployment calibration to climatological correction