SuBastian Scientific Instruments

All sensors are in a constant calibration rotation, latest calibration documents for each sensor are available in cruise data structure at CruiseData/Docs/ROV - SuBastian Sensors Calibration Documents/

CTD

CTD SBE 49 FastCat

Seabird FastCat49 CTD

Conductivity is a measured value - this is the preferred source

Sound velocity is derived from temperature and conductivity - use uvSVX values if available

Internally OutputFormat=3(default, engineering units in decimal), ProcessRealTime=Y(cell thermal mass effect aligned, filtered, and corrected for), OutputSal=Y(realtime salinity output), OutputSV=Y(realtime sound-velocity output)

Outputs and Specifications

Files:

FKtYYMMDD_sb_ctd_sbe49_SXXXX.txt

Timestamp Temperature_C Conductivity_S/m Pressure_dbar Salinity_ppt SoundVelocity_m/s
2024-06-01T10:40:10.482364Z 17.3875 3.06420 1.716 22.6797 1499.997

FKtYYMMDD_sb_ctd_sbe49_depth_corr_SXXXX.txt

Timestamp Depth_m
2024-06-01T10:40:11.173719Z 1.71
Output Unit Accuracy Resolution Range
Temperature deg C +-0.002 0.0001 -5-35
Conductivity S/m +-0.0003 0.00005 0-9
Pressure dbar +-0.1% range 0.002% range 0-7000
Salinity PSU derived derived derived
SoundVelocity m/s derived derived derived

CTD uvSVX

Description

Secondary CTD Valeport uvSVX

Sound velocity is a measured value - preferable over SBE49

Salinity and density are not measured values - derived from sound velocity - use SBE49 values if available.

Files

FKtYYMMDD_sb_ctd_uvsvx_S0XXXX.txt

Timestamp Header Instrument_Address Serial_Number Pressure_dbar Depth_m Depth_unit Temperature_C SoundVelocity_m/s Salinity_PSU Density_kg/m3 Checksum
2024-06-01T10:37:48.175763Z $PVSVX 10 81173 9.957 -0.181 d 15.864 0.000 0.000 0.000 *37

FKtYYMMDD_sb_ctd_uvsvx_depth_corr_SXXXX.txt

Timestamp Depth_m
2024-06-01T10:37:50.011630Z 9.9

Fields

Output Unit Accuracy Resolution Range
SoundVelocity m/s +-0.02 0.001 1375-1900
Pressure dbar +-0.01% range 0.001% range 0-6000
Temperature deg C +-0.01 0.001 -5-35
Salinity PSU +-0.05 derived derived
Density kg/m3 +-0.05 derived derived

Dissolved Oxygen

Aanderaa DO optode 4831

Engineering data are calculated by firmware in the sensor based on measured raw data and sets of calibration coefficients stored in the sensor:

  • The Oxygen content is presented in µM (1 Molar = 1 mole/litre). Conversions to other commonly used engineering unit values are:
    • 1 ml/l = 44.66 µM, (real gas STP)
    • 1 mg/l = 31.25 µM.

A simple “plug & play” deployment without in situ reference easily gives data biased by >20 hPa (>10% O2 saturation, >20–30 μmol kg–1)

Please observe that to obtain absolute concentrations of oxygen these values need to be salinity and pressure compensated (see below).

  • The relative Air Saturation is presented in % relative to the nominal air pressure (1013.25 hPa). These values do not need to be salinity compensated.
  • The ambient Temperature is presented in ºC.

The optode raw data are the phase and amplitude of the returned signal after the luminophore quenching:

  • CalPhase(deg): Calibrated phase
  • TCPhase(deg): Temperature compensated phase
  • C1RPh(deg): Phase measurement with blue excitation light
  • C2RPh(deg): Phase measurement with red excitation light
  • C1Amp(mV): Amplitude measurement with blue excitation light
  • C2Amp(mV): Amplitude measurement with red excitation light
  • RawTemp(mV): Voltage from thermistor bridge.

Air Saturation Values Exceeding 100%

Reasons for air saturation values in excess of 100% from the Aanderaa DO optode 4831:

  1. Supersaturation: In some aquatic environments, especially in highly productive areas with significant photosynthetic activity, the dissolved oxygen concentration can exceed the equilibrium saturation level. This means that the water contains more dissolved oxygen than it would if it were in equilibrium with the atmosphere at the given temperature and pressure. Supersaturation can occur when oxygen production by photosynthesis outpaces the rate of oxygen exchange with the atmosphere.

  2. Pressure effects: The optode measures the partial pressure of oxygen, which is then converted to air saturation percentage relative to nominal air pressure (1013.25 hPa). However, if the actual ambient pressure is higher than the nominal pressure (e.g., at greater depths), the air saturation value can exceed 100%. This is because the higher pressure allows more oxygen to dissolve in the water than would be possible at the nominal pressure.

  3. Temperature effects: The solubility of oxygen in water decreases with increasing temperature. If the water temperature decreases rapidly (e.g., due to upwelling or mixing of cold water), the actual dissolved oxygen concentration can temporarily exceed the equilibrium saturation level at the new temperature, resulting in supersaturation.

  4. Calibration and compensation: As mentioned in the sensor description, to obtain absolute concentrations of oxygen, the measured values need to be salinity and pressure compensated. If these compensations are not applied or are inaccurate, the calculated air saturation values may exceed 100% even if the actual dissolved oxygen concentration is not supersaturated. See comment above on expected accuracy of optode measurements.

It’s important to note that while air saturation values above 100% can occur in real-world situations, it’s always a good idea to double-check the sensor calibration and data processing steps to ensure the accuracy of the measurements.

Files

Raw data from sensor serial feed in:

FKt240524_sb_oxygen_S0679.txt

Timestamp Header Measurement Measurement Concentration_umol Saturation_% Temperature_C CalPhase_deg TCPhase_deg C1_RPH_deg C2_RPH_deg C1_AMP C2_AMP RawTemp_mV
2024-06-01T10:37:48.750825Z MEASUREMENT 4831 582 304.878 101.721 17.335 33.220 >

Corrected/compensated data in:

FKt240524_sb_oxygen_corr_S0679.txt

Timestamp O2_Concentration O2_Saturation
2024-06-01T10:40:11.452531Z 256.32 98.13

Aanderaa O2 Optode Salinity and Pressure Compensated Data

When available, ROV CTD data are used to provide corrected data.

Oxygen correction implementation available for review on GitHub

Solubility and salinity compensation calculation based on:

Garcia and Gordon (1992). “Oxygen solubility in seawater: Better fitting equations.” Limnology and Oceanography, 37(6): 1307-1312.

Pressure Compensation based on:

Hiroshi Uchida, Takeshi Kawano, Ikuo Kaneko, and Masao Fukasawa (2008). “In-Situ calibration of optode-based oxygen sensors.” Journal of Atmospheric and Oceanic Technology, December 2008.

Potential density not compensated for.

Salinity and Solubility Constants \[ \begin{aligned} B_0 &= -6.24097 \times 10^{-3} \\ B_1 &= -6.93498 \times 10^{-3} \\ B_2 &= -6.90358 \times 10^{-3} \\ B_3 &= -4.29155 \times 10^{-3} \\ C_0 &= -3.11680 \times 10^{-7} \\ S_s &= 0 \quad \text{(Salinity setting, default 0)} \end{aligned} \]

Raw values from sensor and CTD \[ \begin{aligned} c_i &= x \quad \text{(O}_2 \text{ concentration uncompensated)} \\ t &= x \quad \text{(Temperature)} \\ D &= x \quad \text{(Depth)} \\ S &= x \quad \text{(Salinity)} \\ P_{\text{atm}} &\approx 1013.25 \quad \text{(Atmospheric pressure)} \end{aligned} \]

Scaled temperature \[ t_S = \log \left( \frac{298.15 - t}{273.15 + t} \right) \]

Pressure compensation factor \[ CF_P = \left| \frac{D}{1000} \right| \times 0.032 + 1 \]

Salinity compensation factor \[ K_{\text{sal}} = \exp \left( (S - S_s) \left( B_0 + B_1 t_S + B_2 t_S^2 + B_3 t_S^3 \right) + C_0 (S^2 - S_s^2) \right) \]

Salinity and Pressure compensated O2 in μmol \[ \begin{aligned} c_{ic} &= c_i \times K_{\text{sal}} \times CF_P \\ c_{ic} &= \text{round}(c_{ic}, 2) \end{aligned} \]

02 Solubility product constant (the atmospheric pressure part cancels out as we operate at sea-level and factoring in real-time pressure effect is 0.0032% per dbar so it is disregarded here) \[ \begin{aligned} K_{sp} &= \left( \frac{P_{\text{atm}}}{1013.25} \right) \times 44.659 \times \exp \left( 2.00856 + 3.224 \log \left( \frac{298.15 - t}{273.15 + t} \right) + 3.99063 \left( \log \left( \frac{298.15 - t}{273.15 + t} \right) \right)^2 + 4.80299 \left( \log \left( \frac{298.15 - t}{273.15 + t} \right) \right)^3 + 0.978188 \left( \log \left( \frac{298.15 - t}{273.15 + t} \right) \right)^4 + 1.71069 \left( \log \left( \frac{298.15 - t}{273.15 + t} \right) \right)^5 + S \left( -0.00624097 - 0.00693498 \log \left( \frac{298.15 - t}{273.15 + t} \right) - 0.00690358 \left( \log \left( \frac{298.15 - t}{273.15 + t} \right) \right)^2 - 0.00429155 \left( \log \left( \frac{298.15 - t}{273.15 + t} \right) \right)^3 \right) + -0.00000031168 S^2 \right) \end{aligned} \]

Compensated O2 airsat \[ O_2 \text{sat} = 100 \times \frac{c_{ic}}{K_{sp}} \]

Depth

Pressure-depth conversion

Different instruments from various manufacturers use different math to convert pressure to depth thus all pressure sensors are logged as or converted back to pressure (dbar) and then converted to depth using Gibbs Seawater Equations TEOS-10 height from pressure 75-term equation gsw_z_from_p. This ensures uniform application of constants and corrections and makes readings from different instruments comparable and repeatable.

SOI pressure to depth conversion python implementation is public on GitHub.

Simplified equation is used where:

\(z_{corr}\) = gsw.z_from_p(pressure, latitude, geo_strf_dyn_height=0, sea_surface_geopotential=0)

It is traditional to ignore dynamic height anomaly and sea surface geopotential \(\Psi + \Phi_0\) when converting between pressure and height, and this can be done by simply calling this function with only two arguments, as in gsw_z_from_p(p, lat). Ignoring \(\Psi + \Phi_0\) makes a difference to \(z\) of up to 4m at 5000 dbar [1].

The calculation is reversible to machine precision with the exact inverse function gsw_p_from_z.

Pressure Depth Sensor Paroscientific DigiQuartz 8000 Series

Initial accuracy 0.01% of full scale 10000psia

Depth(instrument internal calculation pressure psi x 0.7030696 = depth meters) and absolute pressure(psi)

_corr values calculated with constant pressure(psi) x 0.689475728 = pressure(dbar), then Gibb’s Seawater Equations used(see above)

No tare value is set - atmospheric pressure needs to be substracted

Files

FKt240524_sb_paro_S0679.txt

Timestamp Header Pressure_psia
2024-06-01T10:37:48.608260Z $PARO 14.434
2024-06-01T10:37:48.610970Z $DPT -0.182393

FKt240524_sb_paro_depth_corr_S0679.txt

Timestamp Depth_m
2024-06-01T10:37:50.049288Z 0.16

High Temp Probe

Temperature probe – PT100 up to 600℃

Files

FKt240524_sb_hightemp_pt100_S0679.txt

Timestamp Header Temperature_C
2024-06-01T10:37:48.641677Z $HT 16.2598

pH Sensor

AMT Deep Water Sensor Outputs raw sensor counts that need to be calibrated against a known solution at the beginning of every cruise(preferably every dive) it is used.