SeapHOx

Instrument

Instrument produced by Todd Martz laboratory at Scripps San Diego. It consist of a bundle of a Durafet pH sensor (Honeywell Process solutions, USA) an oxygen optode Aanderaa 3835 (Aanderaa Data Instruments AS, Norway) and a SBE 37 Ð SI CTD ( Sea-Bird electronic Inc., USA) The circulation of water between the SBE 37 Ð SI and the optode and Durafet is achieved by a SBE 5M pump (Sea.-Bird electronic Inc., USA). The coordination of pumping and recording by the SBE 37 and the optode is insured by a homemade data logger. To prevent from fouling on sensors, the SBE 37 Ð SI was equipped with tributyltin tablets and copper tubing linked the SBE 37 Ð SI and the flow cell to the pump. Calibration of the pH sensor is achieved by comparison with a reference seawater sample measured for pH by spectrophotometry.

Technical Data

See manufacturer for the Aanderaa O₂ optode and Seabird pump and CTD. The technical data regarding the use of Durafet pH in seawater can be found in Martz et al., 2010; Testing the Honeywell Durafet for seawater pH applications; Limnology and Oceanography: Methods.

Performance

Accuracy in test tanks at varying salinity, temperature and pCO₂ (bubbling in the tank of CO₂ modified air): using as base Baltic Sea water.

Only the pH sensor was tested for performances. The sensor shown some difficulties to perform on the long term in a fluctuating environment within an acceptable precision pH, i.e. < 0.001. Discrepancy between pH calculated from discrete samples (pH=f(TA, DIC) at high pCO₂ also appears to be a problem coming from the discrete samples analysis and / or calculations and not from the sensor. The sensor needs a monthly cleaning.

Applications

Long and short term measurement of pH, O₂, temperature and salinity in seawater. Requires homemade battery packs 18V: 3x parallel 12 D-Cells in serie (36 in total).

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