In-situ conductivity, temperature, and dissolved oxygen (CT-DO) sensor system for marine measurement

• Main Author: Huang, Xi
• Format: Thesis
• Language: English
• Published: 2011
• Subjects: Greenland; Indian
• Online Access: https://eprints.soton.ac.uk/336240/; https://eprints.soton.ac.uk/336240/1/Xi_Thesis-final_clean.pdf

This works describes the development of a miniature high accuracy, low power CT-DO sensor system for in-situ oceanographic measurements. The sensors were fabricated on glass wafers, using micro-fabrication techniques. Three chip designs were made. The sensors in designs 1 and 2 include a seven-electrode conductivity sensor set in a flow channel, a four-ring-electrode open conductivity sensor, a Platinum Resistance Thermometer (PRT) bridge temperature sensor, and a DO sensor based on a platinum electrode inset into five 25 ?m deep wells. A 16-bit impedance measurement circuit was made to support the CT sensor. This has a typical battery life of one month with 10 s sampling interval. The initial CT accuracies are ±0.01 mS/cm and ±0.003 ºC respectively. The seven-electrode conductivity sensor with channel suffered from a durability problem, which was discovered on a mid-Atlantic deployment. This problem was solved with the four-ring-electrode open conductivity and PRT bridge temperature sensor. Dock deployments and 8-week test in a calibration lab showed that the monthly drift was 0.02 mS/cm for the conductivity sensor, and less than 0.01 ºC for the temperature sensor. The DO sensor was calibrated to have an initial accuracy of ±5 µm. A simple analytical model is proposed to estimate the effect of fluid flow. Tests show that the measured flow effect leads to an error of 1% DO, compared with an estimate of 10%. The complete CT-DO sensor was tested during a 75 day Indian Ocean cruise. A novel method of bio-fouling mitigation was tested utilizing electro-chemical reactions on the electrodes of the conductivity and DO sensors, and first results are promising. The CT system was also modified to measure low-conductivity solutions. This system was deployed in Greenland and results showed that the CT sensor can also work in fresh water and harsh environments. Future plans are to integrate the electronics into an ASIC, and to include a miniature sensor chip (design 3), to make a package the size of a pen for fish tag ...