An investigation of the characteristics of thick film amperometric dissolved oxygen sensors
The measurement of dissolved oxygen in natural waters is vital as an indicator of pollution incidents, water quality and for the general maintenance of water courses and reservoirs. Commercially available instruments used in such applications tend to be bulky items which require frequent calibration and are prone to fouling problems. Consequently, they are incapable of providing continuous accurate in situ measurements. One of the most predominant problems though is the extreme cost of these instruments, resulting from the employment of expensive fabrication techniques. Thick film technology is a bulk batch automatic fabrication process which affords reproducible, uniform and miniature surfaces at low cost, making it an ideal technique for sensor production. This thesis describes the application of thick film technology to the production of miniature amperometric dissolved oxygen sensors based on a three electrode potentiostatic mode of operation, for initial use hi the water industry. The devices have been designed and fabricated with gold electrodes covered by a gel electrolyte held intact by an oxygen permeable cellulose acetate membrane. Novel screen-printable membrane and gel thick film pastes have been realised in the process. An external silver/silver chloride reference electrode completes the electrochemical cell. When powered, the working electrode of a device is held at an applied potential of-0.6V versus the reference electrode. A full investigation of device performance has been carried out including sensitivity, linearity and response times. The oxygen permeable membrane has been shown to act as a protective layer as well as providing a diffusion barrier to the electrodes. Methods of reducing instability effects and characteristics caused by variations hi physical parameters have been studied. As a consequence an in depth knowledge of the behaviour of membrane covered dissolved oxygen sensors has been acquired. Results show that the use of a more negative applied potential, alternative membranes (such as PVC and PTFE) and a microelectrode configuration reduce instability and fouling problems, maintaining oxygen sensitivity and minimising secondary operating characteristics caused by the effects of parameters such as flow and temperature. This in turn improves the lifetime of thick film dissolved oxygen devices as they are continuously powered.