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The sensitivity of an optical backscatter sensor (OBS) to bubbles is tested using a variety of bubble generating mechanisms that span the range of bubble sizes typically observed in surf zone studies. Over 1200 one-minute runs were performed in a stirred test tank by altering the bubbler type, water type, air-flow rate and sand or mud concentration. The laboratory data shows a strong dependence of OBS response to bubble size and concentration. For sand or mud sediments, the optical backscatter sensor voltage is increased by the presence of bubbles and smaller bubbles had a larger effect. The effect was more pronounced in salt water than fresh water. We conservatively report that OBS output voltage is increased due to the presence of bubbles by 25% for the trials undertaken here. In the cases performed in vigorously stirred salt and synthetic salt water, bubbles were smaller than those measured under quiescent water conditions due to bubble shattering at the bubbler orifices and by the pumps used for stirring. It was found in those cases that the bubble effect on OBS was larger than the 25% reported here, but the extension of this finding to a natural setting, such as in the direct vicinity of the breakpoint or a bore, is not known. |
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