Detection of Evaporation Reducing Monolayers on Open Water Surfaces

Abstract

On large water storage surfaces, the evaporation reducing ability of monolayers depends on the coverage which can be maintained. Automatic monolayer dispensing systems are cost effective; however require an automatic detection system to advise on the condition of coverage so that appropriate dosing can be used. In this work, existing monolayer detection methods and novel techniques were tested for reliability, robustness and potential for automation when applied on water storages. The main existing techniques; oil indicator, and wave damping, proved to be fairly limited in the range of conditions for which they can be successfully used. Surface tension, wave damping, absorption and radiation of electromagnetic radiation, contact temperature measurements at a depth, remote surface IR temperature measurements and the effects of wind were explored to determine a reliable indicator of monolayer coverage. In this work measurement of temperature, either with shallow depth thermocouples or surface IR thermometer has shown to be the most reliable indication of the difference in evaporation rate which occurs with monolayer coverage. Additionally, these temperature changes are augmented by the application of artificial wind, and with measurements taking place in a confined, small volume of water. Using these findings, a floating wind assisted surface probe (WASP) was developed which can test the condition of coverage of the surface. Small scale tests indicate that by measuring the temperature changes, using a either contact or IR sensors the presence or absence of coverage can be deduced for a range of humidity conditions. The characteristics of monolayers on large surfaces, are also investigated, with explanations of other effects monolayers have on storages, such as changes to local wind structure, viscous impairment of convection currents and alterations to wind induced drift velocity. Understanding how monolayers behave on a storage surface may improve coverage with correct positioning of dispensers and detectors, exploiting the spreading behaviour of wind while ameliorating the destructive effects.