| Abstract: | Tracers, such as ?uorescein dye, are widely employed to measure overland ?ow speeds by time‐of‐travel along measured ?ow paths. Among several disadvantages of this method are the involvement of human reaction time when using stop‐watches, and the relatively long travel path that is consequently needed for reliable timing. Long ?ow paths mean that local variability along the ?ow path cannot be detected. This paper describes a new optical tachometer that overcomes these limitations, as well as offering other advantages. It is based on the use of a small ?oating re?ector target that is carried on the surface tension ?lm, and which passes between two re?ective sensors mounted above the ?ow. The new device allows virtual ‘spot’ measurements of surface ?ow speed over a path as short as 1 cm, and eliminates the in?uence of human reaction time. The new device is battery powered and portable, and provides an improved alternative to dye timing in many ?eld and laboratory applications. Its use will allow the collection of more re?ned data than have hitherto been easily achievable. Copyright © 2003 John Wiley & Sons, Ltd. |
