In this method, a tracer is continuously injected at a constant rate into the flow, and tracer dilution by the metered flow is monitored at a downstream point. If a tracer absent in the meter flow is used, the following relationship applies
Qd = flow upstream qT = tracer input flow
CT = tracer input concentration
CD = tracer concentration downstream
The dilution method has some definite advantages, because it is independent of flow characteristics, does not interfere with the flow and, consequently, does not cause any head loss. Using fluorescent dyes and ensuring complete tracer mixing, the method has a good range of measurement (1000 : 1), and can be fairly accurate (5%) (Alley 1977). Disadvantages are the discrete nature of measurement, as opposed to the preferred continuous measurements; the problem with automating the method; and the need for well-trained personnel. Consequently, the dilu tion method is mostly used for in-situ calibration of conventional flowmeters.
Basic characteristics of flow measurement methods discussed are summarized in Table 9.22.3.
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