In applications where pitot taps cannot be installed in a straight run of pipe, dye-dilution flow rate techniques may be a viable alternative. Dye-dilution flow measurements are especially well suited for "once-through" applications to determine pump, condenser or helper cooling tower flow rates. The flow rate accuracy does not depend on pipe cleanliness, pipe geometry or turbulence at the measurement point and flow measurements can typically be performed with existing taps.
In this technique, fluorescing Rhodamine WT dye is introduced at a precisely known injection rate into the flow stream under investigation. At a downstream location after the dye is thoroughly mixed, the concentration of the dye in the system water is measured with a calibrated fluorometer. The downstream concentration is proportional to the mass flow rate of the water.
Calibrations of the fluorometer are made via very accurate dilutions of the Rhodamine WT dye using water from the system to be evaluated. Because the calibrations are made with the water from the tested system, the calibrations compensate for any background fluorescence or interferences due to sedimentation in the tested water. Temperature corrections are also made to address dye fluorescence response with temperature.
Because chlorine in the system water will react with the dye, chlorination or other oxidizing chemical treatment systems must be taken out of service prior to the generation of calibration solutions and throughout the dye dilution test efforts.
Rhodamine WT was developed for water flow studies and has been chosen by the Army Corp of Engineers for inflow studies and is approved by the EPA for water flow measurements. Typical test concentrations at the sampling station range from 10-20 ppb. The test concentration becomes further diluted upon discharge into a receiving body of water.