Quantitative Measurement of the Mixing Time

After performing the decolorization technique to provide qualitative understanding of the mixing behavior, the method of mixing time measurement must be 

Great care must be taken to prevent unwanted chemical reactions occurring in the mixing vessel. The use of reactive tracers such as nitric acid and sodium hydroxide, rather than common salt, causes some problems. Materials that may normally be regarded as inert, since they react so slowly as not to cause significant drift over the time of the experiments, may start to react more rapidly. Materials that are initially inert may also start to react after a period of time when protective oxide films are broken down. 

5 Processing Mixing Time Data. Data collected by the conductivity, thermocouple, or pH techniques must be processed to obtain a characteristic mixing time for the system under investigation. The analysis is described below in terms of conductivity readings, but is equally applicable to the other data types. 

The data must first be normalized to eliminate the effect of different probe gains. The data are normalized between an initial zero value measured before the addition of tracer, and a final stable value measured after the test is complete. These values are typically obtained by measuring the probe outputs over 30 s. 

More stringent criteria of 99% or 99.9% mixing time are used in certain applications where a higher degree of uniformity is required (e.g., paint manufacture or pharmaceutical production). These values can also be obtained from the log variance graph. The linear response of log variance as a function of time is due to the exponential decay in concentration fluctuations. This relationship also allows one to calculate an arbitrary degree of uniformity from the 95% nfixing time  

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