Understanding of and experience with a wide variety of analytical techniques is a necessary component of a quality undergraduate curriculum in Chemical Engineering. Students often gain such experience in some sort of unit operations course, using commercially manufactured analytical equipment. However, such equipment is costly and may often be regarded as merely a black box source of data by students, sometimes leading them to erroneous conclusions. Therefore, it is important to introduce students to economical and effective means of data collection that also highlight the electronics and basic physical principles influencing the path between the actuality of physical samples and the ultimate quantified data.
For our unit operations course, our students were in need of a real-time estimation of yeast concentration within a bioreactor. In the past, such estimation had been accomplished offline, using measurements of optical density from a spectrophotometer. We constructed a rudimentary flow cell and photometer using a LED light source and an amplified photodiode. Our design was ran in series with a commercial spectrophotometer, worth several thousands of dollars. The constructed photometer cost approximately $8.00 and performed as well as the commercial option at estimating yeast cell concentration, and detecting the transition from the stationary phase to the exponential growth phase. Results were also validated using a fluorescent staining method within a commercial cell counter. By switching out LED's of different peak wavelengths, this simple, inexpensive and compact optical analytical device will be used on a variety of student projects. Its operation will be further demonstrated with a CSTR using the reaction between dilute NaOH and Crystal Violet.
The construction of this device and its comparison with more elaborate equipment provided a valuable education for our students regarding the electronics, optics, and costs that may be hidden within the physical measurements on which they will rely in their professional lives. Furthermore, the device may serve as a compact and easily-understandable means of quantifying the results from classroom demonstrations of several chemical engineering areas, such as kinetics and mixing.
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