Thomasson and the team agreed that measuring optical density of algal cells as they grew in water was the most straight-forward and readily achievable way to go, he said.

 “(In research situations) optical density is typically measured by collecting a sample of aqueous solution, taking the sample to the laboratory, and using a spectrophotometer to measure the absorbance of the sample at a specific wavelength,” Thomasson said. “OD probes are available and could potentially be incorporated into a process control system, but those currently on the market had not been proven to be of acceptable accuracy, ruggedness, etc., for a process-control system for open-pond algae production.”

To be suitable for commercial production, the optical sensor needed to be mountable at a fixed location in a production facility, operate without human intervention and provide measurements that could be used to adjust production in “real time,” Thomasson explained in a paper the team presented at the 2010 annual meeting of the American Society of Agricultural and Biological Engineers last summer.

Real time is a computer science term for systems that function instantaneously or nearly so.

In addition to all of the above parameters, Thomasson thought it a good idea not to take measurements in just one wavelength of light as did the laboratory tests. Multiple measurements at different wavelengths would ensure increased reliability, he said.
Therefore, the team experimented with wavelengths ranging from 250 nanometers to 2,500 nanometers, from the ultraviolet through infrared.

After successfully comparing sensor results to actual lab analyses, the team designed, constructed and tested a prototype sensor on an algae-production raceway in June 2010. The sensor took measurements at five-second intervals. Field tests at the AgriLife Algae Research and Development Facility near Pecos showed the prototype sensor could “very accurately” and consistently measure the optical density in the raceway and therefore enable real-time growth management, Thomasson said.

Because the team has applied for a patent, the actual wavelengths used are subject to intellectual-property protection, Thomasson said. But he could disclose that one wavelength in the near-infrared region was used to accurately measure the slow-and-steady increase of optical density during the growth process, as well as the drop in optical density when more water and nutrients were added.