Other Auxostats
Control Based on Specific Gravity
When metabolism converts dissolved nutrients to carbon dioxide that escapes
from the liquid, the specific gravity lessens. The change is small, and the
method of measurement must be sensitive. The manometric method shown in
the figure had one leg inclined at ten degrees from horizontal to magnify
changes in the level (Mormino and Bungay, 1993). One side connected to a
glass tube positioned near the bottom of the bioreactor while the other side
connected to a glass tube submerged just below the surface of the medium.
Each tube had its own valve to adjust its bubbling rate. The manometer was
filled with dilute NaCl. An electrode was submerged in the vertical leg and
another was adjustable in the inclined leg. Pumping of fresh medium
counteracted the decrease in specific gravity due to metabolism; as the
specific gravity decreased and the level in the inclined end rose, shorting
the electrodes was sensed by a computer that turned the feed pump on.
Runs for greatly extended periods of time were possible, but dilution rate
was erratic. A more sensitive measurement would be needed to improve control.
This method should perform better in a deep bioreactor where the manometer
pressure differences would be greater.
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(c) 1995 H. Bungay