The first report of a product auxostat held the rate of production of carbon dioxide constant Watson 1969).
The most popular type of auxostat is the pH auxostat which is indirectly a nutrient controller. As the microorganisms consume a sugar (usually glucose), organic acid intermediates are produced that tend to lower the pH. This is countered by adding fresh medium that dilutes and buffers the acids. In other words, more acid equates to more medium. Because the volume of the vessel is fixed, the dilution rate increases and the residence time decreases when the feed rate rises.
Practically any nutrient can be fed in auxostat mode; the problem is devising a rapid assy method. We used an inexpensive gas sensor to develop an Ethanol auxostat . Other groups have devised a Dissolved oxygen auxostat .