NEW BRUNSWICK SCIENTIFIC BIOFLO III, Руководство по эксплуатации

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addition. If the system is overshooting the set point in either direction the system
could be made to control better by diluting the appropriate reagent (acid if
undershooting and base if overshooting).
The last factor that can be controlled is the amount added over a given period of time.
Since in this system the pump is a fixed speed pump and the "ON" time is controlled
over a fixed pulse width if the flow rate is too small or too large, the controller cannot
adjust properly. Therefore, the flow rate should be adjusted by changing the bore size
of the tubing used for the peristaltic pump addition. Usually the tubing is too large
and should be adjusted downward to effect better control if unit is not controlling
properly.
Under proper conditions, pH may be controlled to within
±
.02 units. However, if the
flow rate or acid/base concentration are too high, the control shown in Figure 23A
may be observed. Here, control at pH 6.00 (from 7.18) after 24-48 hours of
Saccharomyces cerevisiae fermentation was attempted using 2N HCL and 2N NaOH
with a flow rate of approximately 7.3 mL/min (9.5 mL/min for maximum flow rate).
After an initial period of accurate control, it became apparent that the ratio of flow rate
(or concentration) to bioreactor volume was too high. In this case, control to
±
0.12
unit was obtained. If the flow rate is increased still further to 13.5 mL/min by
increasing the internal diameter (I.D.) of the tubing, then control to
±
0.19 units
results. This is shown in Figure 23B. On the other hand, if the flow rate is decreased
to 4.1 mL/min by a reduction in tubing I.D., then accuracy is increased to
±
0.02 units,
after a slightly longer initial lag time (Fig. 23C).
The effects of agitation rate (or inversely, viscosity) may also be seen in Figure 23D,
where the pH control of a 1L fermentation of the same organism is attempted. The
difference in control accuracy at different agitation rates is evident (flow rate = 1.9
mL/min). Although a less optional deviation from the set point value, as shown in the
examples above, may be more than acceptable for a particular application, it is
generally desirable to eliminate such deviations in order to minimize the excessive use
of acid and base. Furthermore, the addition of excess Na+ and CL- ions (as NaOH
and HCL), along with increase in medium volume, are factors which one might also
wish to consider.
The dissolved oxygen control loop may have similar control difficulties if the probe or
electrode is not maintained on a regular basis.
M1226-0050 23