After transfer of cell culture medium into the Wave bag, the pH is more alkaline than the set-point. Switching on the pH control results in maximum CO2 output. On several instances the pH then dropped up to 0.3 units below the set-point and it took several hours until the pH reached the acceptable range and the cells could be transferred to the bag.

Answer

CO2 control of the pH in Cellbag Bioreactors occurs via the headspace. Depending on gas flow, bag size and agitation conditions some delay will occur until the gas composition in the headspace is exchanged and the CO2 has dissolved in the cultivation medium. Suggested gas flows and agitation conditions for specific Cellbag Bioreactor sizes can be found in the user manual.
The concentration of CO2 that is required to reach a certain pH, is dependent on the bicarbonate concentration in the medium and the agitation conditions. For a specific set of conditions it can easily be determined experimentally. Once the appropriate CO2 concentration is established, the Cellbag should already be inflated using this gas composition. Starting any controller near the set point of the target variable minimizes the risk for overshooting and ensures that stable operating conditions are reached quickly.
The PID controller parameters for pH and DO control are accessible in WAVEPOD II and WAVE 25. Suggested settings are listed in the user manuals. Experienced users have the option to further tweak the controller settings. One way to speed up the controller response is to increase the P factor. This should be done carefully to avoid oscillation of the controller output. Further information can be found in technical literature about PID tuning.