If your system is such that the uncontrolled inflows and outflows do not follow a trend or pattern (e.g., noisy around a mean, or just short-term fluctuations), then a Proportional Controller with a constant Bias (and Proportional Gain) can often be used to control the system (i.e., smoothly approach and maintain the process variable at the target).
These two parameters need to be “tuned” to achieve good performance. This is done by selecting their values so that they provide good control. To do so, what should the values be? This will require experimentation as they will obviously differ for each system, but the following two points should be noted:
Bias: If this is an Outflow Controller, the Bias can be thought of as the (mean) sum of the expected uncontrolled inflows minus other expected uncontrolled outflows. If this is an Inflow Controller, the Bias can be thought of the (mean) sum of the expected uncontrolled outflows minus other expected uncontrolled inflows. That is, if the error was currently at zero, the Bias is the flow needed to maintain a zero error given other expected uncontrolled inflows and outflows.
Proportional Gain: This represents how strongly the Controller responds to the error. The larger this number, the quicker it responds. A smaller value approaches the target more slowly and smoothly. A larger value responds very abruptly. If it is too small, the response will be slow and ineffective. If it is too large, it will result in oscillations.