The fan is operated at a constant RPM, so the airflow is constant, independent of battery or mains voltage. Of course, if you took it onto a high mountain where the air density was much different, there might be a small change in the actual flow rate.
You cannot replace the make-up water in the make-up reservoir with PBS, since it will result in increasing sample salinity with long operating time. We maintain a constant level of sample water in the collector by periodically adding small increments of distilled water to it. However, you could inject PBS into the sampler's inlet throat initially, which would provide the saline environment you want, and then the distilled water automatically provided by our unit would maintain the sample level. If you do this, you might want us to initially set the system up for a 'dry' start-up. Normally, the unit injects a full charge of distilled water immediately upon turn-on to minimize start-up time. With the proposed scenario, you would be providing that start-up fluid charge manually. By the way, the PBS cannot have surfactant in it, since an excessive amount of foam would be created during operation. At least one other customer is doing something like this, but they have not confided details of their procedures to us. You might also want to wash the interior of the SASS from time-to-time so that salts did not build up near the fan entrance, or inside the fan.