How often do you take on-site parameter readings?

Generally speaking, I prefer to use a conservative approach at typical flow rates of anywhere from 200-500 mL/min of purging, and take readings about 3-5 minutes apart. The further apart the readings are, the better your chances of seeing the truly stabilized conditions in the well. For example, readings that are one minute apart may have the appearance of stabilization when, in fact, that's not actually occurring. Also, make sure that between readings you purge at least 1-2 volumes of the flow cell so you are taking independent readings.