We know little of the specifics, and even less of the accuracy that is to be achieved so it doesn't hurt to keep in mind a possible source for error.
Flow rate does not need to be "fairly constant" at all, it can vary wildly. The only consideration is how often you sample that analog, and that consideration can only be applied when you know all of the factors involved, most of which are outside the PLC.
The PLC can only process the data it is presented with, at a rate you determine, based on the capabilities of the hardware and software configuration the system is built with.
Sources of error can not always be eliminated, just reduced to an acceptable level, and that might involve some sort of filtering, smoothing, or signal rejection algorithm.