Just how accurate is the frequency of US Wall Outlets?
Typical accuracy of the AC Mains in North America is extremely accurate. Over a long period of time (hours), total error is such that an electric clock with synchronous motor has exactly zero error.

The instantaneous frequency may vary by some tiny fraction of one Hertz but the long term error is actively corrected.

I'm assuming that your impeller pump uses an induction motor. The slip in that motor will introduce far more error in your measurement than frequency changes of the AC Mains.