For the Swoffer current meters that we use for wade-across flow measurements, we currently calibrate each rotor/prop unit individually twice a year.  When we calibrate, we test each rotor/prop at both a fast and a slow speed (2 and 0.5 ft/sec), then average the two for the calibration number used in flow calculations. 


We devised this approach after realizing that the unit turns about 10% less efficiently at 0.5 ft/sec than at 2 ft/sec, dropping off significantly below about 1 ft/sec; and even in the winter and spring, half of our wade-across flow cells have velocities < 1 ft/sec.  2 ft/sec seemed like a good midpoint for the upper range of speeds, and 0.5 ft/sec for the lower range.


This “average” calibration number is more accurate on the whole than using only “fast” or “slow” trials to calibrate, but it becomes less accurate as the velocities in a given reach become more extreme toward either the fast or slow end.


In order to improve the accuracy of our flow measurements, we’d like to suggest the following:


  1. Test each unit at both a “fast” and a “slow” speed (perhaps 1.5 & 0.5 ft/sec to best bisect the fast- and slow-speed ranges encountered in our streams and the Swoffer’s accuracy ranges).
  2. When calculating flow in the database, use the “fast” calibration number for flow cells with velocities >= 1 ft/sec.  Use the “slow” calibration number for velocities < 1 ft/sec.
  3. (Optional:) For an even more accurate reading, use the floating-object method instead of the flow meter for cells with velocities < 0.5 ft/sec.  This might be a bit ungainly to do in actual conditions, but we could try coming up with a procedure that would be easy to implement.
  4. (Alternative, simpler option:) Calibrate the units at only 1.5 ft/sec and use that calibration number for all readings above 0.75 ft/sec. At slower speeds, use a floating object.

We’d appreciate your feedback!