EQUIPMENT AND SUPPLIES--Polymer Current-Meter Rotors

In Reply Refer To:                               August 8, 1989
WGS-Mail Stop 415



Subject:  EQUIPMENT AND SUPPLIES--Polymer Current-Meter Rotors

There is mounting evidence that current meters equipped with
polymer rotors consistently under-register velocity when there is
a vertical component of flow, such as might occur in turbulent
conditions.  The meters may measure too little velocity in steep,
rocky streams or in more placid streams where there are bed forms
or where irregular or rough beds exist.  As reported in Office of
Surface Water Technical Memorandum No. 89.09, the mean difference
between average measured velocities between meters equipped with
polymer rotors and those equipped with metal-cup rotors was
2.4 percent over a wide range of measuring conditions.  Less
velocity was measured with polymer-rotor meters than with metal-
cup meters in each of the four districts that participated in this
paired measurement experiment.  The difference was larger, on the
average, for measurements where the hydrographer reported
turbulent conditions.

Subsequently, experiments in a flume and in a tow tank have shown
that the polymer-rotor meters register significantly less velocity
than would be expected when the meter is rotated (tilted) through
various positions from horizontal to up and down 90 degrees
relative to the flow.  With angles (tilt) as small as 10 degrees
from the horizontal, the polymer-rotor meters measured velocities
of about 86 to 92 percent of the theoretical horizontal component
of the flow.  At angles of 30 degrees, the measured velocities
were as small as 50 percent of the theoretical horizontal
component.  It seems reasonable to expect the angles of flow
approaching a meter under turbulent conditions to fluctuate as
much, or more, than 10 degrees from the horizontal.  The metal-
rotor meters apparently did not follow the theoretical value
perfectly either, but they gave much better results than did the
polymer-rotor equipment.

Based on this preliminary evidence, the Office of Surface Water
recommends that polymer-rotor meters not be used.  In performing
station analyses, consideration should be given to disregarding
negative shifts to measurements made previously with these meters
under turbulent conditions or where prominent bed forms, such as
dunes, are present.  Under non-turbulent conditions, however, the
polymer-rotor-meter measurements are probably within acceptable
accuracy limits.

The Hydrologic Instrumentation Facility has ordered a supply of AA
and pygmy metal-cup rotors.  These will soon be available to fill
needs in the field.

Additional study of this problem is ongoing.  As more information
becomes available it will be distributed as soon as possible to
the field.

                                       Charles W. Boning
                                       Chief, Office of Surface Water

WRD Distribution:  A, B, S, FO, PO