EQUIPMENT AND SUPPLIES--Polymer Current-Meter Rotors

In Reply Refer To:                               July 11, 1989
WGS-Mail Stop 415


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

The purpose of this memorandum is to convey the status of investigations of
the differences in performance of polymer solid-cup rotors as compared to
metal open-cup rotors under field conditions.

Office of Surface Water (OSW) Technical Memorandum No. 88.09 reported that the
results of tow-tank testing of a production run of polymer rotors
statistically fitted an already established standard rating for AA current
meters equipped with polymer rotors.  As a result the polymer rotors were
released for general use in the field.

Subsequently, Technical Memorandum No. 88.10 noted that differences were
reported in velocities obtained in the field using polymer as opposed to metal
rotors.  Further study was promised.

A study involving a district in each region--Indiana, Florida, Wyoming, and
Idaho--was planned and conducted.  The experimental design was to have the
districts make paired measurements--4 pairs at 10 sites each--using polymer
and metal-cup rotors.  The meters with polymer rotors were individually rated
in the tow tank at Stennis Space Center; the metal-cup meters were used with
the standard rating.  This plan was essentially followed, and 116 pairs of
measurements are being used for analysis.  These measurements had average
velocities ranging from 0.17 to 4.45 feet per second.

Preliminary results show that the polymer rotors generally measure less
velocity than do the metal-cup rotors, averaging 2.4 percent less.  By
district the results are as follows (negative values indicate the polymer
rotors measured less velocity):

                                Difference      Standard Deviation
    District       Pairs       (in percent)        (in percent)

    Florida          9             -0.1               4.1
    Idaho           35             -2.8               2.0
    Indiana         32             -l.7               2.5
    Wyoming         40             -2.3               4.7

Actual differences in mean velocities between pairs of measurements ranged
from -15.4 to 7.7 percent for the data set.

It has been suggested that differences between the rotors are associated with
conditions of turbulence.  Based on the hydrogra-pher's notes, the data were
divided into measurements where flow conditions were reported as turbulent or
non-turbulent or not reported as either.  The results are as follows:

                                 Difference   Standard Deviation
        Flow           Pairs    (in percent)      in percent)

    Non-turbulent       55         -1.8              3.3
    Neither             34         -2.2              3.6
    Turbulent           27         -3.1              3.5

Although these results are preliminary, one might conclude that there is a
statistically significant difference between the performance of the rotors
under most field conditions.  More data collection at higher velocities seems
warranted.  There seems to be some factor associated with turbulence that
influences the magnitude of the difference.  It was anticipated that vertical
components of flow might have less effect on the polymer rotor performance.
In any case, the consistent bias of the velocity data from these studies and
from results reported of other work is certainly a matter of concern.

The Office of Surface Water is planning further study to determine which rotor
provides the most accurate velocity data and to relate the difference between
rotors to some causative factor at the measurement site.  Similar work will
have to be done on the pygmy meter rotors, also.  These plans are being
coordinated with the National Research Program and the Hydrologic
Instrumentation Facility.  Any individual or office that has additional
paired-measurement data or other information should bring it to the attention
of OSW.

Until more information is available, please be advised that a difference may
be noted upon switching to polymer rotors.  Care should be taken to use the
correct standard rating and the correct tail fin for the polymer-rotor current
meters.  Be sure measurement notes indicate the type of rotor used.
Additional information will be provided as soon as it becomes available.

                                       Ernest F. Hubbard
                                       Acting Chief, Office of Surface

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