EQUIPMENT AND SUPPLIES--Use of Polymer Current-Meter Rotors Pending Further Investigation

In Reply Refer To:                               June 6, 1988
WGS-Mail Stop 415


Subject:  EQUIPMENT AND SUPPLIES--Use of Polymer Current-Meter Rotors Pending
                                  Further Investigation

Samples from a supply of polymer AA current-meter rotors have been rated in
the tow tank at the Office of Surface Water (OSW) hydraulic laboratory at the
John C. Stennis Space Center.  The results of these tests have been statisti-
cally analyzed by the OSW.  The results of the analysis appear in OSW
Memorandum No. 88.09.  Essentially, the memorandum reports that the fit of the
data from this group of rotors to the standard rating is statistically the
same as the fit of the data from the original set of rotors from which the
rating was developed.  The memorandum authorized release of the rotors for
supply to the field.

Another concern, however, has surfaced in making the transition from open-cup
to polymer rotors.  Several independent sets of data have indicated that the
polymer rotors may register significantly less velocity than the open-cup
rotors under some field conditions.  There is speculation that the difference
may be because the polymer rotors are less affected by vertical components of
velocity, which are significant in turbulent flow, but this is unproven.

The OSW is starting a field experiment to learn if a significant difference
really exists under field conditions.  The results of this study may tell us
if a difference truly exists and if it is significant.  It will not tell us
which rotor cup is more accurate.  Further investigation will be required if a
difference is found.  The field experiment is expected to take about 6 months,
so results should be available late in calendar year 1988.

In the meantime, if a current-meter measurement is made with a polymer rotor,
this should be indicated by the notation "PAA" in the "Meter Type" space or
after the meter number on the front sheet of the measurement notes (form 9-
275).  Then, if adjustments or corrections are needed later, it will be
possible to determine which measurements are affected.

As well as possible velocity differences between open-cup and polymer rotors
under identical field conditions, a few polymer rotors had tow-tank results
that were 2 to 5 percent different from the standard rating.  Be watchful for
polymer-rotor meters that consistently give different results when compared
with other meters or with a stable stage-discharge rating.

If a polymer rotor appears to be giving questionable results, it should be
returned to the OSW hydraulic laboratory for inspection and possible individ-
ual rating in the tow tank at the expense of the OSW.  The complete meter
should be sent to William R. Kaehrle, Office of Surface Water Hydraulic
Laboratory, Building 2101, Stennis Space Center, Mississippi 39529, with a
memorandum explaining the circumstances.  A copy of the memorandum should be
directed to the Chief of the Office of Surface Water, Reston, Virginia.  The
data obtained may be valuable in assessing the accuracy of the new rotors.

As you receive the polymer rotors, please be aware that they cannot be used
for cable suspension with standard tail fins.  The underwater balance of the
meter is greatly affected by the polymer rotor.  The standard tail fins are
too heavy, causing the meter to angle upward to the front.  Do not attempt to
modify the standard tail fins until a modification is recommended by the
Hydrologic Instrumentation Facility (HIF) or this office.  Instead, use the
special shortened tail fins, which are available from HIF.

                                       Ernest F. Hubbard, Jr.
                                       Acting Chief, Office of Surface Water

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

This memorandum does not supersede any previous OSW Memorandum.