EQUIPMENT AND SUPPLIES--Polymer Current-Meter Rotors

In Reply Refer To:                               October 12, 1989
WGS-Mail Stop 415



OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 90.01

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

This memorandum establishes the policy that current meters
equipped with polymer rotors should not be used by Water Resources
Division (WRD) personnel for measurement of velocity in stream-
gaging activities.  This policy is based on data from laboratory
and field experiments, indicating that the velocities provided by
polymer-rotor meters are sufficiently flawed to preclude their
use.

As discussed in the Office of Surface Water (OSW) Technical
Memorandums Nos. 89.09 and 89.14, polymer rotors typically under-
register when used in measuring velocity in streams with moderate
to high turbulence.  Although many measurements may be within
accuracy tolerances allowed by the Division, the bias toward
under-registration supports the established policy.

This policy extends to and includes the measuring of velocities
under ice cover.  Ice measurements are generally regarded to have
a lesser degree of accuracy than open-water measurements and flow
under ice may, depending on the characteristics of the ice surface
in contact with the water, be only mildly turbulent.  However,
determination of the turbulence characteristics under ice is not
usually possible.  Therefore, it is not possible to determine if
and when a polymer-rotor meter is providing acceptable accuracy
for ice measurements.  Thus, for consistency, polymer-rotor meters
should not be used for making ice measurements.

WRD offices should discard existing stocks of polymer rotors at
their discretion.  The Hydrologic Instrumentation Facility (HIF)
is procuring metal rotors for AA and pygmy meters.  Offices
desiring to purchase metal rotors should check with the Chief,
Field Service and Supply at HIF regarding the status of these
procurements.  The HIF has informed OSW that they will provide
credit for previous purchases of polymer rotors when orders are
placed for metal rotors.  The short tail fin assembly that was
required for use with polymer-rotor meters will balance the optic-
head metal-rotor AA meter, provided the balance weights are
properly adjusted.  However, such an assembly has not yet been
rated.  If and when a standard rating becomes available, that
assembly may be used in the field.  The short tail fin requires
excessive weight, however, to balance a standard cat-whisker-head
metal-rotor AA meter and should not be used with such meters in
the field.

The OSW reiterates concern regarding discharge records produced
from data collected with polymer-rotor meters.  Offices are
requested to critically review such station records to assure that
negative shifts, or rating curves that have been revised, are not
the result of erroneous velocities measured with polymer-rotor
meters.

The OSW will continue to investigate accuracy of a variety of
meters and will continue to explore opportunities to provide a
superior meter to replace the metal-rotor AA and pygmy meters, as
well as to provide alternative means of determining velocity in
streams.  The OSW also is continuing to conduct and support
research to improve the ease and reliability of ice-affected
measurements.




                                       Charles W. Boning
                                       Chief, Office of Surface Water

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