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