EQUIPMENT AND SUPPLIES--Use of Polymer Current-Meter Rotors Pending Further Investigation In Reply Refer To: June 6, 1988 WGS-Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 88.10 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.