EQUIPMENT AND SUPPLIES--Polymer Current-Meter Rotors In Reply Refer To: July 11, 1989 WGS-Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 89.09 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): Mean 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: Mean 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 Water WRD Distribution: A, B, S, FO, PO