In Reply Refer To: November 20, 1991 Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 92.04 Subject: EQUIPMENT--Current Meters for Measurement of Flow During Ice Conditions There is considerable confusion about the correct current meter to use for discharge measurements when ice is present. The purpose of this memorandum is to provide guidelines on this subject. Vane-type meters were developed by WRD in an attempt to find a meter rotor that did not fill with slush ice as open metal cups tend to do. Several vane-type meters, with slightly different vane and yoke configurations, have been used. While these meters are effective at shedding slush, their low-velocity performance is poor. In 1988 these meters were discontinued from the Hydrologic Instrumentation Facility (HIF) warehouse. Office of Surface Water Technical Memorandum No. 88.18 stated that the preferred metering equipment for ice conditions is a type AA current meter built with a Water Survey of Canada (WSC) winter- style yoke with a polymer rotor. The WSC winter-style yoke had proven itself for many years with the conventional metal-cup rotor. The polymer rotor had gained popularity with field personnel for making discharge measurements during ice conditions because it does not fill with slush as does the metal-cup rotor, and because ice does not easily adhere to the polymer material. Office of Surface Water Technical Memorandum No. 90.01 established policy that meters with polymer rotors, including those for measuring during ice conditions, should not be used because of inaccuracies in measured velocities where the flow is noticeably turbulent. This policy has now been revised, but only for making measurements during ice conditions. The superior ability of the polymer rotor to shed slush ice and retard freezing is considered to be more important than turbulent-flow considerations on ice- covered streams. This memorandum revises policy to permit the use of polymer rotors in winter-style AA meters for conditions where slush ice is present. The winter-style AA meters with metal-cup rotors are preferred for slush-free measurements during ice conditions. Regular AA meters (with metal-cup rotors) are also acceptable for slush-free conditions, if cutting the required larger holes through the ice is not a problem. Winter-style meters with both polymer and metal rotors are available from the HIF. Winter rods in both 1/2- and 1-inch diameters are available for suspending this style of meter. Where cable suspension is required, tilting harness assemblies for 30- and 50-pound standard sounding weights are available. These harness assemblies are designed to pass the weight and meter through an 8-inch diameter hole. Under-ice video taping of various types of sounding weights by WSC during the winter of 1991 shows this weight-meter assembly to perform acceptably. All winter-style meters must be individually rated with the suspension device that will be used with it. Winter-style meters and related equipment are available from the HIF, which will arrange for individual ratings. The following are appropriate stock numbers and descriptions Meter, Winter-Style, Metal Rotor, No Rating 1104001 Meter, Winter-Style, Polymer Rotor, No Rating 1104002 Rod, 1/2-inch, Extension Section 2102005 Rod, 1/2-inch, Lower Section for Winter Meters Only 2102013 Rod, 1-inch, Extension Section for Winter Meters Only 2103002 Rod, 1-inch, Lower Section for Winter Meters Only 2103003 Foot, fits 1/2-inch or 1-inch rods 2103001 Harness, Tilting, for 30 lb. weight 3305001 Harness, Tilting, for 50 lb. weight 3305002 For special applications, please call Russ Wagner at 494-1580 or (601) 688-1580, and for special ratings call Bill Kaehrle at 494-1526 or (601) 688-1526. The Office of Surface Water is currently engaged in an extensive review and testing of all available current meters. This study may provide information on better equipment for all field conditions, including meters for making measurements during ice conditions. However, there is no certainty that a better meter will be found and stocked in the next several years. The above recommendations provide for use of consistent equipment until improved instrumentation is obtained. Charles W. Boning Chief, Office of Surface Water WRD Distribution: A, B, PO, FO This memorandum supersedes parts of Office of Surface Water Technical Memorandum 90.01.