June 2, 1999
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 99.06
Subject: Care and Maintenance of Vertical-Axis Current Meters
The Office of Surface Water (OSW) began the Meter-Exchange Program in 1988 aimed at evaluating the reliability of current meters used in the field. Large percentages of both AA and pygmy meters failed to calibrate within the long-established tolerances for the standard rating tables. The OSW and Hydrologic Instrumentation Facility are currently addressing problems associated with the errors. Most of the errors are thought to result from differences in the performance of meter components, particularly bucket wheels, manufactured in the last 10 years, or more. Part of the errors, however, is attributable to maintenance, repair, and adjustment problems.
This memorandum is to clarify and establish Water Resources Division policy in regard to the care and maintenance of vertical-axis current meters, principally the Price AA and pygmy current meters. It also establishes new requirements for current-meter logs. Existing policy is set forth in TWRI Book 8, Chapter B2, p. 11-12, WSP 2175, Vol. 1, p. 93-94 and OSW Technical Memorandum No. 89.07. This present memorandum reduces the requirement that a meter be disassembled and cleaned after each measurement as stated in the TWRI. It adheres, however, to the policy stated in the WSP paper that a meter shall be cleaned after each day of use (or after each measurement in water heavily laden with sediment). The policies on spin tests appearing in Technical Memorandum No. 89.07 are unchanged, but the spin-test logs are expanded to provide more information to identify bucket wheels and concerning major repairs.
A key concept in maintaining the accuracy with which a current meter measures water velocity is keeping the meter in the same good condition as when it was obtained from the manufacturer or was recalibrated. Cleaning, adjustment, and repair maintain this condition. The following procedures should be followed:
After a day of use in the field
After each field trip
The meters used should be completely disassembled, inspected, and cleaned. Repairs should be made if needed. Attachments 1a, 1b, and 2 provide instructions on how to disassemble, inspect and adjust Price type AA, both standard and magnetic head, and pygmy current meters. These instructions reflect more than 25 years combined experience of individuals in the OSW Hydraulic Laboratory who have repaired and adjusted current meters to calibrate within close tolerances of the standard rating table. These instructions are intended to supplement the instructions found in TWRI, Book 8, Chapter B2, Calibration and maintenance of vertical-axis type current meters.
Prior to being stored, a meter should have been disassembled, inspected, and cleaned as described above. If the period of storage is less than 1 year, the meter may be used without further maintenance if an inspection and a spin test indicate it is operating properly. If the meter has been in storage longer than 1 year--or an indeterminate period--cleaning and adjustment (as would be done after a day of use) should be done before using the meter.
Since 1989 current-meter logs have been maintained to record spin-test data and repairs. (See OSW Technical Memorandum No. 89.07.) Consistent with modern quality-assurance/quality-control requirements, the current-meter logs should include two additional pieces of information: the date of disassembly, cleaning, and inspection (including any significant observations or actions) and the designation stamped on the bucket wheel. This designation is important because it has been found that some groups of bucket wheels have different performance characteristics than do others. Since about 1967, the bucket-wheel designation has been in the form of S-89. The S indicates that the bucket wheel is for a standard rating table. The 89 are the last two digits of the year in which the contract was let for procurement of the bucket wheel (or the meter with which it was assembled). Please note that this designation appears under the hub nut on pygmy current meters and cannot be seen unless the meter is disassembled. If a bucket wheel has a letter other than S, or no letter designation, please consult the OSW to determine if it may be used with a standard rating table.
Attachment 3 is a suggested format for the current-meter log to include the new information requirements. A district may use this form or a version modified to meet local needs. A district may also use the old form, provided the essential information is recorded. The new meter-log information will give a higher level of current-meter traceability in which maintenance and inspections are recorded and the bucket-wheel group is identified.