Care and Maintenance of Vertical-Axis Current Meters

In Reply Refer To: June 2, 1999

Mail Stop 415

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 requi- rement 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 buc ket 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

1. The pivot and contact-chamber cap should be removed to clean and light ly oil upper and lower bearing surfaces with current-meter oil. (Do not use "3-in-1" type oil, which becomes gummy when exposed to water.)

2. The bearing surfaces should be examined for wear or damage, especially the pivot point. The pivot should feel sharp, as opposed to rounded or dull. It should not have a burr detectable visually or to the fingernail A magnifying glass is helpful for the visual inspection.

3. The pivot should be replaced if needed and the adjustment of the cat whisker(s) checked. The cat whiskers should touch lightly to give a distinct signal and to minimize drag. Any old-style cat whiskers with beaded contacts should be removed and replaced with the currently available cat whiskers, which have no bead. The penta-count mechanism should be operating freely with no indication of binding or misalignment.

4. After replacing the contact-chamber cap, the meter should be spun to observe that it is operating properly. This should not necessarily be a timed spin test, but the meter should be observed as turning freely, coming to a smooth and gradual stop, and not rotating with a discernible wobble. If one of these conditions is observed, or some other malfunction or damage is noted, further adjustment or repair is required to ensure proper operation before the meter is used again.

5. During the day, before making a discharge measurement, the meter should be inspected for damage, fouling, and water on the bearing surfaces. (When inspecting for water, look for a milky emulsification of oil and water on the lower bearing and pivot and droplets of water in the contact chamber.) If water is found on the bearing surfaces, they should be dried and re-oiled, because the presence of water on these surfaces will affect the lubricant viscosity, significantly changing the performance of the met er. The lower bearing is probably more vulnerable to the entrance of water.

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, bot h standard and magnetic head, and pygmy current meters. These instructio ns reflect more than 25 years combined experience of individuals in the O SW Hydraulic Laboratory who have repaired and adjusted current meters to calibrate within close tolerances of the standard rating table. These in structions are intended to supplement the instructions found in TWRI, Book 8, Chapter B2, Calibration and maintenance of vertical-axis type current meters.

Inactive 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.

Record keeping

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-met er logs should include two additional pieces of information: the date of disassembly, cleaning, and inspection (including any significant observa- tions 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 informa- tion will give a higher level of current-meter traceability in which main- tenance and inspections are recorded and the bucket-wheel group is identified.

Thomas H. Yorke

Chief, Office of Surface Water

Distribution: A, B, FO, PO

Attachments