In Reply Refer To:
June 2, 1999 Mail Stop 415
Office of Surface Water Technical Memorandum No 99.05
Subject: Development of New Standard Rating Tables for the Price Type AA and Pygmy Current Meters
This memorandum is to inform you of the development of new standard rating tables for the Price type AA and pygmy current meters. Copies of the new rating tables are attached. The Hydrologic Instrumentation Facility (HIF) plans to make laminated copies available. The HIF is also preparing to make available or install, new chips for the USGS current-meter digitizers (CMD's) that will contain the algorithm for the new rating tables. These tables should be placed in use immediately for AA and pygmy meters with cat-whisker contact chambers and for AA meters with magnetic heads.
Since 1988 surface-water technical reviewers have selected two current meters, one of each type, from the district being reviewed for calibration testing in the Office of Surface Water (OSW) Hydraulic Laboratory. Results from this Meter-Exchange Program have shown that changes from the standard ratings have occurred since the original standard rating tables for AA and pygmy meters were released in about 1970 and 1980, respectively. This conclusion is based on the calibration testing of the meters after they were disassembled, inspected, repaired, and adjusted by laboratory personnel.
Data from acceptance testing of HIF procurements of current meters and bucket wheels also show that changes from the standard rating tables have occurred. AA meters tend to run more slowly than predicted by the standard rating, and pygmy meters run too fast. Representative data from the Meter-Exchange Program and the procurement testing, weighted to reflect the estimated current composition of the populations of AA and pygmy meters, were used to develop the new rating tables.
At a velocity of 1.5 feet per second (fps) the new AA-meter standard rating equation gives a result that is 0.74 percent more than given by the old standard rating and at 8 fps, 1.4 percent more, compensating for the AA meter turning more slowly than predicted by the old standard rating.
The Meter-Exchange Program and procurement-testing data for pygmy meters show that these meters tend to run faster than predicted by the standard rating table. For the average pygmy meter, a correction is needed of 1.2 percent less at 0.75 fps, ranging to 1.6 percent less at 3.0 fps.
A valid question is whether changes from the standard ratings averaging between 1 and 2 percent are worth the trouble of making the change. A good discharge measurement is only supposed to be within plus or minus 5 percent. Other errors associated with making discharge measurements are generally of about the same magnitude or larger than these current-meter rating errors; however there is a difference. The other errors are thought to be random in nature. Random errors over time will "average out", having little effect on the stage-discharge relationship and leaving the stage-discharge relation as an unbiased estimator. The average error of a group of meters relative to the standard rating table is a systematic error, which will not "average out". This average meter error will cause the stage-discharge relationships to be in error a like amount.
Using the new rating tables, a reanalysis of 2 years' record at three gaging stations in West Virginia shows an increase in annual average discharge of 1.2 percent for a large stream, 1.0 percent for a medium-sized stream, and 0.6 percent for a small stream. The increase in daily-maximum discharges for these gages during the 2-year period ranged from 1.1 to 1.3 percent. Minimum- daily discharges ranged from a decrease of -0.8 percent to an increase of 0.5 percent. Changes in monthly-average discharges ranged from -1.5 percent for the smallest stream to 1.3 percent for the largest. None of the errors meet our customary criteria for publishing revisions and no revisions are planned.
In addition to the new standard ratings, the OSW recommends the following measures to reduce potential bias in streamflow records, resulting from current-meter error:
o Provide training to field personnel on the care, maintenance and repair of current meters (This training has already begun, with current-meter workshops being held in conjunction with district surface-water technical reviews.)
o Establish more formal procedures for meter maintenance and for recording repairs and adjustments, along with spin tests, on the current-meter logs
o Consider developing a plan for rotation of meters among personnel within a district (Rotating meters will reduce the impact of a single biased meter on individual stage-discharge relations, much in the same way that rotating field trips does.)
o Increase calibration testing in the Meter-Exchange Program to be able to detect small changes in the national biases of AA and pygmy meters (Actions involving better care of current meters should result in reducing the national bias as will use of the new standard ratings.) The increase in calibration testing is anticipated to be about 80 meters of each type per year and to extend to faster velocities than the usual maximum of 8 ft/s.
The division has accepted the bulleted recommendations above and development of new standard ratings as an interim course of action. A reassessment is planned in 2 years to see if further actions are necessary. A major factor in accepting this course of action, as opposed to going back to individual current meter ratings, is preserving the ability to make a major repair to a current meter without having it recalibrated.
Two sources of error may be considered in using current meters to measure velocity. There is error associated with the average performance of a group of current meters being different from the standard rating. The new standard rating tables will correct this error. There is also the variability of individual meters within the group. For the representative samples from which the new rating tables were prepared, there is considerable variation among individual meters. About 95 percent of the AA meters fall within 1.5 percent of the new standard rating. For pygmy meters this variability is about 2 percent.
For some applications districts may wish to avoid this additional uncertainty in meter performance. They can obtain individually rated new meters by special request from the HIF. The districts may also have meters belonging to them individually rated at the Hydraulic Laboratory. The additional expense of individual ratings, either from the HIF or the Hydraulic Laboratory, are currently $250 for a AA meter and $125 for a pygmy meter, which are the costs of calibration. A plan for modernization is being implemented in the laboratory, which may materially reduce these costs.
Individually rated new meters are usually available from stock at the HIF. The time needed to have a meter individually rated at the laboratory depends on work load at the time and is scheduled on an first-come-first-served basis. With advance notice, a meter can be calibrated within 2 weeks of being received in the laboratory.
Thomas H. Yorke
Chief, Office of Surface Water
Distribution: A, B, FO, PO
Attachments (Standard Rating Table No. 2)
AA Current Meters
Pygmy Current Meters