Calibration and Quality Assurance of Mag-Head Pygmy Meters
In Refer Reply To: July 29, 2003
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 2003.07
Subject: Calibration and Quality Assurance of Mag-Head Pygmy Meters
This memorandum sets forth OSW policy for use of mag-head pygmy meters.
Because of concern that use of mag-head pygmy meters may bias velocity
measurements, the policy is that mag-head pygmy meters may be used only if
they have been rated at the Hydrologic Instrumentation Facility (HIF)
Hydraulics Lab tow tank.
Background:
In order to facilitate the utilization of electronic computational devices
for current-meter discharge measurements, some vendors have developed kit
assemblies for the pygmy meter that convert the cat-whisker contact
chambers into magnetic encoders (mag-heads). However, preliminary testing
in the HIF Hydraulic Laboratory indicates that the JBS kit-installed
mag-heads might change the performance of the meter. These preliminary
test results indicate a measurement bias when compared to the standard
rating and larger measurement variability than cat-whisker pygmy meters.
The test results suggest that the velocity estimates using the mag-head
pygmy meters and the standard rating are consistently 0.5 to 0.7 percent
too high for high-range velocities. This bias is significant when
compared to the rating limits of +/- 2 percent. The preliminary results
also show a one percent larger variability for velocities less than 1
ft/s. The larger variability may be the result of the magnetic switches
having more manufacturing variation than the cat whisker. The bias and
larger measurement variability are a source of concern where measurement
conditions would ordinarily yield measurements rated "good" or
"excellent". Such conditions include steady velocities and smooth channel
beds. Measurements made in poorer conditions of irregular beds and
unsteady flows usually have larger uncertainties and flow disturbances may
compensate or at least over shadow this bias.
The preferred approach to addressing this apparent bias would be to
develop a standard "mag-head pygmy meter rating". Additional tow-tank
testing and data analysis is planned for the near future that will
determine if this approach is needed. In the interim, individual mag-head
current-meter ratings should be developed and used. The Hydrologic
Instrumentation Facility (HIF) already sells pygmy meters converted to
mag-heads, and provides individual ratings with each meter. Some
districts are purchasing the kits directly from external vendors and are
doing the installation themselves, however, they are not sending the units
to the Hydraulics Lab for calibration in the tow tank. This omission
could call into question streamflow data collected with these instruments.
Conclusion:
Pending further guidance, only mag-head pygmy meters that have been rated
at the Hydraulics Lab tow tank should be used. Districts thus have the
option of (1) obtaining a mag-head pygmy meter with its individual rating
from the HIF, or (2) sending their self installed mag-head pygmy meters to
the Hydraulics Lab tow tank for calibration and provision of an individual
rating.
/signed/
Stephen F. Blanchard
Chief, Office of Surface Water
Distribution Code: A, B, DC, CD, District SW Specialists, OSW Staff All
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Steve Blanchard Chief, Office of Surface Water
U.S. Geological Survey Phone: 703-648-5629
415 National Center Fax: 703-648-5722
Reston, VA 20192 Cell: 571-216-1423
sfblanch@usgs.gov
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