Calibration and Quality Assurance of Mag-Head Pygmy Meters 

In Refer Reply To:			July 29, 2003  
Mail Stop 415 


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.   


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. 


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 


Stephen F. Blanchard 
Chief, Office of Surface Water 

Distribution Code:  A, B, DC, CD, District SW Specialists, OSW Staff All 

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