TM No. 377 



Modified Orthogonally Mounted Ducted Meter ( OMDUM II ) 



As stated previously, the electronics of OMDUM I were judged inadequate 

 because of circuit failures and unclear- trace generation. This detector was 

 therefore replaced by simple electronic circuitry, consisting of a coil and. 

 magnets. 



Design Characteristics — In the resulting OMDUM II system, small alnico 

 magnets (10 by 5 by 2 mm) weighing about 0.7 gm each are mounted in the impel- 

 ler blades in place of the smaller and lighter iron slugs used in OMDUM I. 

 These magnets were provided by the Magnetic Seal Corp., Barrington, R. I. A 

 miniature induction coil (replacing the entire transistorized circuit in 

 OMDUM I) is installed in the pillbox. The coil (manufactured by Precision 

 Electronics Company, Marshfield, Mass.) is open-ended and wound with #36 

 insulated copper wire. Its U-shaped geometry is designed to provide maximum 

 voltage pickup as the impeller magnet moves by the open side of the potted 

 coil. The coil is positioned on its side so that one arm of the U lies 

 parallel to the tangent of the circular trajectory inscribed by the outer 

 magnet tip at the point opposite the coil housing. With this geometry, the 

 sign of the induced voltage pulse indicates the sense of the angular velocity 

 of the impeller. Figure II-7 shows the voltage spike patterns generated by 

 each of the two outputs of the OMDUM II system. The upper trace is the out- 

 put of the vertical flow sensor (w meter), and the lower trace that of the 

 horizontal flow sensor (u meter). The dominant oscillatory motion of the 

 w component is clearly evident. The motion of the u component is that of 

 the wave motion superimposed on a tidal current of about 20 cm sec~l. This 

 record was obtained while the meter was at a depth of 5 meters below the 

 mean wave trough level. At this depth much of the surface wind wave motion 

 is filtered out. The gentle (7-8 second period) oscillations in velocity 

 are mainly caused by swell, which was visually observed during the period 

 of the measurements. 



This system was a modified version of the original OMDUM I. An exterior 

 modification was made by reducing the height of the side pillboxes from 

 k cm to 2 cm in an effort to reduce drag. This was possible sines the induc- 

 tion coil required much less space than the original electronics. 



The original needle bearings had become oxidized and deteriorated during 

 the preliminary tests. The Braincon Corp. of Marion, Mass., replaced them 

 with bearings used in their Savonius rotor type current meters. 'This bearing 

 consists of a hard chrome steel pin with a hemispherically polished end. The 

 pins are mounted at each end of the Delrin (Teflon) impeller shaft. Each pin 

 seats against a polished, machined flat, sapphire wafer and is supported by a 

 cylindrical quartz collar with a sideways clearance of about 0.2 mm. 



Calibration and Response Studies — The use of relatively heavy magnets 

 in lieu of the thin iron slugs was expected to increase the moment of inertia 

 of the impeller, and hence to increase proportionally the response time (see 

 equation (A-12) in appendix A). It was even more Important to ascertain the 

 effect of this modification upon the minimum or threshold velocity at which 

 the impeller would commence to spin. 



25 



