









o 

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A 



mplifier 







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"i 1 

 Oscillograph 



Figure 3 - Bridge Circuit Used to Study the Stability 

 of a Hot Wire in Water 



quality insulating material* to cover all dissimilar metals. It was almost impossible to effec- 

 tively coat the plated portions of the wires if the old technique of mounting the wire was used. 

 Ideally the same metal should be used throughout and the wires should be welded in place. At 

 present, however, the technique for welding tungsten wire to tungsten has not been developed. 

 It may prove worthwhile to reconsider other wire materials which would be easier to weld. For 

 example, platinum or certain nickel alloys might be suitable. 



If all the above mentioned precautions are taken it is possible to obtain a wire which 

 is stable for a reasonably long time in well-filtered water. In ordinary water found in test fac- 

 ilities, however, an instability develops as the wire accumulates dirt and hair-like fibers from 

 the water. Dr. Hubbard has found that the film formation is slower in highly turbulent water 

 and is retarded by shaping the wires in a V.^ In slowly moving streams as in the TMB channel 

 the angle of the wire did not seem to delay the dirt accumulation. Part of the dirt could be 

 swept away by a small stream of water from a syringe, but none of these methods were adequate 

 to maintain a wire calibration which could be relied upon for a practical length of time. It was 

 still necessary to remove the wire at frequent intervals for a thorough cleaning. There had 

 been, however, a large gain in stability over the direct-current wire used in the early work. 



DYNAMIC CALIBRATION TECHNIQUES 



As it is usually not practical to remove the wire for cleaning very often, it may be nec- 

 essary to develop a dynamic method of calibration. For example, it may be feasible to super- 

 impose on the flow a known turbulence field immediately before or after each reading. It is 

 important, however, that the imposed turbulence be uncorrelated with the field of turbulence 



♦The only satisfactory insulating material on hand at TMB for this purpose was liquid Neoprene which requires 

 several hours to dry. Faster drying materials, such as Glyptol or Tygon, were not sufficiently waterproof. 



