same offset as for CS-2, to 0.05 knots or less. 

 Deviation of the data points from the smooth 

 curve is relatively large and is attributable to 

 the close proximity of the signal pickup to one 

 of the rotor plates . It was also noted during 

 the response tests that the rotation rate of 

 ST-2 decayed much more gradually than CS-2. The 

 k- tier model (ST-3) is significantly less effi- 

 cient than CS-2. At 0.35 knots the output of 

 ST-3 is 13$ below CS-2 and at 0.1 knots it is 

 down 50$. This seems to indicate that retarding 

 fluid stress on the drag area (tier separators) 

 of the standard diameter rotors is significant at 

 speeds less than about 1 knot. 



The calibration curve for rotor ST-k (double 

 height) corresponds closely to CS-2. However, 

 the longer rotor is consistently more efficient 

 by about 10$ above 0.1 knots and less efficient 

 as the threshold is approached. This is consis- 

 tent with the slight reduction in drag area com- 

 pared to torque area; the reverse of the 4- tier 

 rotor. 



SURFACE ROUGHNESS 



One of the most pertinent and least investi- 

 gated aspects of current measurement with rotor 

 or impeller devices is the degree to which meter 

 performance is affected by change in surface 

 roughness, especially by marine fouling. The 

 nature of biological fouling precludes very def- 

 inite or quantitative answers but orders of mag- 

 nitude are important. Prior to the experiments 

 in July a production meter, HT-1, of the same 

 design as CS-2, was suspended in San Diego Bay 

 for about k weeks . It accumulated a very even 

 coat less than l/8-inch thick of natural fouling 

 that was left undisturbed for part of the cali- 

 bration experiments. Fig. 9 is a picture of the 

 unit after being in the fresh water tank over- 

 night. Much of the fouling had deteriorated and 

 sloughed off the vertical surfaces but the origi- 

 nal fouling coat can still be seen on the hori- 

 zontal plates. 



The dashed curve of Fig. 10 is for HT-1 with 

 the light coat of fouling. Unfortunately, the 

 bearings of HT-1 apparently were affected by 

 fouling or corrosion so the calibration curve 

 for the rotor with cleaned surfaces deviated 

 markedly from the CS-2 curve below 0.15 knots. 

 From these 2 curves it is found that the thin 

 coat of fouling caused a kofy reduction in rotor 

 efficiency at 0.25 knots. Perhaps more interest- 

 ing is the decreased slope of the curve in the 

 low speed range, indicating that the fouled rotor 

 operated more efficiently at low speeds and had 

 a lower threshold than after cleaning. 



Also shown in Fig. 10 are calibration results 

 for CS-2 after the rotor was lightly coated with 

 petroleum jelly. Although this clearly improved 

 rotor efficiency, the increase was only 5$ at 

 0.1 knots and became less significant as speed 

 was increased. Petroleum jelly was selected to 



Fig. 9- Rotor unit HT-1 with coat of biological 

 fouling present during calibration tests 

 in July 1962. 



simulate an anti-fouling aerosol coating that has 

 proven moderately successful. 



RESPONSE 



The dynamic response of an instrument in most 

 cases is as important as the steady state output 

 and much more difficult to evaluate experimentally. 

 The necessity for a quantitative knowledge of 

 rotor response to shifts in mean flow is obvious; 

 more subtle is the probable dependence of mean 

 output on the spectrum of turbulent flow. 



Three kinds of still water acceleration- 

 deceleration experiments so far have been 

 attempted: (l) approximate step change from stop 

 or low speed to higher speed, (2) approximate 

 step change from a steady speed to stop and (3) 

 harmonic cycling. It is apparent that techniques 

 (2) and (3) involve relative movement of the 

 rotor through its own wake each time a decelera- 

 tion or travel direction reversal occurs. During 



121 



