the potential for improvement is partly based on this material, with 

 the exception of opinions which may have pertained to one particular 

 specimen of unknown history and which are as likely to reflect a user's 

 skills and prejudices as the quality of an instrument. 



Many of the data shown in Table III-l and III-2, mostly in the 

 manufacturer's own terms, are very much defined ad hoc, and they are dif- 

 ficult to translate into absolute terms. The "accuracy" of velocimeters 

 is usually defined as the maximum deviation of measured velocity values, 

 which have been derived according to a calibration formula similar to 

 Equation (1), from "true" values. The so-called "true" velocity in the 

 calibration tank of distilled water is calculated (Reference 1, 17, 33) 

 from pressure and temperature. This method of calibration is, however, 

 difficult due to the strong dependence of velocity on temperature and due 

 to the high demands on purity of the water. The temperature must be known, 

 for example, within 0.02° at 0°C to avoid velocity errors greater than 

 0.1 meters per second from this source alone (see Equation 1). 



B. Tabular Listing of Velocimeters 



The following, Tables III-l and III-2, give a tabular listing 

 of velocimeters followed by explanatory notes. 



28 



Arthur ZD.lUttlcJnr. 



