DISADVANTAGES 



The induced voltage is usually independent of the electrical properties of 

 the fluid. However, this is not true for all designs or conductivities. ^^ jf y^g 

 meter requires salt water (conductivity, a= 4 mho/m) for acciu'ate calibration 

 instead of fresh water (tap water a = 10-2 „iho/ni), the choice of tow tanks is 

 severely limited. Two or three orthogonally mounted meters might interfere 

 hydrodynamically with the flow, depending on the size and shape of the sensors 

 and on the distance between the electronics package and the sensors. 



DIRECTIONALITY 



A serious problem results from the use of the tube to obtain directionality. 

 The direction response is unlikely to be a cosine function for two reasons: 



1. As the meter is turned thi'ough an angle with reference to the flow, 

 the mean volumetric flow will probably not be proportional to cos G for hydrody- 

 namic reasons. 



2. As the meter is turned thi-ough the angle G, the velocity distribution no 

 longer remains symmetrical about the meter axis. 



Thiirlemann (results summarized by Shercliff^M proved that in a transverse - 

 field flowmeter with circular cross section, the voltage induced in the electrodes 

 is proportional to the mean volumetric rate of flow so long as the velocity distri- 

 bution is symmetrical about the axis of the tube. Serious errors can result from 

 flows with asymmetrical velocity distributions.^ Failure to produce a cosine 

 response would lead to the same problems in resolving the instantaneous magni- 

 tude and angle of the thi-ee-dimensional vector as was found with the ducted 

 impeller meters. 



NONCOSINE DIRECTION RESPONSE. A noncosine direction response 

 can represent a serious problem. Since instantaneous velocity is not required for 

 this application, the question is posed as to whether the filtered output will 

 indicate the steady flow correctly when given both an oscillating and steady flow. 

 The noncosine response will be assumed to be the only fault of the meter. The 

 meter will be considered perfect in other respects: linear output, perfect frequency 

 response; and no zero drift. The ability to sense flow direction will be assumed. 

 Let the meter be placed at a given depth with its axis parallel to the y-axis (fig. 9). 

 A typical flow direction would be as follows: 



Let U be the instantaneous resultant flow at angle (3 to the y axis. The 

 velocity components U^, U^, U^, and U^ are shown. Let plane surface waves be 

 traveling in a direction parallel to the U^ component and give rise to the oscillating 



17 



