15 



indicated above that the extent of exposure provided by the vertical falling film is relatively 

 unimportant. 



The connection of the sight tube to the side of the chamber instead of to the top 

 eliminates the need for the spray baffle. 



The internal core tapers to a smaller diameter immediately below the intended level 

 of the liquid surface. The resulting lowering of the velocity of outflow facilitates the rapid 

 return of entrained bubbles to the surface. The lower bulbous portion is retained as an 

 additional precaution. 



A stopcock connected to the upper part of the chamber is useful for venting the gas 

 space for the purpose of establishing nearly the equilibrium pressure before starting. This 

 is desirable because, even with the method of compensation described above, the instrument 

 begins to give accurate indications only after the equilibrium condition has been approximated 

 and the time required for this is small if the starting condition roughly approximates the 

 equilibrium condition. An additional reason for the provision of such a means for venting is 

 the possibility that some relatively insoluble gas might be carried into the gas space in un- 

 dissolved state and accumulate there, thus producing an erroneously high reading for a con- 

 siderable period thereafter. This, of course, is hardly possible where the body of liquid is 

 exposed only to atmospheric air, as in the present application. 



From the observed performance of the arrangements tested, e.g.. Runs 7 and 8, the 

 following operating characteristics may be expected for the meter chamber designed accord- 

 ing to the sketch. Figure 4c: 



Flow rate: 600 cc/min with 3 No. 4 nozzles 



Air-space volume: 110 cc 



Mixing efficiency: 0.75 



Response time (Nitrogen, 20°C): 15 min. (uncompensated) 



These characteristics are suitable for indicating the air content of the water used in 

 a hydrodynamic facility such as the flow facility or a large water tunnel. If a shorter response 

 time is required, or if the presence of undissolved air in the inflow requires a higher value 

 of the mixing efficiency, a different compromise in design is desirable. Thus, an increase in 

 the flow rate will reduce the response time, though at some detriment to mixing efficiency, 

 whereas the use of finer spray nozzles might improve the mixing efficiency but require a 

 higher nozzle pressure. Similarly, a reduction in the dimensions of the meter chamber would 

 reduce the response time but with attendant difficulties associated with control of the flow 

 rate and gas-space volume. It appears that no absolute limit exists with regard to an approach 

 to a mixing efficiency of unity or increase in 'the rapidity of response but that practical con- 

 siderations such as those indicated will dictate the optimum compromise for a given applica- 

 tion. 



For practical use, it is desirable that the meter provide a continuous indication while 

 unattended. For this purpose a servomechanism arranged to maintain constant gas-space 



