16 



volume is necessary. A simple control system designed for the Model Basin installation pro- 

 vides two-point control of the water level in the sight tube. Electrical contacts made between 

 the water and metal electrodes fused into the sight tube actuate, through relays, a motor- 

 driven rheostat differentially controlling the fields of the two motors driving the inlet and out- 

 let pumps. The servo system has not been tested at this writing so that no experimental data 

 concerning the adjustments required for satisfactory operation are available, but no extreme 

 difficulties in this regard are anticipated. 



SUM MR Y 



The air-content meter described in this report determines the concentration of dissolved 

 air in water by measuring the air pressure which is in equilibrium with the water. In this way 

 the meter can be used to indicate directly the degree of saturation of the water, relative to 

 any arbitrary air pressure. If it is desired to express the air content in terms of volume or 

 weight concentration, these quantities can be calculated from the measured equilibrium pres- 

 sure by using well-known tables. 



The meter operates continuously and can be used to provide a continuous indication of 

 air content. However, the meter is limited in the rapidity with which it can follow a change 

 in air content. For the present design, the response time is about 15 minutes but, by a meth- 

 od of compensation discussed in the text, it may be reduced effectually to about 5 minutes. 

 This response is obtained with the sample water flowing at 600 cubic centimeters per minute. 

 For applications where faster response is necessary, modifications in design have been sug- 

 gested. 



Comparison of the indications given by the continuously operating meter with those of 

 the more conventional Van Slyke apparatus show good agreement over the restricted range of 

 air content considered. 



