Ttie method was developed and the instrument constructed for use with the Taylor 

 Model Basin Flow Facility in the study of cavitation phenomena under the Bureau of Ships 

 Fundamental Hydrodynamics Research Program (NS715-102). 



Operation of the instrument is based on certain well-known principles concerning the 

 solution of gases in liquids. The first of tiiese principles relates to the condition for equili- 

 brium between the concentration of dissolved gas in solution and the pressure exerted by the 

 gas on the exposed surface of the liquid. At a given temperature, the concentration of dis- 

 solved gas will be, at equilibrium, a function of the pressure exerted by that gas on the ex- 

 posed surface of the liquid. If the pressure of the gas on the exposed surface is made higher 

 than the equilibrium pressure corresponding to the existing concentration of dissolved gas, 

 more gas will go into solution; if the pressure is reduced below the equilibrium value, gas will 

 be evolved from the liquid. 



For most gases and liquids at moderate pressures, the relation between the concentra- 

 tion of dissolved gas and the equilibrium pressure is a simple proportionality. This generali- 

 zation, known as Henry's Lav/,^ while not essential to the operation of the instrument des- 

 cribed here, permits a simplified description of its behavior and will be assumed in the subse- 

 quent discussion. It will likewise be assumed that the perfect gas law applies. Henry's Law 

 states, then, that at equilibrium, the mass of gas dissolved in a given volume of the liquid is 

 proportional to the pressure exerted by that gas on the surface of the liquid. Since the mass 

 of free gas contained in a fixed volume at a given temperature is, by the simple gas law, like- 

 wise proportional to the pressure, the solubility of a gas in a liquid may be expressed as a 

 solubility coefficient B defined as the ratio, at equilibrium, of the amounts (i.e., the mass) of 

 gas contained in equal volumes below and above the surface. The ratio (3 for each combina- 

 tion of a gas and a liquid depends upon the temperature. Table 1 gives values of the solubi- 

 lity coefficients for nitrogen and for oxygen in water.* 



The rapidity of establishment of equilibrium between the concentration of a gas in solu- 

 tion and the pressure exerted by the gas on the surface of the liquid is limited by the rates of 

 diffusion obtaining on both sides of the gas-liquid boundary. For oxygen and nitrogen in the 

 usual atmospheric proportions in contact with water, the imposed limitation ordinarily arises 

 principally in the rates of diffusion of the dissolved gases within the liquid. Since the mole- 

 cular diffusivity of oxygen and nitrogen in water is very low for most practical purposes,** 

 aeration and de-aeration of water are ordinarily effected by some combination of spraying, 

 bubbling, agitation, and other means for increasing the area of contact and for accelerating 

 mixing within the liquid.^ 



*The values shown in the table were derived from data in the "Handbook of Chemistry and Physics" which 

 gives values of an absorption coefficient a whose definition is such that /3/Ot =T/T^. Here T is the absolute 

 temperature at which the values of Of and j3 apply and T^ is the absolute temperature corresponding to 0°C. 



**The observed values of the molecular diffusivity for oxygen and nitrogen in water are of the order of 2*10 



2 , 

 cm /sec. 



