MINIATURE CIRCULATING SYSTEMS FOR SMALL LABORATORY AQUARIUMS 45 



for the water of that one to depart from 

 the characteristics of the rest since it is 

 moving freely from one aquarium to the 

 other and is beino- continually and effec- 

 tively mixed. 



An especially useful arrangement for 

 some purposes is one in which the flow 

 between aquariums may be continuously 

 varied from maximum in one direction 

 through zero flow to maximum flow in the 

 opposite direction. This may be readily 

 accomplished by the adjustment of four 

 valves while the pump runs continuously 

 in one direction at constant speed. The 

 details of the arrangement of these valves 

 are shown diagrammatically in figure 6. 



Figure 7 is a photograph of such a de- 

 vice. In operation the action is as follows. 

 With valves A2 and Bl closed and the 

 others open, the flow is out through pipe 

 A and returns through pipe B, as indi- 

 cated by the arrows, at maximum flow. If 



these valves are reversed so that Al and 

 B2 are closed and the others open, the 

 floAv through pipes A and B is reversed, 

 although the flow through the pump re- 

 mains as indicated by the arrow on it. To 

 pass uniformly from the first position, as 

 shown in figure 6, through a state of no 

 flow to the reverse, either valve A2oy valve 

 Bl can be gradually opened. This reduces 

 the speed of water movement because of 

 "back leakage." After one of them has 

 been opened fully, the opening of the other 

 can further retard flow. When it, too, has 

 been fully opened, that is, with all valves 

 fully open, there should be no flow 

 through pipes A and B, as there is as much 

 pump pressure in one branch of both A 

 and B pipes as in the other. Then by be- 

 ginning to close either valve Al or valve 

 B2 the flow begins to move in the opposite 

 direction. When these two are fully 



Figure 6. — IMagram of piping and valves for continuously variable flow from maximum in one 

 direction through zero flov^^ to maximum in the opposite direction. 



