through the bottom screen with the spray gun. 

 Two or three short blasts are usually sufficient. 

 There is no back blast out of the funnel or 

 disturbance to the water in the jar because 

 the screen itself dissipates the force of the 

 spray markedly. The adjustment of the spray 

 gun is not particularly critical, but the instru- 

 ment seems most effective when the atomizer 

 is set to emit a relatively heavy or "wet" 

 spray under moderate pressure. The pressure 

 for the gun is set by the valve at the outlet 

 of the vessel's compressed air system to which 

 a hose from the gun is attached. 



The spray gun is also used to clean the main 

 filtering funnels when they become heavily 

 clogged with phytoplankton. This has to be 

 done at least every 3 or 4 hours and in actual 

 practice it is usually done at intervals of 1 or 

 2 hours during convenient breaks between 

 sampling sequences. Water entering the fun- 

 nels forms a rapidly swirling vortex near 

 the lower edge of the screened windows when 

 the funnels are clean. Over a period of a 

 few hours phytoplankton clogs the screens 

 progressively from the bottom, raising the 

 level of the vortex. A funnel is always cleaned 

 before the vortex level rises to two-thirds the 

 height of the screened windows. 



To be cleaned, a funnel is uncoupled from 

 the double-throw valve and removed from 

 the trash can. The screens are gone over 

 thoroughly from the outside with the spray 

 gun while the funnel is held over a drain 

 trough. Spray-gun pressure is increased con- 

 siderably for this operation. After spraying is 

 completed, loose, flocculent clumps of phyto- 

 plankton remaining on the inner surfaces of 

 the screens are washed down from the inside 

 with a moderately forceful stream of water. 

 On some occasions the phytoplankton was 

 washed directly into jars for later inspection. 



It takes 10 or 15 minutes to clean each 

 filter in the manner described above, thus 

 making it impossible to clean them after 

 every 6.5-minute sampling interval. For 

 present purposes such frequent cleaning is 

 not necessary, but should it be desirable for 

 future operations, a quick-cleaning mechanism 

 could undoubtedly be incorporated into an 

 improved model of the filtering unit. 



PERFORMANCE 



With approximately 100 feet of hose out the 

 collector tows at a depth of 5 or 6 meters. 

 This was determined early in the fall of 1961 

 by signals telemetered from a Bourns pres- 

 sure potentimeter mounted on the lower hose- 

 cable terminal assembly. The device was 

 removed because of frequent malfunction, but 

 since vessel speed (9 knots) and length of hose 

 out (approximately 100 feet) were kept con- 

 stant during the later surveys, it is assumed 

 that all samples were collected at the same 

 depth. It should be possible to achieve greater 

 depth in the future by increasing the depressing 

 force and length of the hose. 



Water is delivered to the filtering unit at 

 the rate of 9215 liters per minute which is 

 about 16 percent greater than the rate at which 

 water would freely enter a 3/4-inch diameter 

 orifice moving through the sea at a speed of 

 9 knots. The free flow rate of any orifice is 



TT r^d k 



■ liters per minute 



where r is the radius of the orifice, d the 

 number of feet traveled per minute at speed x, 

 and k the factor for conversion from cubic 

 feet to liters. Since the pump orifice is 

 three-quarters of an inch in diameter and the 

 cruising speed of the vessel is 9 knots 



2 

 3.1416 X 0.0312 X 912 x 28.32 = 78.95 Uters 

 per minute • 



Thus there should be a field of suction ahead 

 of the collecting orifice, and the diameter of 

 this field would be the diameter of the core 

 of water being sampled. 



Tests carried out on the collector in a 

 laboratory trough with a flow of 3 knots 

 suggests that the diameter of the core is 

 well under 2 inches at a speed of 9 knots. 

 Dyes and particulate matter released in the 

 tT'rough clearly defined a bulbous zone about 2 

 inches in diameter ahead of the orifice. Partic- 

 ulate matter was instantly pulled into the pump 

 if it drifted into this zone. If it missed the zone, 

 it drifted on past the collector undisturbed. 



13 



