SEA-WATER SYSTEM IN SHELLFISH LABORATORY 



157 



the tank near the bottom — and there- 

 fore never liable to be nncovered. This 

 thermostat, set at about 60° C, operates 

 through a contactor which, when energized 

 from the thermostat, interrupts the cur- 

 rent supply to all the heaters. The sup- 

 ply can only be manually restored. 



This tank, with a volume of about 150 

 liters, produces about 200 liters per hour 

 of sea water at 22° C. from water at 2° to 

 4° C, and although there are regular fluc- 

 tuations in the temperature of the water, 

 as it leaves the tank, due to the switching 



of the thermostat and the time taken for 

 the heaters to warm up, these are damped 

 in the 40-liter aquariums into which water 

 is running at 10 liters per hour. A flow 

 rate of 200 liters per hour raised through 

 20° C. is about two-thirds of the maximum 

 possible with an input of 9 kilowatts. This 

 is an adequate margin for cold weather or 

 exceptionally large requirements. The 

 degree of temperature control given by 

 this system is shown in the thermograph 

 record from one of the aquariums, repro- 

 duced in figure 2. 



22 



/ 



/ 



/ 



I 



I 



I 



20^ 



18- 



I 



I 



I 



I 



I 



I 



/ 



Figure 2. — Temperature record for 5 consecutive days in a 40-liter aquarium — flow rate about 10 

 liters an hour and the incoming temperature about 5° C. before heating. 



The warmed sea water flows by gravity 

 into the tankroom where, after it has been 

 enriched with algal cultures, it is distrib- 

 uted by a polythene manifold to a series of 

 aquariums. The water is enriched, us- 

 ually with Phaeodactylum or DunalieUa, 

 because in the winter months the natural 

 flora of the water is at too low a level for 

 filter-feeding animals which are being in- 

 duced to grow and breed by raised tem- 

 peratures. 



The arrangement for continuously en- 

 riching the water is outlined in figure 3. 

 The incoming sea water enters a wooden 

 header tank through a hard-rubber ball 

 valve (Dexine Rubber and Ebonite Ltd., 

 Eochdale, Lanes. ) . This tank is at a high- 

 er level than the aquariums. Immediately 

 above the header tank is a plastic tank 

 which is filled daily with 20 liters of algal 

 culture. This has to be stirred contin- 

 uously to prevent settling of the algal 

 cells. The culture leaves the tank through 

 a tube set flush in the bottom to which is 

 attached a rubber tube, which passes 

 through a solenoid-controlled pinch clip 



(Londex Ltd., London), and then into the 

 header tank. The culture, therefore, 

 passes into the header tank when the sole- 

 noid is energized for a few seconds every 

 15 minutes^ — the length of time being ad- 

 justed so that the algal storage tank takes 

 24 hours to empty. 



The timing device (fig. 4) has been con- 

 structed from a synchronous motor (Sang- 

 amo Weston Ltd., London), with a spin- 

 dle speed of 1 revolution per hour. Four 

 brass arms with adjustable points are at- 

 tached to the spindle. A mercury tilt 

 switch (I.A.C. Ltd., London) is arranged 

 on a cradle with an adjustable arm so that 

 each of the spindle arms in turn briefly 

 touches the arm of the mercury switch and 

 gives it sufficient tilt to allow the contacts 

 to close. This opens the solenoid-con- 

 trolled pinch clip for a period, the length 

 of which is dependent on the adjustment 

 of the contacts. The turbulence in the 

 header tank provides sufficient mixing, and 

 the following counts of the density of 

 Phaeodactylum in the water entering an 



