Marine Biology (2+1)

OPERATION PROCEDURE OF FOERST CENTRIFUGE

The centrifuge which rests on a rubber mat is set up along with the funnel as shown. After the instrument is switched on, a suitable operating speed is first selected with the help of the graduated rheostat knob. Then the stopper of the sample bottle is removed and with a quick motion, it is inverted into a funnel with the mouth about half way into it. The bottle is then slipped in place in middle ring and the upper ring is brought down over the bottom of the bottle in supporting position. As the inflow of the sample into funnel depends upon the outflow, which is facilitated by a pinch-cock, the delivery to centrifuge is automatic, and the overflow from the funnel is averted.

After the sample bottle is emptied, the centrifuge is stopped. Now the delivery tube is removed from the centrifuge and with back forth rotary motion, the cap is removed from the top of the centrifuge, thereby exposing the interior of the bowl. The upper rim of the rotating cup in bowl is held tight and is removed by clockwise turning and unscrewing. The cup may contain small amount of water and the plankton gets accumulated into the angle between the floor and sides of the cup. The plankton is then made loose by a piece of soft rubber tubing and is then transferred carefully to a container. The bowl may also be washed subsequently with one ml of distilled water for removing the adhered plankton, if any. The bowl after repeated washings with distilled water, may be replaced in the centrifuge for another run.

Though a number of advantages like portability, compactness, small weight and capture of all fractions of plankton are with this type of centrifuge, it has certain disadvantages too. That is the Forest centrifuge needs electricity for its use and collects all particulate matter including detritus thus making the plankton sample unsuitable for gravimetric or volumetric studies.