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The double bucket system consists of two large vessels each with a volume of approximately 75L. The two buckets sit on a wheeled platform at equal height joined by a U-tube running beneath the two vessels. There is also a hose joint to one of the buckets with a pump system attached. This pump is able to draw water from the bottom of this bucket. This bucket is refered to as the salt water bucket; the other bucket is refered to as the fresh water bucket. To obtain stratified fluid the two buckets are filled with fresh water to the appropriate height. Stoppers are placed in the various hose/tubes to avoid mixing. In the salt water bucket, salt is added to achieve a desired density of the experiment parameter. For every gram of salt that was added to the salt water bucket, one gram of fresh ater is added to the fresh water bucket. This is done to avoid salt water rushing into the fresh water bucket when the stoppers are removed. The salt is mixed in thoroughly and once the double bucket system is in place near the tank, the stoppers are removed.
With the tank in place and stopppers removed, the pump is set in motion. The pump draws water from the salt water bucket, through the hose and into the tank. The hose lies on a floating sponge like apparatus. The purpose of this is two-fold; one, the hose always lies on the top of the surface of the water in the tank and two, the flowing water passes through the sponge in an effort to minimize disturbances of the stratified fluid.
The mechanics of the double bucket system are as follows. As the pump draws salt water from the bottom of the salt water bucket and out the hose, fresh water from the fresh water bucket will flow through the U-tube and into the salt water bucket. This reduces the relative salt concentration of the salt water and in turn it's density. Infact, for every infinitesimally small volume of salt water which is removed from the salt water bucket, half that amount in weight will be replaced with fresh water by the fresh water bucket. As this process continues the double bucket system will fill the tank with the hose gently pouring water onto the surface of the water. The water leaving the hose progressivley grows less and less dense until the both buckets are nearly empty and the density is approaching that of fresh water. When properly executed, this procedure will create a perfectly linear density profile.