J. Arthur Campbell and J. Leland Hollenberg
Portable Down-Draft Table for Lecture Experiments
Harvey Mudd College Claremont, California
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Many classroom experiments reauire venting of noxious products. Yet few rooms have hoods, and special installations have been expensive in the past. A readily movable down-draft table, which was designed for use under a wide variety of studio conditions in CHEM Study motion pictures, should be equally useful in classrooms. The down-draft table was built by a conventional sheet metal shop at a total cost of about $230.00. A 6-in. squirrel-cage type blower was chosen to assure a good flow of air through the 25-ft length of 6-in. diameter flexible exhaust duct. A shorter duct distance to a window or other vent exhaust opening would produce even better ventilation.
about 45' angles inside the table make a funnel-like entrance to the blower for smooth airflow. These baffles are not essential. In fact, when very bright lights arc used, it is better to remove them to avoid excessive heating. The removable grill was made of expanded metal with diamond-shaped holes 3 j 4in. X 1 'A in. High air velocities can be achieved by placing pieces of glass or cardboard (depending on the lighting desired) on top of or under the grill. This can provide a concentrated draft at a given portion of the table top. The effectiveness of the draft can be seen by comparing Figure 2 (blower off) and Figure 3 (blower on). I n the absence of the down-draft, ammonium chloride smoke forms as ammonia diffuses toward and into the 'hydrogen chloride beaker. With the down-draft operating, almost no smoke forms since the volatiles are efficiently removed as soon as they escape from each beaker. We recommend that the exhaust duct be let out one end of the table. This makes both front and back identical, and one can place the exhaust to the demonstrator's right or left, thus fitting any classroom arrangement. An added feature is a folding or removable backdrop-white on one side and black on the other By swivel-mounting the backdrop, the demonstrator can turn toward the audience the side which prolides the better background for the lighting and experimental setup.
Figure 1.
Schematic construction o f down-draft table
The general construction is shown in Figure 1. Four swivel sockets on movable bases, for photoflood reflector lamps, were provided. The use of lighting from below is especially valuable when bubbles or other small details are to be viewed. Two glass baffles mounted a t
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Figure 2.
Blower off.
Figure 3.
Blower on.
