Laboratory hoods are often called "fume hoods" even though laboratory work rarely produces fumes. They will be called "laboratory hoods" and "chemical hoods" here. When used correctly they contain vapors, dusts, aerosols, and, indeed fumes if any are produced.
They do this by enclosing a ventilated work area on all sides except the front, and providing a front opening that is never particularly large, and which can be made quite small.
Not finished....
In the simplified drawing below, the left hood has the hood sash lowered most of the way. In the right hood the sash is raised most of the way. A hood sash is a very heavy window in general, depends on the size of course, but glides easily up and down if it's working correctly. The sash counterweight, or just "sash weight" is the reason.
If we could see through to the back it would look something like these below. Weights hanging from cable that travel in pulleys is not the only way it's done, but is extremely common in most of the hoods that are over 10 years old. "B." and "C." represent slightly different sash weight configurations. Note that in C. there is reduced likelihood of all of the counterweight being detached suddenly by one break in a cable.
Red lines are the stranded cables which are attached to the sides and/or top of the sash, run across pulleys and are attached to the sash weight. The sash weight is the counterweight which weighs almost as much as the hood sash and makes raising and lowering the sash a near-effortless operation. In this configuation the sash weight is attached at two corners. In many installations there is a single center point of attachment.
If the cable on one side breaks (below), the hood sash might hang crooked and will be difficult or impossible to raise and lower. PUT A SIGN ON IT and be very careful. Do not raise or lower it until it has been fixed.
Repeat -- If there's reason to suspect that the cable on one side has broken, do not raise or lower the sash until it has been fixed. CONTACT YOUR BUILDING DEPUTY ABOUT REPAIRS.
It's quite important to be very careful and not move the sash if a cable is worn, broken, or breaking. In the 'half broken" state, the wear and tear on the unbroken cable will be extreme, increasing the chance that it will break too, and if it breaks, the sash comes down hard. Bones have been broken, and glassware smashed and flung far. DO NOT PLACE ANY BODY PART under the hood sash if there is any question about the integrity of the counterweight system.
If the sash comes down hard onto a power cord, it is likely that it will be ruined and should be replaced immediately. Crushing of an electrical conductor changes its properties depending on the damage done and the change to its shape. Fires start in cords like this.
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November 2012 -- WTHR 248 hood. The sash weight fell and broke the bench. In the full size photo it is possible to see the little roller on the sash weight through the space between the hood back and the caulk gun, between the cut-away panel at left and the baffle bracket at the right. The panel has been cut away because (1) it has proven impossible to retrieve the downed sash weight from above, and (2) removing the entire back of the hood is extremely difficult to do safely without more than two strong people. the metal brackets at left and right of the cut-away hole have been added just now, in preparation for replacing the removed piece. |
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At the right I have attempted to draw in the weight and pulleys to represent the finished repair. Click on the photo to see the drawn in lines. They don't show up at all well in reduced version. If a separate weight were attached to each side, a break on one side would likely still allow the sash to fall, but with a bit less force since half the counterweight would still be attached.
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