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Name: Bill
Status: educator
Grade: 9-12
Location: MA
Country: N/A
Date: 4/7/2005


Question:
Science experiment: Put a digital scale into a vacuum chamber when the scale is turned off. Turn on the chamber and evacuate the air inside. Turn on the scale so it tares and reads zero. Now open the chamber to the outside air. Does the scale read anything? Will it show a value equal to the force of the atmosphere times the area of the pan on the scale?


Replies:
No. Scales are not air-tight. If they were, their readings would change with atmospheric pressure. Anyway, if the scale were air-tight, the air behind the pan would remain when the chamber was evacuated.

Tim Mooney


That scale will continue to read zero. The air gets under the pan and pushes up exactly as much as it pushes down, so it makes no change.

To make your scale read (atmosphere X pan_area) as you mention, suppose your pan sits on a box with the same area. (Practically all scales have a bottom part under the pan.) You would need to seal all the way around the rim of the pan with an air-tight rubber membrane or bellows. The bottom of this bellows would seal to the main box of the scale, which in turn would need to have no leaks. Then the added air cannot get between the pan and the chassis, and the air-pressure outside would be measured, squeezing the pan on top and the bottom of the box towards each other.

If there is no seal, the air gets in and pushes them apart with the same pressure and force, causing complete cancellation.

Jim Swenson


A pan scale will not show a noticeable reading. When air pressure increases above the pan, it also increases below the pan.

If you could somehow maintain the vacuum below the scale, you would see a significant reading. Most would incorrectly call this suction pulling the pan down. It is really air weight pushing the pan down. I do not know of any workable way to maintain the vacuum below the pan.

Ken Mellendorf
Math, Science, Engineering
Illinois Central College



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