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Mass Measurement and Buoyancy
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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Update: June 2012
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