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Manometers and
barometers
A manometer is used to measure gas pressures.
The U tube contains a liquid. One end is
connected to the gas supply. The height
difference between the liquid in the two
arms is a measure of the gas pressure.
U tube
gas supply
If a manometer is used to measure large
pressure differences the liquid is usually
mercury. Mercury is 13.6 times more dense than
water, so the pressure cannot support as much
of it. The height difference is 13.6 times smaller
using mercury instead of water, and so a smaller
manometer can be used.
A mercury barometer can also be used to
measure pressure. A tube is completely filled
with mercury so that all the air is removed. It is
then turned upside down and placed in a tank
of mercury.
liquid such as
water or mercury
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mercury
barometer
1m
vacuum
(empty space)
Atmospheric pressure pushes down onto the
mercury in the tank, and stops the mercury
falling out of the tube. There is a column of
mercury left in the tube with a vacuum above
it. If there is any change in the atmospheric
pressure the height of the mercury column in
the tube will change. At a pressure of one
atmosphere or 100 kPa there is 76 cm of
mercury in the tube.
?
pressure
reading
in mm of
mercury
mercury
1 What is a manometer?
2 If you are measuring the gas pressure at the gas taps in the lab, which liquid
would you use in the manometer? (Hint: There is likely to be only a small
difference between air and gas pressure.) Explain your answer.
3 Why would you use a mercury barometer rather than a water one to
measure atmospheric pressure?
4 a What is the height of mercury in the tube at a pressure of one atmosphere?
b How much less dense is water compared with mercury?
c What would the height of water be at one atmosphere? (Hint: Multiply
your last two answers together.)
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Manometers and barometers
(continued)
The barometer on the previous page is too large to have on the wall at home or in the
office. An aneroid barometer is much smaller than a mercury barometer, but is less
accurate. Aneroid barometers are easier to read and much less liable to damage.
740
scale reading in
mm of mercury
760
pointer
spring
720
780
spring
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air pressure
a sealed box with
most of the air removed
Aneroid barometer.
An altimeter is a special type of aneroid barometer used by pilots and mountaineers.
It has a scale which gives the height above sea level. This is calculated from the
difference between air pressure where the aeroplane or mountaineer is, and the
normal sea level pressure (1 atmosphere).
?
5 Construct a table to compare three ways of measuring gas or atmospheric
pressure. Include advantages and disadvantages in your table.
6 Which type of barometer would a helicopter pilot use?
7 Try to find out what aneroid means. Explain why it is called an
aneroid barometer.
S
knowledge, literacy, numeracy, research
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