UNDERstanding devices that measure PRESSURE
The open-tube manometer is another device that can be used to measure pressure. The open-tube
manometer is used to measure the pressure of a gas in a container.
The pressure of the gas is given by h (the difference in mercury levels) in units of torr or mmHg.
Atmospheric pressure pushes on the mercury from one direction, and the gas in the container pushes
from the other direction. In a manometer, since the gas in the bulb is pushing more than the
atmospheric pressure, you add the atmospheric pressure to the height difference:
Pgas > Patm
Gas pressure = atmospheric pressure + h (height of the mercury)
Pgas < Patm Gas pressure = atmospheric pressure - h (height of the mercury)
You could be asked about a closed-tube manometer. Closed-tube manometers look similar to regular
manometers except that the end that’s open to the atmospheric pressure in a regular manometer is
sealed and contains a vacuum. In these systems, the difference in mercury levels (in mmHg) is equal to
the pressure in torr.
Closed tube
Open Tube
Pgas=755mmHg + 24mmHg=779mmHg
Open Tube
Pgas=763mmHg - 35mmHg=728mmHg
How a barometer works:
The piece of lab equipment specifically designed to measure the pressure of gases is known as the
barometer. A barometer uses the height of a column of mercury to measure gas pressure in
millimeters of mercury or torr (1 mmHg = 1 torr). The mercury is pushed up the tube from the dish
until the pressure at the bottom of the tube (due to the mass of the mercury) is balanced by the
atmospheric pressure.
Interesting Note-The weight of the column of mercury is equal to the force of the atmospheric
pressure. It the column was water the column would have to be 34 feet tall.
Modern Technology and Barometric Pressure
The majority of modern weather instruments — such as weather stations — can measure barometric
pressure, but use a digital barometer that uses electrical charges to measure air pressure. This enables
them to take multiple accurate recordings of the pressure and produce more accurate weather forecasts.
Using Atmospheric Pressure to Forecast the Weather
Changes in atmospheric pressure are one of the most commonly used ways to forecast changes in the
weather because weather patterns are carried around in regions of high and low pressure. Weather
maps use lines of equal pressure called isobars to indicate areas of equal pressure.
A slowly rising atmospheric pressure, over a week or two, typically indicates settled weather that will
last a long time. A sudden drop in atmospheric pressure over a few hours often forecasts an
approaching storm, which will not last long, with heavy rain and strong winds.
Pioneering meteorologist Vice-Admiral Robert Fitzroy once noted: "long foretold, long last; short
notice, soon past". By carefully watching the pressure on a barometer, you can forecast local weather
using these simple guidelines:









Decreasing barometric pressure indicates storms, rain and windy weather.
Rising barometric pressure indicates good, dry, and colder weather.
Slow, regular and moderate falls in pressure suggest a low pressure area is passing in a nearby
region. Marked changes in the weather where you are located are unlikely.
Small rapid decreases in pressure indicate a nearby change in weather. They are usually
followed by brief spells of wind and showers.
A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours.
Large, slow and sustained decreasing pressure forecasts a long period of poor weather. The
weather will be more pronounced if the pressure started rising before it began to drop.
A rapid rise in pressure, during fair weather and average, or above average pressure, indicates a
low pressure cell is approaching. The pressure will soon decrease forecasting poorer weather.
Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is
likely.
A large, slow and sustained rise in pressure forecasts a long period of good weather is on its
way.
Barometers are widely used and generally quite reliable at forecasting the weather – and they also
make a great gift!