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Transcript
Barometry
The art or science of barometric
observation
History
• Giovanni Batista Baliani – observed that
syphon pumps could not pump water higher
than ~ 34 feet
• Galileo - proposed it was due to a vacuum
• Gasparo Berti – created the first working
barometer sometime between 1640 and 1643
• Evangelista Torricelli – credited with inventing
the barometer in 1643
Atmospheric Pressure
• The atmosphere exerts a pressure on the
surface of the Earth equal to the weight of a
vertical column of air
Static Pressure Calculation
• To calculate static pressure (p) at the surface,
use: p = ρ(z)g(z)h
• ρ(z) = air density at altitude (height) of
measurement
• g(z) = gravitational acceleration at altitude of
measurement
• h = height above sea level
Units of Pressure
•
•
•
•
•
SI unit of pressure is the pascal (Pa) kg m-1 s-2
Preferred unit in meteorology is the millibar
1 mb = 1hPa = ___Pa
1 in. Hg (@ 273.15 K) = _____ hPa
1 standard atmosphere (sea level) =
______hPa = _______ mb
Three Types of Pressure Measurements
• Absolute – total static pressure (ie barometric
pressure)
• Gauge – pressure relative to ambient
atmospheric pressure
• Differential – pressure relative to some other
pressure
Static vs Dynamic
• Static pressure is actual air pressure
• Dynamic pressure is pressure exerted by wind
flow
• Dynamic pressure can produce errors in static
pressure measurement
Direct (In-situ) Measurement of Pressure
• Balancing the force of the atmosphere against
the weight of a column of fluid (Fortin
barometer)
• Balancing the force of the atmosphere against
the force of a spring (Aneroid barometer)
Mercury Barometers
• Manometer is the simplest form of a mercury
barometer
• Can be either open-ended (Differential
pressure measurement) or close-ended
(Absolute pressure measurement)
• Can be awkward to use since manual
measurements of fluid height in each arm and
the difference calculated to get the raw
output
Fortin Barometer
• Improved version of the manometer offering
high accuracy and easy calibration. Somewhat
portable. Excellent long-term stability
• A column of mercury is enclosed in a glass
tube that is sealed at the top with a reservoir
of mercury at the bottom.
• A vacuum is created at the top of the tube
Fortin Barometer
• Height of the column of mercury is
determined using the attached scale
• The height of the mercury in the reservoir
must be adjusted upwards using an adjusting
screw to the fiducial (reference) point
• The measurement is then taken from the
attached scale
Fortin Barometer Measurements
• Open the case and immediately read the
temperature
• Use the bottom screw to adjust the height of
the mercury to the fiducial point.
• Adjust the scale index to the top of the
mercury column. Keep your eye level with the
mercury meniscus in the tube.
• Read the pressure using the vernier
• Lower the level of the mercury in the cistern
Why Use Mercury?
• Has a high density (14x heavier than water)
leading to a column that is of reasonable
length
• Low vapor pressure
• Easily purified and chemically stable
• Remains liquid for a wide range of
temperatures (-38.87C – 356.58C)
Sources of Error for Mercury Barometers
• Dynamic wind pressure can produce (positive
and negative) errors on the static
measurement on the order of several millibars
• Density of mercury is a function of
temperature so temperature effects must be
compensated for
Sources of Error for Mercury Barometers
• Since the force of the atmosphere is balanced
against the weight of the mercury in a column,
local gravity must be known and a
gravitational correction calculated
• The presence of gas (other than mercury
vapors) in the vacuum portion of the tube will
cause an error in measurement
Sources of Error for Mercury Barometers
• The surface tension of mercury will cause a
depression in the mercury column of smallerbore tubes. The correction for this is usually
incorporated into the index correction
(calibration).
• The barometer must be kept vertical
• Impurities affect the density and
measurement
Temperature Correction
• The temperature correction for a Fortin
barometer is CT = _________
• β=
Volume and Linear Expansion Coefficients
•
•
•
•
•
Mercury (β): 1.818 x 10-4 K-1
Aluminum (α): 23.0 x 10-6 K-1
Brass (α): 18.9 x 10-6 K-1
Steel (α): 13.2 x 10-6 K-1
Iron (α): 11.4 x 10-6 K-1
Calculating Local Gravity
• Start by calculating gravity at sea level at the
barometer latitude, ϕ
• gϕ =
Calculating Local Gravity
• Then calculate the elevation effect
• gL =
Correcting for Local Gravity
• The correction factor for local gravity is given
as:
• CG =
Corrected Station Pressure
• The raw barometer reading is converted to
station pressure by:
• ps =
Aneroid Barometers
• Consists of an evacuated (vacuum) chamber
with a flexible diaphragm that moves in
response to an applied pressure. The restoring
force is a spring that may be part of the
diaphragm
• Aneroid – “without fluid”
• Two types – Metallic-diaphragm and silicondiaphragm
Calibration Equation for Aneroid
Barometers
• p=
Bourdon Aneroid Barometer
• Consists of a flattened tube with round ends
bent in a circular pattern. The tube is open to
the ambient pressure but is enclosed in an
evacuated box.
• As pressure increases, the tube tries to
assume a circular form, causing it to straighten
out. This movement can be correlated to
pressure.
Sources of Error for Aneroid
Barometers
•
•
•
•
Exposure Errors
Temperature-induced errors
Hysteresis Effects
Drift
Indirect Measurement of Pressure
• Indirect – measurement of a variable other
than pressure that is a function of pressure
• Boiling point of a liquid
– Boiling point of pure water at standard sea-level
pressure is 373.15 K
– Decreases with increasing height
Hypsometer
• Height Meter
• Contains a flask of fluid, heated to maintain
continuous boiling with a temperature sensor
• Need to know the relationship between vapor
pressure and temperature to derive pressure
Vapor Pressure
• The pressure of a vapor in equilibrium with its
non-vapor phases
• Dependent on temperature
• Related through the Clausius-Clapeyron
Equation
Hypsometer Pressure Calculation
• p=
Comparison of Barometer Types
• Mercury Barometers (Fortin)
–
–
–
–
–
–
–
Simple physical concept
Require no calibration
Difficult to automate and transport
Must be kept vertical
Needs temperature and gravity corrections
Mercury vapor is toxic
Improper handling may introduce bubbles into
mercury column
– Height of column cannot be changed
Comparison of Barometer Types
• Aneroid Barometers
–
–
–
–
–
–
–
Very small size, easily portable
Easily automated
Insensitive to orientation, motion and shock
No gravity correction needed
No toxic chemicals
Concept is simple, but calibration is always required
Temperature sensitivity is high, no simple or
predictable correction
– Subject to unpredictable drift
Comparison of Barometer Types
• Hypsometer
– Small size, reasonable portable
– Easily automated
– Sensitive to orientation
– No gravity or temperature correction needed
– No drift or hysteresis
– Concept is simple, no calibration required
Exposure Error
• Barometers are designed to measure static
pressure
• Need to be isolated from dynamic effects
• Impractical for barometers to be inside
buildings unless equipped with a static port
• Static port needs to extend beyond the
pressure field of the building
Exposure Error
• Pressure field of building can be 2.5x building
height vertically and 10x height of the building
horizontally
• Still need static port for mounting outside on
towers
• Vertical static port still doesn’t completely
reduce the dynamic error, but keeps it to a
minimum as long as the static port is kept
vertical
Exposure Error
• Can be very problematic for pressure
measurements on buoys