Barometry

1
Barometry

• The art or science of barometric observation

2
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

3
Atmospheric Pressure

• The atmosphere exerts a pressure on the surface
  of the Earth equal to the weight of a vertical
  column of air

4
Static Pressure Calculation

• To calculate static pressure (p) at the surface,
  use p ?(z)g(z)h
• ?(z) air density at altitude (height) of
  measurement
• g(z) gravitational acceleration at altitude of
  measurement
• h height above sea level

5
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 (_at_ 273.15 K) _____ hPa
• 1 standard atmosphere (sea level) ______hPa
  _______ mb

6
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

7
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

8
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)

9
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

10
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

11
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

12
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

13
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)

14
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

15
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

16
Sources of Error for Mercury Barometers

• The surface tension of mercury will cause a
  depression in the mercury column of smaller-bore
  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

17
Temperature Correction

• The temperature correction for a Fortin barometer
  is CT _________
• ß

18
Volume and Linear Expansion Coefficients

• Mercury (ß) 1.818 x 10-4 K-1
• Aluminum (a) 23.0 x 10-6 K-1
• Brass (a) 18.9 x 10-6 K-1
• Steel (a) 13.2 x 10-6 K-1
• Iron (a) 11.4 x 10-6 K-1

19
Calculating Local Gravity

• Start by calculating gravity at sea level at the
  barometer latitude, ?
• g?

20
Calculating Local Gravity

• Then calculate the elevation effect
• gL

21
Correcting for Local Gravity

• The correction factor for local gravity is given
  as
• CG

22
Corrected Station Pressure

• The raw barometer reading is converted to station
  pressure by
• ps

23
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
  silicon-diaphragm

24
Calibration Equation for Aneroid Barometers

• p

25
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.

26
Sources of Error for Aneroid Barometers

• Exposure Errors
• Temperature-induced errors
• Hysteresis Effects
• Drift

27
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

28
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

29
Vapor Pressure

• The pressure of a vapor in equilibrium with its
  non-vapor phases
• Dependent on temperature
• Related through the Clausius-Clapeyron Equation

30
Hypsometer Pressure Calculation

• p

31
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

32
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

33
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

34
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

35
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 doesnt completely
  reduce the dynamic error, but keeps it to a
  minimum as long as the static port is kept
  vertical

36
Exposure Error

• Can be very problematic for pressure measurements
  on buoys