ATMS 320 � Meteorological Instrumentation by 2CxzIx

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									   ATMS 320 – Meteorological
       Instrumentation
• Barometry objectives:
   – Learn some of the methods used to
     measure the static pressure exerted by
     the atmosphere
   – Understand the
     advantages/disadvantages to each of
     the methods
   – Experiment with exposure and
     calibration issues related to measuring
     atmospheric static pressure
          ATMS 320 – Barometry
• A review on pressure

Static pressure- Force/Area against
any surface in the absence of air
motion.


Dynamic pressure- Force/Area due
to air motion.                        http://www.godchecker.com/pantheon/greek-mythology.php?deity=ATLAS
          ATMS 320 – Barometry
• The Earth’s atmosphere
  exerts a static pressure on
  the surface of the Earth
  equal to the weight of a
  vertical column of air of                     W&H f2-1 here
  unit cross-section. . .
                 
    p(0)    
             0
                     g ( z )  ( z ) dz


       Since air is a fluid, this pressure, or force, is exerted
       equally in all directions.
       ATMS 320 – Barometry
• If the wind is blowing,
  it exerts a dynamic
  pressure which
  introduces a static
  pressure error…
          1
      p  C  V 2

          2                 http://news.bbc.co.uk/1/hi/in_depth/photo_gallery/3085722.stm
          ATMS 320 – Barometry
• Physical principles
  employed for
  measuring
  atmospheric pressure:
   – Direct techniques
   – Indirect techniques


                           http://www.rube-goldberg.com/html/pencil_sharpener.htm
          ATMS 320 – Barometry
• Direct – mercury
  barometers:
  – Balance the force due
    to atmospheric
    pressure against the
    weight of a column of
    mercury
            ATMS 320 – Barometry
• Direct – mercury
  barometers (cont.)
   – Difficult to automate
   – Not suitable for field
     experiments
   – Health risk
            ATMS 320 – Barometry
• Direct – mercury
  barometers (cont.),
  why mercury?
   – High density (small
     column height)
   – Low vapor pressure
     (little evaporation into
     vacuum)
   – Chemically stable
   – Liquid for a wide range
     of temperatures            http://www.gormangiftgallery.com/tormerbar.html
       ATMS 320 – Barometry
  F  ma
 m h Ac  mass of mercury
       F
  p1 
       Ac
  p1   m g h   Mercury barometer calibration equation
           ATMS 320 – Barometry
• A nice web page to
  help with the
  understanding of how
  mercury barometers
  work…



       http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Barometer/Barometer.html
              ATMS 320 – Barometry
• Main sources of error in a
  mercury barometer:
   – Dynamic wind pressure
     (alleviate via a static port)
   – Density of mercury (and of
     glass tube) are functions of
     temperature
   – Local gravity must be
     known accurately (a
     function of latitude)
   – Presence of air or water
     vapor in “vacuum”
   – Surface tension effects
   – Barometer must be kept
     vertical
   – Impurities in the mercury
           ATMS 320 – Barometry
• Advantages of
  mercury barometers:
  – Simple in concept (can
    visualize how it works)
  – Easy calibration
            ATMS 320 – Barometry
   Getting the atmospheric pressure reading “right”…

                                            Brhf2-4 here…
    Thermal correction equation

     p2  p1  C x  CT

Altitude and latitude correction equation

        ps  p2  CG
            ATMS 320 – Barometry
• Direct – aneroid
  (without fluid)
  barometers:
   – Balance the force due
     to atmospheric
     pressure against the
     restoring force of an
     “elastic” material (e.g.
     metal)
             ATMS 320 – Barometry
                                         y = deflection of diaphram center, t = diaphram thickness

• Aneroid barometers:
     – By changing the shape                                            A
        of the material surface
        used in the aneroid         yr                                        B
        barometer, we can                   Brf2-6 here
        improve the sensitivity
        of the barometer at
        high atmospheric

                                                            
        pressures
                     dy
static sensitivit y  r
                             p    c0 yr  c1 yr3                   Curve A or B??
                     dp
                          p  Q yr                                  Curve A or B??
              ATMS 320 – Barometry
• Other aneroid
  barometers:
  –   Stacked aneroid cells (2-7)
  –   Aneroid capsule (2-8)
  –   Silicon diaphragm (2-9)
  –   Bourdon tube (2-10)
              ATMS 320 – Barometry
• Main sources of error in
  aneroid barometers:
   – Same exposure errors as
     mercury barometers (e.g.
     dynamic pressure)
   – Temperature-induced error
   – Error arising from defects
     or irregularities in the
     diaphragm material and/or
     shape
   – Sensitivity to pressure is
     non-linear
   – Diaphragm “creep” (causes
     drift, a long-term change in
     the sensor sensitivity)
          ATMS 320 – Barometry
• Advantages of aneroid
  barometers:
  – Very small size
  – Readily automated
  – Insensitive to
    orientation, motion,
    and shock (portable)
  – No gravity correction
    required
  – Users not exposed to
    toxic materials
                    ATMS 320 – Barometry
• Indirect* –
  hypsometers:
      – Pressure sensor that
        utilizes the property of
        the decreasing boiling
        point of a liquid with
        decreasing pressure in
        order to determine
        pressure
*A pressure measurement technique is call indirect if it does not
respond directly to the force due to atmospheric pressure but,
instead, responds to some other variable that is a function of
pressure.
          ATMS 320 – Barometry
• Hypsometers:
  – Must somehow provide
    heat to get liquid to
    boil
  – If the liquid has a
    boiling point below the
    air temperature, a
    heater is not required
    (Freon-13; 191.75 K)
                              http://www.chefscatalog.com/store/catalog/silo.jhtml?itemId=cat000106&parentId=cat000000
           ATMS 320 – Barometry
Hypsometer equations:

  Clausius-Clapeyron equation…
       d ln  p / p0     L
                       
            dT           RT 2

  Calibration equation…
                L  1 1 
    p  p0 exp              Transfer equation…
                 R  T0 T 
                                        T0
                                 T 
                                        R T0  p 
                                     1     ln  
                                               p 
                                         L      0
           ATMS 320 – Barometry
The static sensitivity of hypsometers
               dT
               dp
changes over the range of typical
atmospheric pressures. If large static
sensitivity is good, at what range of
pressures do hypsometers perform
“good”?

       at HIGH pressure or at LOW pressure ?

                 Cast your votes now…
             ATMS 320 – Barometry

A pressure observing network
1-2 punch!!




(1) hypsometer on a radiosonde


(2) aneroid barometer at low altitude
            ATMS 320 – Barometry
• Main sources of error
  in hypsometers:
   – Sensitive to orientation
     of instrument
   – Extreme non-linearity
     at sea-level pressure
             ATMS 320 – Barometry
• Advantages of
  hypsometers:
   – Small size
   – Can be automated
   – Reasonably portable
   – No gravity or temperature
     correction required
   – Simple physical concept
     (does require careful
     implementation)
   – No drift
               ATMS 320 – Barometry
• All barometers are subject                         +Z
  to dynamic wind effects
  (e.g., air flow, building air
  conditioning or
  ventilation). A static port           tilt angle

  is designed to reduce
  dynamic error for                  wind vector
  barometers located inside
  shelters.
   Static port:
             Must be located outside of the significant pressure field caused by the
   shelter. Field impacts 2.5 – 10 times shelter height.
             Should be kept at a tilt angle of less than 10 degrees.
      ATMS 320 – Barometry
• Barometer project
  – Which type of
    barometer in Chapter 2
    is most like your
    spaghetti sauce jar
    barometer?
                             http://www.atomicmuseum.com/tour/manhattanproject.cfm

								
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