# 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
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
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
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
barometers:
– Very small size
– 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 ?

ATMS 320 – Barometry

A pressure observing network
1-2 punch!!

(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
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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