• 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 –
Meteorological
Instrumentation
• A review on
pressure
ATMS 320 – Barometry
http://www.godchecker.com/pantheon/greek-mythology.php?deity=ATLAS
Static pressure- Force/Area against
any surface in the absence of air
motion.
Dynamic pressure- Force/Area due
to air motion.
• 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 unit
cross-section. . .
ATMS 320 – Barometry
0
)
(
)
(
)
0
(
dz
z
z
g
p
W&H f2-1 here
Since air is a fluid, this pressure, or force, is exerted
equally in all directions.
• If the wind is
blowing, it exerts
a dynamic
pressure which
introduces a
static pressure
error…
http://news.bbc.co.uk/1/hi/in_depth/photo_gallery/3085722.stm
ATMS 320 – Barometry
2
2
1
V
C
p
• 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
ATMS 320 – Barometry
http://www.gormangiftgallery.com/tormerbar.html
ATMS 320 – Barometry
h
g
p
m
1
mercury
of
mass
c
m
A
h
a
m
F
c
A
F
p
1
Mercury barometer calibration equation
• A nice web page
to help with the
understanding of
how mercury
barometers
work…
ATMS 320 – Barometry
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Barometer/Barometer.html
•
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
ATMS 320 – Barometry
G
s
C
p
p
2
T
x
C
C
p
p
1
2
Thermal correction equation
Altitude and latitude correction equation
Getting the atmospheric pressure reading “right”…
Brhf2-4 here…
• 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
• Aneroid
barometers:
– By changing the
shape of the
material surface
used in the
aneroid barometer,
we can improve
the sensitivity of
the barometer at
high atmospheric
pressures
ATMS 320 – Barometry
Brf2-6 here
3
1
0
r
r
y
c
y
c
p
r
y
Q
p
Curve A or B??
Curve A or B??
dp
dy
r
y
sensitivit
static
A
B
y
r
y = deflection of diaphram center, t = diaphram thickness
• 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
ATMS 320 – Barometry
*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.
• 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)
ATMS 320 – Barometry
http://www.chefscatalog.com/store/catalog/silo.jhtml?itemId=cat000106&parentId=cat000000
ATMS 320 – Barometry
2
0
/
ln
T
R
L
dT
p
p
d
Clausius-Clapeyron equation…
Hypsometer equations:
Calibration equation…
Transfer equation…
T
T
R
L
p
p
1
1
exp
0
0
0
0
0
ln
1
p
p
L
T
R
T
T
ATMS 320 – Barometry
The static sensitivity of hypsometers
dp
T
d
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
• 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 to dynamic
wind effects (e.g., air
flow, building air
conditioning or
ventilation). A static
port is designed to
reduce dynamic
error for barometers
located inside
shelters.
ATMS 320 – Barometry
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.
wind vector
tilt angle
+Z
ATMS 320 – Barometry
• Barometer
project
– Which type of
barometer in
Chapter 2 is most
like your
spaghetti sauce
jar barometer?
BAROM
ETER
http://www.atomicmuseum.com/tour/manhattanproject.cfm