Transcript No Slide Title

Barometry
Evangelista Torricelli credited with
invention of barometer.
Gasparo Berti may have been first.
Atmospheric Pressure
Pressure is a force applied to, or distributed
over, a surface, measured as a force per unit
area.
The pressure of air is due to the “hits” or
impacts of the air molecules on the surface,
or other molecules, on which the force is
exerted.
Pressure can also be thought of as energy
per unit volume since the pressure exerted
on a surface is a result of millions of these
“hits” by molecules and the number of hits,
(which is determined by density), and the
force with which the molecules hit,
(determined by their kinetic energy),
determines the pressure exerted.
We can then say that atmospheric pressure
is dependent on:




Speed of the molecules of air
Mass of the molecules of air
Frequency of impact of the molecules of air
gravity
Sea Level Pressure
Using the Hydrostatic Equation and the
General Gas Law Equation one can arrive at
the following equation:
Psf c  Pst ne
gzstn
Rd T 
From which the pressure at sea level (Psfc)
can be determined. Note that it includes the
height of the station and the average
temperature.
ASOS Sea Level Pressure
Calculations
NWS uses the following:


Sea Level Pressure (mb) = 33.864 • Ps • R
Where,
• Where Ps is the pressure at ground in inches of
mercury.
• R is a value dependent on the average 12 hour temperature,
determined by taking the current temperature and the
temperature 12 hours ago and averaging them.
• R values are calculated for each station by NWS
• Ps is determined from the equation:

H 
0.00813 
T 

Ps  P  10
• where
– P is the pressure measured at the instrument in inches of
mercury
– H is the height of the instrument above the ground, usually
5 feet,
– T is the 12 hour average ambient temperature in degrees
Rankin.
– Rankine scale is the Fahrenheit equivalent of the Kelvin
scale.
o
F  459.69o o R
General Errors of Pressure
Measurements
Wind

Ideally the atmosphere would be static. However, wind
produces dynamic pressure errors, a “pumping” action on the
barometer.
Temperature

Changes in physical characteristics of the instrument due to
temperature changes affects the pressure value obtained.
Instrument Error 
errors in construction or calibration
Direct and Indirect Pressure
Measurements
Direct:

Measures pressure due to weight / force directly
exerted by atmosphere.
Indirect:

Measures pressure by sensing a change
(produced by pressure) to some characteristic
of a material.
A. Direct Pressure Measurements
Piston Gauge:




Primary standard barometer.
Used to calibrate secondary barometers under
laboratory conditions.
Not used to directly measure atmospheric
pressure.
Total weight of piston, weight carrier and added
weights determines the pressure against which a
secondary barometer is calibrated.
1. Mercury Barometers
Work on the principle of balancing the
weight of a column of the atmosphere
against the weight of a column of mercury.

Cistern barometers
• Adjustable - Fortin
• Fixed - Kew

Without cistern
• Siphon Barometer
Characteristics of Mercury
High Specific Gravity (density),

13.5939 g/cm3 at 0oC
Low vapor pressure, 0.000185 mm Hg at
0 oC
Small volume coefficient of expansion,

1.818 x 10-4 cm3/oC
Low specific heat, 0.0335 cal/g oC
Low freezing point, -38.87oC
High boiling point, 356.58oC
Corrections to Mercury Barometers
Instrument


Determined by calibration against a standard
barometer.
Correction table provided by manufacturer.
Thermal

To account for changes in volume of mercury and
scale length changes due to temperature.
Gravity

To account for changes in gravity from standard
gravity resulting from elevation of the barometer and
latitude of the barometer
Thermal Correction
General Equation
s(T  Ts)  m(T  Thg )
Ct  {
}R s
1 m(T  Thg )






s = coefficient of linear expansion of scale
m = coefficient of volume expansion of
mercury
T = temperature of thermometer
Thg = standard temperature of mercury
Ts = standard temperature of scale
Rs = observed height of mercury column:
If standard temperature for both mercury
and scale are both 0oC, then the pressure
corrected for temperature is:
(s  m)T
Rt  [1 
]Rs
1  mT
An error of about 1oF in temperature will
result in about 0.1 mb error in pressure.
Gravity Correction
Value of gravity for latitude change
g = 980.616 (1 - .002637cos 2 = 5.9 x 10 cos 2 )
-6
Value of gravity for altitude change
g  g  0.0003086 h  0.0001118(h h')
Pressure corrected for instrument,
temperature and gravity is:
gl  gs
Rg  Rt  [
]Rt
gs
2
Common Mercurial Barometers
Adjustable Cistern

Normal Barometer
• Secondary Standard in England
• Used to calibrate other barometers
Diagram of the National Physics
Laboratory Normal Barometer
The separation of the
two mercury surfaces
is determined using
two micrometer
microscopes and cross
wires.
New Normal Barometer
A modern
Normal
Barometer
using lasers to
determine the
separation of
the mercury
levels.
Another Adjustable Cistern Barometer

Fortin Barometer
• A secondary Standard in the U.S.
Reading the Fortin Barometer
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