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Φυσική Προσανατολισμού Β Λυκείου
20/11/2014
Κωνσταντίνος Χ. Παύλου
Φυσικός – Ραδιοηλεκτρολόγος (MSc)
12
Η μονάδα πίεσης 1
mmHg
20‐Νοε‐14
(c)
Κωνσταντίνος Χ. Παύλου
23
The millimeter of mercury (symbol: mmHg) is
defined as the pressure exerted at the base of a
column of fluid exactly 1 mm high, when the density
of the fluid is exactly 13.5951 g/cm
3
, at a place
where the acceleration of gravity is exactly 9.80665
m/s
2
.
There are several things to notice about this definition:
1.
A fluid density of 13.3951 g/cm
2
was chosen for this definition
because this is the approximate density of mercury at 0 °C.The
definition, therefore, assumes a particular value for the density of
mercury.This assumption limits the precision of any pressure
measurement (in mmHg) to six significant digits.
2.
The definition assumes a particular value for the acceleration of
gravity: the standard acceleration g
n
= 9.80665 m/sec
2
. In practice, of
course, measurements are made using local values.
3.
These assumptions limit both the validity and the precision of the
mmHg as a unit of pressure. No metrology laboratory measures or
calibrates pressure directly in these terms. It would be extremely
difficult to find a fluid with exactly this density, and a place where g
was exactly 9.80665 m/s
2
.
Ατμοσφαιρική πίεση
20‐Νοε‐14
(c)
Κωνσταντίνος Χ. Παύλου
24
Atmospheric pressure is the pressure at any point in the Earth's atmosphere. In most
circumstances atmospheric pressure is closely approximated by the hydrostatic
pressure caused by the weight of air above the measurement point. Low pressure
areas have less atmospheric mass above their location, whereas high pressure areas
have more atmospheric mass above their location. Similarly, as elevation increases
there is less overlying atmospheric mass, so that pressure decreases with increasing
elevation. A column of air 1 square inch in cross section, measured from sea level to
the top of the atmosphere, would weigh approximately 14.7 lbf. A 1 m² (11 sq ft)
column of air would weigh about 100 kilonewtons (equivalent to a mass of 10.2
tonnes at the surface).
Κλάσμα της
1 atm
Μέσο ύψος
(
m)
1
= 760 mmHg
0
1/2
5.486
1/3
8.376
1/10
16.132
1/100
30.901
1/1.000
48.467
1/10.000
69.464
1/100.000
96.282