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The absolute pressure p
with respect to the zero pressure in a vacuum. At-
mospheric pressure p
amb
and above the absolute pressure. The difference
between the atmospheric pressure and absolute
pressure is called pressure over atmospheric p
Pressure over atmospheric is positive when the air
pressure is less than the absolute pressure and nega-
tive otherwise. If it is negative then it can be called
underpressure or partial vacuum.
This manometer is a U-tube open at both ends and
partially filled with a liquid impervious to air. It is
mainly used for measure small pressures and pres-
sure differences. The pressure in a closed container
to be measured acts on the fluid in one arm of the
tube. The other open arm is subject only to atmos-
pheric pressure. The fluid then rises on one side so
that there is a difference in height Δh between the
two sides. From Δh and the density ρ of the fluid in
the tube, it is possible to calculate pressure over
atmospheric p
in the closed container:
e
p
(mbar) = g·ρ (g/cm³)·Δh (mm)
e
Pa
1 Pa
1
5
1 bar
10
2
1 mbar
10
1 Torr
133
1 atm
101325
1 at
98100
Elwe Didactic GmbH • Steinfelsstr. 6 • 08248 Klingenthal • Germany •
3B Scientific GmbH • Rudorffweg 8 • 21031 Hamburg • Germany •
is the pressure measured
abs
is the pressure of air over
.
e
2
6. Conversion table
bar
-5
10
1
-3
10
-3
1.33*10
1.01325
0.981
Rights to amend technical specifications reserved
© Copyright 2008 3B Scientific GmbH
For the liquid in the manometer it is possible to use
distilled water with coloured dye, ethanol and special
oils, or even mixtures of silicone and anti-freeze
fluid.
•
Remove the rubber bung.
•
Slowly pour the liquid into the overflow reservoir
until the columns of liquid in both limbs of the
manometer are on the zero mark.
•
Attach the tubing and connect it to the vessel for
which the pressure is to be measured.
Read the height difference Δh, estimating tenths
•
of the scale divisions.
•
Calculate the pressure using equation 2.
mbar
Torr
-2
-3
10
7.5*10
3
10
750
1
0.75
1.33
1
1013.25
760
981
736
5. Operation
atm
at
-6
9.87*10
1.02*10
0.987
1.02
-3
0.987*10
1.02*10
-3
1.32*10
1.36*10
1
1.033
0.968
1
www.elwedidactic.com
www.3bscientific.com
-5
-3
-3
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