3B Scientific U-Tube Manometer S User Manual
Page 4
Elwe Didactic GmbH • Steinfelsstr. 6 • 08248 Klingenthal • Germany •
www.elwedidactic.com
3B Scientific GmbH • Rudorffweg 8 • 21031 Hamburg • Germany •
www.3bscientific.com
Rights to amend technical specifications reserved
© Copyright 2008 3B Scientific GmbH
The absolute pressure p
abs
is the pressure measured
with respect to the zero pressure in a vacuum. At-
mospheric pressure p
amb
is the pressure of air over
and above the absolute pressure. The difference
between the atmospheric pressure and absolute
pressure is called pressure over atmospheric p
e
.
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
e
in the closed container:
p
e
(mbar) = g·
ρ (g/cm³)·
Δh (mm) 2
5. Operation
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.
6. Conversion table
Pa
bar
mbar
Torr
atm
at
1 Pa
1
10
-5
10
-2
7.5*10
-3
9.87*10
-6
1.02*10
-5
1 bar
10
5
1 10
3
750 0.987
1.02
1 mbar
10
2
10
-3
1 0.75 0.987*10
-3
1.02*10
-3
1 Torr
133
1.33*10
-3
1.33
1
1.32*10
-3
1.36*10
-3
1 atm
101325
1.01325
1013.25
760
1
1.033
1 at
98100
0.981
981
736
0.968
1