3B Scientific U-Tube Manometer S - 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
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© 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