3B Scientific Critical Point Apparatus - Manual (Page 17)

3
5. Technical data
Sulphur hexafluoride:
Critical temperature: 318.6 K (45.5°C)
Critical pressure: 3.76 MPa (37.6 bar)
Critical volume: 197.4 cm 3 /mol
Critical density: 0.74 g/mol
Maximum values:
Temperature range: 10-60°C
Maximum pressure: 6.0 MPa (60 bar)
Threshold value for
safety valve:
6.3 MPa (63 bar)
Theoretical long-term
pressure:
7.0 MPa (70 bar)
Theoretical rupture
pressure:
>20.0 MPa (200 bar)
Materials:
Test gas: Sulphur hexafluoride (SF
6
)
Hydraulic fluid: Castor oil
Measuring cell: Transparent acrylic
Temperature coating: Transparent acrylic
Recommended
thermal medium:
mixture of water and anti-
freeze in the ratio 2:1
Determination of volume:
Piston diameter: 20.0 mm
Piston surface: 3.14 cm 2
Displaced volume: 3.14 cm 2 × displacement
Maximum volume: 15.7 cm 3
Scale division for
displacement: 0.05
mm
Maximum displacement: 50 mm
Determination of pressure:
Manometer: Class 1.0 (max. 1% deviation
from full scale value)
Measured quantity: Excess pressure
Indicator: 60 bar max.
Manometer diameter: 160 mm
Connections:
Hole for
temperature sensor:
6 mm dia.
Connections for
thermal medium:
7 mm dia.
Connection for
regulating valve:
1/8’’ dia.
Gas connection: 1/8’’ (3.17 mm) dia. (as
supplied)
General specifications:
Dimensions: 380 x 200 x 400 mm 3
Weight: 7 kg approx.
6. Volume calibration
6.1 Preliminary notes:
A
S
R
K
L
J
G
H
I
Q
B C E F
N O P M
D
Fig. 1: Cross-section of apparatus with measuring cell (A),
conical seal (B), oil chamber (C), piston (D), cylin-
der (E), heat casing (F), silicone seal (G), end
plate (H), square grommet (I), piston cover (J),
threaded axle (K), gasket (L), manometer connec-
tion (M), guide tube (N), spring (O), sleeve (P), hole
for temperature sensor (Q), circular grommet (R) and
valve plate (S)
One turn of the handwheel winds the piston into/out
of the cylinder by means of a threaded axle. This
leads to a change of volume in the oil chamber (see
Fig. 1). Since oil is practically incompressible and all
the other components other than the conical seal are
almost rigid, a change in volume in the oil chamber
causes the conical seal to deform, thereby creating an
almost equal change in volume
ΔV
G
in the measuring
cell. As a first approximation for
ΔV
G
, we can assume:
s A V
Δ ⋅ = Δ
G
(1)
where
2
cm 14 3. A
=
and
Δs = displacement of piston.
The piston displacement is shown in divisions of
2 mm on the fixed scale. Intermediate values are read
on the vernier scale in divisions of 0.05 mm.
The fixed scale can be moved by loosening the two
knurled screws. The vernier scale can be repositioned
and turned around the threaded axle on loosening
the grub screw (between scale positions 0 9 and 1 0).
6.2 Zero point calibration:
The zero point for the volume scale must be deter-
mined by conducting a calibration.
For this, we take advantage of the fact that in a pres-
sure range of 1-50 bar and in a temperature range of
270-340 K, air acts as a near-ideal gas (the real gas
factor has a deviation of less than 1% from 1). There-
fore, at a constant temperature (e.g. room tempera-
ture) for two piston displacements s
0
and s
1
and for