3B SCIENTIFIC PHYSICS U104001 Instructions D'utilisation page 16

2. Safety instructions
When used properly, the operation of the critical
point apparatus is not dangerous, since both the
experimenter and the equipment are protected by a
safety valve. However, it is extremely important to
observe a few precautionary measures:
•
Read the instruction sheet thoroughly and follow
the instructions therein.
•
Do not exceed the maximum permissible values
for pressure and temperature (60 bar and 10-
60°C).
•
Do not operate the equipment without qualified
supervision.
•
Always wear safety goggles.
Only increase the temperature at low pressure with
pure gas phase in the measuring cell.
•
Before increasing the temperature, wind the
handwheel outwards so that maximum volume is
attained in the measuring cell.
When conducting adjustments, make sure that the
safety valve does not point in the direction of people
who could be injured or objects that could be dam-
aged if the valve cover shoots out. When conducting
experiments, pay special attention too to the align-
ment of the safety valve.
•
When setting up the apparatus, make sure that
the safety valve does not point in the direction of
people who could be injured or objects that could
be damaged.
•
When adjusting the safety valve, wrap your arms
around the apparatus to reach the valve at the
back.
If the conical seal is overtaxed, it could get damaged
or even destroyed.
•
Never set a pressure above 5 bar if the regulating
valve or the flush valve is open, i.e. if there is no
back pressure from the gas in the measuring cell.
•
Never create underpressure by turning the hand
wheel inwards when the valves are shut.
In the frame there is a threaded bush, which is to be
regarded as a safety-related feature (see section 9).
•
Lubricate the threaded bush every 100 cycles.
•
Inspect the threaded bush annually.
To prevent damage by corrosion inside the instru-
ment,
•
use a 2:1 mixture of water and anti-freeze fluid as
the thermal medium.
3. Description
The critical point apparatus allows us to investigate
the compressibility and liquefaction of a gas. Meas-
urements allow determination of the critical point for
the gas as well as the recording of isotherms for an
adiabatic p-V diagram (Clapeyron diagram). The gas
used for testing is sulphur hexafluoride (SF
critical temperature of 318.6°K (45.5°C) and a critical
pressure of 3.76 MPa (37.6 bar) which makes for a
simple experiment set-up.
The critical point apparatus consists of a transparent
measuring cell of particularly well sealed, pressure-
resistant design. The volume of the measuring cell
can be modified by turning a fine-adjustment wheel
and can be read by means of a fixed scale and a rotat-
ing vernier scale to an accuracy of one thousandth of
the maximum volume. The pressure is applied via a
hydraulic system using castor oil approved for me-
dicinal use. The measuring cell and hydraulic system
are isolated from one another by a conical rubber
seal which rolls up when there is an increase in pres-
sure. This design means that any pressure difference
between the measuring cell and the oil reservoir is
negligible in practical terms. A manometer measures
not the pressure of the actual gas but that of the oil,
thus eliminating any need for a space within the
measuring cell. When observing transitions from gas
to liquid or vice versa, the lack of such a dead space
means that the development of the very first drop of
liquid as well as the disappearance of the last bubble
of gas can be observed. The measuring cell is sur-
rounded by a transparent chamber of water. A circu-
lating thermostat arrangement (water bath) means
that a constant temperature can be maintained dur-
ing the experiment with a high degree of accuracy.
The temperature can be read and monitored using a
thermometer.
The fact that volume, pressure and temperature can
all be read with a high degree of accuracy means that
accurate p-V diagrams or pV-p diagrams can be re-
corded without much difficulty. Pressure and tem-
perature-dependent volume correction enable us to
achieve accurate quantitative results which are well in
agreement with published values.
1 Critical point apparatus, filled with hydraulic fluid
(castor oil). With attached gas connection fittings
for MINICAN® gas container and protection for gas
supply connections. Test gas (SF
1 Oil filling device
1 Allen key, 1.3 mm (for grub screw on the vernier
scale)
1 Plastic tubing, 3 mm diameter
1 1/8" tube fitting (wrench width 11 mm)
1 Grease gun
2
). SF
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4. Contents
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