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the corresponding pressures p
air, we get:
⋅
=
⋅
p
s
p
s
0
0
1
1
=
+
Δ
Substituting
s
s
0
1
p
=
Δ ⋅
0
s
s
1
−
p
p
1
0
Rough calibration of scales:
•
Open the regulating valve wide.
•
Loosen the grub screw for the vernier scale by
half a turn (it is now possible to turn the scale
easily on the threaded axle without moving the
handwheel, although a counterpressure acts
against this independent movement).
•
Wind the handwheel out till you detect a notice-
able resistance.
•
Without turning the handwheel, turn the vernier
scale on the threaded axle till the 0.0 mark is on
the top and the fixed scale shows approx. 48 mm.
•
Loosen the knurled screws of the fixed scale and
shift the scale to the side till the 48-mm bar is ex-
actly above the centre line of the vernier scale
(see Fig. 2).
•
Tighten the knurled screws again. In doing so,
make sure that the fixed scale does not press
against the vernier scale.
0
10
20
Fig. 2: Piston position reading at 48.0 mm
Zero correction:
•
Shut the regulating valve (the pressure in the
measuring cell now corresponds to the ambient
pressure p
= 1 bar. To within the accuracy of the
0
measurement, the manometer should display an
excess pressure of 0 bar).
•
Wind the handwheel in till an excess pressure of
15 bar has been reached (absolute pressure
p
= 16 bar).
1
•
Read the piston position s
displacement Δs = s
and p
of the trapped
0
1
s
and rearranging gives:
30
40
50mm
0 0
1 9
1 8
1 7
1 6
1 5
and calculate the
1
– s
.
0
1
•
Calculate the zero corrected piston position s
using Equation 3.
•
Adjust the vernier scale to the corrected value
(2)
and, if necessary, move the scale again.
•
If required, wind the handwheel out a little and
secure the vernier scale with the grub screw.
(3)
Measurement example:
p
= 1 bar, p
= 16 bar, p
0
1
s
= 48.0 mm, s
0
Therefore, s
1, corr
The vernier scale must therefore be adjusted so that
now only 2.97 mm are shown instead of 3.50 mm.
Note:
After calibrating the zero point, it is possible to obtain
qualitatively accurate measured values. With regard
to temperature T and pressure p , it is also possible to
obtain quantitatively accurate measurements of the
isotherms in range around to the critical point where
the two phases exist simultaneously. However, espe-
cially in the liquid phase, the measured isotherms are
rather too widely separated.
6.3 Detailed calibration:
The exact relation between the volume V
measuring cell and the scale reading s is dependent
on the volume of oil in the oil chamber. The oil
chamber also expands marginally in proportion to the
pressure as a result of the spring in the manometer
tube. Additionally, when the temperature is in-
creased, the castor oil expands to a greater extent
than the rest of the equipment. This means that the
pressure rises at a slightly greater rate at higher tem-
peratures. All of these phenomena can be calculated
if appropriate calibration has been effected using air
as an ideal gas.
The ideal gas equation would thus be:
⋅
p
V
=
⋅
n
R
T
R =
with
8.
314
After taking the overpressure reading p
pressure can be calculated from:
p = p
+ 1 bar
e
The absolute temperature is given by:
T = ϑ + ϑ
where ϑ
0
The volume is given by:
=
⋅
V
A
s
G
A =
where
3,
14
displacement.
From the measured displacement s
calculate the effective piston displacement as follows:
4
– p
= 15 bar
1
0
= 3.5 mm, Δs = 44.5 mm
1
= 2.97 mm.
J
K
mol
= 273.15°C
0
2
cm
and s is the "effective" piston
, it is possible to
e
1, corr
in the
G
(4)
, the absolute
e
(6)
(7)
(8)
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