, and the uncertainty is less than 0.1%.
The accuracy of an individual measurement of relative viscosity will be no better than — + 20/ . Observing the above procedure, the reliability should be the same as with the use of this technique at atmospheric pressure i.e. 'Atter than 1%.
Temperature control of the oil bath was to an estimated ±0.2°C.
With the large bulk of the pressure vessel, fluctuations during a series of measurements would be negligible. It can be seen that for the liquids measured, the effect of temperature on the rate of increase in viscosity with pressure is small. Provided the temperature is constant during a run, the relative viscosity will not be altered, even if the actual measurement of temperature was inaccurate. Care was observed that temperature changes in the system due to compression, were dissipated before measurements were made. Furthermore, only
in the runs with the pentanes were pressure increments large.
The accuracy of the relative viscosity measurement is related to that of the density data employed. This data has been reported to a corrected four figures, and presumably an accuracy of
0.1%
is possible along the data isotherms, and probably 0.24 at other temperatures is reasonable after smoothing values.It is difficult to estimate the effect of contamination of the sample with the "red oil" hydraulic fluid. The bellows assembly was sealed at the bottom with a 2BA screw sealing on a PTFE flat
187 ring in a containing groove, which appeared completely satisfactory.
The use of PTFE sealing tape in the thread of the bellows sleeve was less satisfactory, and some results were discarded due to seepage of oil into the bellows. Since there is no pressure differential between the test liquid and the hydraulic fluid, observable contanmination only occurred after an interval of days in the pressure vessel. This was the case when leaks in other
parts of the high pressure equipment had to be remedied. The effect of contamination would be to cause an increase in the rate of the viscosity change with pressure. However, the experimental data when compared with that of Bridgman tends to a lesser rate of change when different. It would appear that contamination has not effected the reported data.
In the light of these comments, the probable overall accuracy in relative viscosity measurements in this work, after having smoothed the data, approaches 1%. To ensure accuracy in the lower pressure ranges, the calibrated 0.40000 psi gauge was employed, and in the range 1000 to 2000 Kg./cm.2
on n and iso-pentane where both gauges were employed, the agreement was very good. The probable accuracy in pressure measurement to 500 Kg./cm.2
is approximately 1% but approaching 0.5% between 500 and 2000 Kg./cm.2
. The accuracy at higher pressures is presumed to be 0.8%, as normally recommended with a calibrated Bourdon gauge of the quality employed.
188
5.7_
Suggested Improvements in the Cell DesignThree important changes are necessary in the construction of the stainless steel bellows assembly which contains the liquid sample. The design of the high pressure cell is currently being revised making these modifications, but the new design is not reported in this work.
The method of sealing the bellows sleeve to the pressure closure is to be improved. The new bellows sleeve will have a flat-faced step beneath the threaded section, and a PTFE square ring will seat between this step and the polished face of the pressure closure.
Screwed to the closure will be a 9/10" dia. stainless steel disc, so that the PTFE washer is essentially contained in a groove.
During testing, three assemblies were damaged due to twisting of the bellows under pressure. Though the plug end of the bellows was fitted with a guide ring with only 10 thou. clearance in the pressure vessel internal diameter, distortion occurred within the three inch length over which the bellows was free moving. In the new assembly, a retractable guide sleeve is to be fitted of 0.D. just less than the bellows internal diameter.
The maximum compression of the bellows employed is 1 inch, which in the initial design of the crystal and bellows assemblies represented a volume compression of 29.4%. Choosing n-pentane as the test liquid, at 0°C for a compression of 10000 Kg./cm.2
, the volume reduction is 26.2%, at 50°C for 8000 Kg./cm.2 30.6(x„ and at 95°C for 5000 Kg./cm.2
$5.65. Therefore, at the test temperatures of 30°C and 50°C, the cell pressure limit is approximately 8500 and 6500 Kg./cm.2
respectively.
It is intended in the new design that this pressure limit in the
normal range of temperatures will be above 10000 Kg./cm.2
, the maximum design pressure for the vessel.
189
CHAPTER
6.
DISCUSSION' OF THE RESULTS 6.1. Comparison of Results6.1.1 Benzene
In fig.
5.9,
the data of Bridgman and Jobling and Lawrence are compared graphically with the results obtained with the torsional crystal. At 30°C, the deviation of the single value of Jobling and Lawrence is 1.5%, though the agreement is better at 50°C. The slope of the Bridgman 30°C isotherm differs significantly being 1.4% lower at 500 Kg./cm.2, and the value at 1000 Kg./cm.2
is 45 higher than the extrapolated crystal curve. The highest pressure measurement with benzene at 30°C was 872 Kg./cm.2
Bridgman3
has reported that the melting point at 1033 Kg./cm.2
is
33.4°
C and from fig.6 in appendix3,
at 30°C the freezing pressure should be 920 atms. or 950 Kg./cm.2The accuracy of Bridgman's pressure measurement or the purity of benzene must be questioned.
The isotherms at 30°, 40° and 50°C are consistent with one
another, with the isotherms of Jobling and Lawrence at 50° and 70°C, and with the curve of Bridgman at
75 o
C. The ratio of increase in viscosity with pressure decreases very slightly with temperature, as readily demonstrated in tab.2e 6.1.
TABLE 6.1 The Effect of Temperature on 77p
o for Benzene
191 6.1.2. Cyclohexane
1
7-P data at temperatures of 30o, 34° and 50°C are compared with Bridgman's resultsi at 30° and 75°C in fig. 5.10. Bridgman reports only a single value at 500 Kg./cm.2
at 30°C which is approximately 4i higher than the value extrapolated from the experimental results.
Considering melting curve date reported by Hamman4 in fig. 6, appendix 3, the freezing pressure at 30°C is 400 Kg./cm.2
, suggesting Bridgman's result is incorrect. In table 6.2, the rate of increase in viscosity with pressure is examined at different temperatures.
Table 6.2 The Effect of Temperature on for Cyclohexane
Temperature o
It will be observed that the rate of increase in viscosity with pressure at 75°C which would be expected from the experimental data is lower than Bridgman's slope.