INDUSTRIAL
and
ENGINEERING
CHEMISTRY ANALYTICAL EDITION
Earrison E. Howe, Editor
Determination of .High Viscosities By Means of the Gardner Mobilometer E. L. BALDESCHWIELER AND L. Z. WILCOX, Standard Oil Development Co., Linden, N. J.
A setup for Gardner mobilometers is described, whereby accurate temperature control and easy operatioa are obtained. The conclusions of Cornthwaiteand Scofieldthat the correlation between absolute viscosity and mobility is a straight line have been checked by the authors for much higher vis-
cosities, at various temperatures, and for different disks. Provided rigid control of time and temperature is obtained and improvements in mechanical construction are made, the mobilometer can be used as a precision instrument for the determination of absolute viscosity.
T
HE mobilometer, originally described by Gardner and Parks (3), was designed to be used as a production control instrument to secure uniformity in consistency between different batches of a given product. The instrument was first recommended for control in paint and lacquer manufacturing. Later, Gardner and Van Hueckeroth (4) extended its use to the testing of food products, mineral oils, vaseline, and coal tar. The instrument has also been described at length by Sward and Stewart (6). While no claim for high accuracy of results obtained with the instrument was made by Gardner and Parks (a), Cornthwaite and Scofield (8) showed that under rigid control of temperature and time the apparatus gave a remarkably close correlation with the absolute viscosity of a number of samples possessing true fluid flow. This correlation resulted in a straight line passing through the origin when plotted on rectangular coordinates. The work of Cornthwaite and Scofield (2) was, however, limited to oils of relatively low viscosities (about 8 poises). It was necessary for this laboratory t o determine a number of mobilities and viscosities of much higher values with a fair degree of accuracy. For this purpose the relationship between viscosity and mobility for true fluids was obtained a t various temperatures, using oils of much higher viscosities. The apparatus is described below.
Timing in this laboratory is obtained by Veeder-Root magnetic counters, reading directly in tenth seconds. These are run by a contactor connected to an American Time Products constantfrequency generator. This generator, which is run by the plant power, allows a fluctuation of *IO volts and a frequency variation of *2 cycles in the current. Under these conditions the frequency does not vary by more than *0.001 cycle. This arrangement has given complete satisfaction, the frequency fluctuations having seldom exceeded the above limits. A more compete description of this equipment will be published in the near uture. Viscosities were carried out by means of Ubbelohde (7) suspended level viscometers, A No. 4 capillary (constant C = 10.04) was generally used, except for the lower viscosities which were obtained with a No. 3 capillary (constant C = 1.015). The results thue obtained were in kinematic units. The absolute viscosities were calculated b multiplying the kinematic results by the density (obtained t y pycnometer) of the individual samples at the various temperaturea. The above rocedure will give kinematic viscosities within 1 0 . 2 er cent w h e the densities were accurate to about 0.001. T i e constant-temperature bath used for the determination of kinematic viscosities can be controlled to within *0.02" F.
Apparatus In order to obtain flexibility and, at the same time, accurate temperature control, the barrel of the mobilometer was provided with a brazed outer brass jacket about 0.5 inch wide fitted with outlets at the top and bottom. These outlets are connected by means of rubber tubing to the circulation outlets of a Hoepplcr (6) thermostat which is capable of controlling the temperature to within *0.02" F. This apparatus provides a very flexible control of the temperature and, by using the proper circulation fluid and regulator in the Hoeppler thermostat, it is also suitable for low-temperature work, as shown by the authors in a previous article (1). The necessity of immersing the whole mobilometer in a bath is thereby avoided, which is an important advantage when working at extreme temperatures.
of
Results The choice of materials available for the determination of viscosities by means of viscometers of the glass capillary type is limited by the following considerations: The material must have true viscous flow-i. e., the rate shear should be proportional to the shearing stress. 2. It must be a true solution, absolutely free of suspended particles. 3. It must be transparent, so that a sharp meniscus can be seen when determining the viscosity. 1.
The heaviest Pennsylvania bright stock available had a viscosity of about 163 poises at 77" F. In order to obtain high viscosities, especially at the higher temreratures, it was necessary to prepare blends of the above mineral oil with various amounts of an isobutylene polymer of very high molecular weight. Thus viscosities as high as 800 poises at 77" F. and 300 poises a t 150" F. were obtained. Determina525
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
TIME WITH ABSOLUTE VISCOSITY VISCOSITIES AT
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tions of both viscosities and mobilities were carried out a t 77", loo", 130", 150", and 210" F. Mobilities were also carried out with both a 51-hole and a 4-hole disk. The results are shown in Figure 1, which is self-explanatory, (Concentric rings indicate determinations made on different materials having the same viscosities.)
Conclusions From an examination of the curves shown in Figure 1, the following conclusions can be drawn: The correlation between Gardner mobilometer time (in seconds) and absolute viscosity (in poises) is a straight line passing through the origin. The above statement holds true for determinations made at various temperatures, and for different disks, although, as should be expected, changing the disk changes the slope of the curves. In other words, each disk has its own curve passing through the origin. The results check the work and conclusions of Cornthwaite and Scofield (2) in every respect. However, the slope of the curve obtained by these authors for their 51-hole disk is not quite the same as that presented in this paper. This is due to the fact that it is mechanically impossible to manufacture disks identical in all respects; small variations in the shape of the disk as well as in the sizes and spacing of the holes will
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cause changes in mobilities and thus alter the slope of the curve. While admittedly the mobilometer is primarily a control instrument, it is susceptible of mechanical improvement. It is based on sound principles and the data presented show that it can be used for the determination of viscosity, being particularly useful for opaque materials of very high viscosities. It is only necessary to calibrate each disk against a material of known viscosity a t any given temperature, plot this point, and draw a straight line through the origin. The sizes and number of holes in a given disk can be varied at will to suit any particular case.
Literature Cited Baldeschwieler, E. L., and Wilcox, L. Z . , IND.ENG.CHEY., -4nal. Ed., 11, 221 (1939). Cornthwaite, C. R., and Scofield, F., Sci. Sect., Natl. Paint, Varnish Lacouer Assoc.. Circ. 547 (1938). Gardner, H: A., and Parks, H. C.; Paint Mfrs. Assoc. U. S., Circ. 265, 414-28 (1936). Gardner, H. A., and Van Hueckeroth, A. W., IND.ENCI.CHEY., 19, 724-6 (1927). Hoeppler, F., Brenn., 14, 234 (1933); 2. tech. Physik., 14, 165 (1933). Sward, 6.G., and Stewart, J. R., Am. Paint Varnish Mfrs. kssoc., Circ. 394, 317-22 (1931). Ubbelohde, L., J. Inst. Petroleum Tech., 19, 376 (1933).
