A Rapid Precision Viscometer OGDEN FITZSIMONS,' Standard Oil Company (Indiana), Whiting, Ind.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
lationship to kinematic viscosity. Then each individual could calibrate his tube and plot a deviation chart, much as we use standard thermocouple tables.
Suspended-Level Instrument For several years accurate viscosity measurements in this laboratory had been carried out in the British Engineering Standards Association (1) type of Ostwald viscometers. Determinations were slow and tedious, however, and the use of the instrument was confined to a few research problems. In 1933 Ubbelohde (8) published an account of his suspendedlevel viscometer, which had such obvious advantages that it was adopted by this laboratory, using a vapor bath for temperature control. Several changes have since been made, the result being an apparatus that excels the Saybolt not only in accuracy but also in speed and simplicity of operation.
well above the upper mark. The suction line and fingers are then removed, establishing the suspended level at N , and the time for the oil to pass between the marks is noted. If the time is less than 100 seconds the oil is immediately drawn up into the smaller (left) tube and the efflux time observed. The kinematic viscosity is obtained by multiplying the efflux time by a factor, determined by calibrating each tube of the instrument on an oil of known viscosity. At short efflux times a kinetic energy correction is necessary, which is always less than 1 per cent for motor oils. The kinetic energy correction is calculated from the dimensions of the instrument. The oil is quickly removed through tube C by a suction line connected to a trap. The instrument is flushed with a narrow cut naphtha with an initial slightly above the bath tem erature. The naphtha is also removed by suction through tube 8 a n d the instrument sucked dry. The entire cleaning operation requires about one minute.
Calibration The equation for all capillary type of viscometers can be expressed by the relation
where r v -0 -C
VOL. 7 . NO. 5
t g
-A
h 1
-E
m
V
mV v = - -?rghr4 -t-81V 8Hlt = radius of capillary in cm. = kinematic viscosity in centistokes = efflux time in seconds = gravity constant in em. per second per second = average effective head in cm. = length of capillary in cm. = volume in cc. = coefficient in kinetic energy term
The kinetic energy term was evaluated from dimensions of the instrument, where m = 0.8 (Q),V = 3.75 cc., and 1 = 12 em. Thus the equation could be simplified into the form 2,
LN
4 &
LD
- . . . - _..
-...- .- __ _ _
I-H
FIGURE 2. DIAGRAM OF APPARATUS
The apparatus, shown in Figure 2, consists of two capillaries of different sizes, A , sealed into a vapor bath, E. Tube C is an air vent for producing the suspended level and tube B is connected to the oil reservoir, D. The bulb is filled with a pure organic liquid with a normal boiling point slightly above the temperature at which it is desired to operate the viscometer, methylene chloride being suitable for use a t 37.78' C. (100' F.) and tertiary amyl alcohol for use at 98.89' C. (210' F.). Vacuum is turned on and regulated by valve I to obtain a slow rise of bubbles up column J. The bulb, H , is heated electrically, causing vapor to rise up the,jacket and down the riser, F , to the condenser, G, and return as liquid to the bulb, H. To obtain exactly the desired temperature, the pressure is adjusted by adding or removing water from the pressureregulating column, J, by means of the leveling bottle, K . By changing the connection to the column from tube L to tube M and substituting an air line for the vacuum line, the viscometer can be operated under pressure; this is occasionally necessary on days of low barometer.
Procedure Oil is poured through a 100-mesh filter down tube B until reservoir D is about one-half full; this requires about 12 cc. After a short wait of 1 or 2 minutes to allow the oil to reach bath temperature, tube C and the smaller capillary, A , are closed off with the fingers and oil is drawn up into the right capillary by suction
=
Clt
C* -t
Since the dimensions have been chosen so that Cz = 1, the correction in centistokes is merely the reciprocal of the efflux time and is easily applied. This correction term, however, is negligible for efflux times in excess of 100 seconds except in the case of the smallest capillary size. The coefficient CI is easily evaluated by the use of an oil of known viscosity.
Results Two instruments were installed in a n inspection laboratory and a short period of instruction given to four non-technical operators (A, B, C, and D). They were furnished 14 samples of various brands of Pennsylvania grade motor oils which had been inspected some months previously on Ostwald viscometers. The result of the survey is shown in Table 11. These operators were accustomed to read the stop watches to even seconds only, and this procedure was not varied, as the watches were in poor condition. The results were reported in Saybolt seconds obtained from tables converting directly from efflux times. This table does not show the greatest degree of accuracy obtainable, but does show the freedom of the instrument from the personal equation under the most unfavorable conditions. I n Table I11 is shown the agreement that can be obtained with other types of viscometers in other laboratories.
Advantages The advantages in the use of the vapor-bath suspendedlevel viscometer are: SPEED, The viscometer equals the Saybolt in speed on light oils but is considerably faster on heavy oils. This speed is due to five factors, (I) the vapor bath affords nearly instantaneous heating, (2) a short cleaning cycle, (3) no necessity to weigh or measure the amount of oil charged, (4) no
ANALYTICAL EDITION
SEPTEMBER 15, 1935
TABLE11. EQUIVALENT SAYBOLT VISCOSITIESUSIXG VAPOR BATHSUSPENDED-LEVEL VISCOMETER Oil
1 2
Temp.
Vapor Bath Suspended-Level Viscometer A B C D
Ostwald
' F.
100
160
210
43.3
100
(89)b 218.5
210
3
100 210
4 5
210 100 2 10
47,5 (96) 194.5 45.6 (101.5) 46.6 188 45.4 (94) 50.2 257.5
160
43.3
160a 43.3a
(89)
(89)
...
45.45
...
50.2a 256'" 50.Za
6 7
210 100 210
50.2 (101)
(102)
8
100
304 52.6 (97) 287 51.4 95.6 374.5 57.5 (102.5) 391.5 59.2 (106) 348a 54,9a (96.5) 49Za 63.7a
302.5a 52.Ba (98) 286 51.4 (95.5) 375 57.5 (102.5) 391 59.1 (105.5) 345 54.8 (96) 493a 63.6O
210
9
100 2 10
10
100 210
11
100 210
12
100 210
13
100
210
218 47.5 196) 194 45.6 (101.5) 46.6a
218.5a 47.P (96) 192.5a 45.6a (102.5) 46.6a
160 43.3 (89)
158.9
218a
218.2 47.4 (95)
47.5a (96) 194" 45.6a
(101.5)
46.7a
...
188a
'
45.4 (94) 60.3Q 255a 50.2'"
45,4a
...
50.2t 256a 50.2a
(102) 305" 52.6a
(102)
304n 52.6a (97) 287a 51 4a (95.5) 374 57.5
(97)
287a 51.4a (95.5)
376 57.5
(102.5)
(102)
390a 59.3" (107) 342" 55.0'" (98.5)
495a 63.6a
3S8a 59. Za
(107) 344a 54.9a (97) 49Za 63.7a (99) 531a 67.7a (104)
43.3 (86)
...
... ... ... 184.2 45.3 (96) 50.2 254.5 50.1 (102.5)
300 52.3 (97) 284 51.2 (95.5) 372.5 57.3 (102.5) 388.4 59.0 (106) 342.3 54.8 (97.5) ...
... ...
... 531a 530a ... 210 67.6a 67.W (104) (104) (104) a Values based on single determinations only. b Values in parentheses are Davis, Lapeyrouse, and Dean viscosity indices ( 8 ) . 14-
100
531.5" 67.7a
...
TABLE111. VISCOSITY OF OIL 15 Kinematic Viscosity in Centistokes 100' F. 210' F6 (37.78O (98.89 C.) C.)
Laboratory Viscometer Bureau of Standards Bingham Laboratory 1 Ostwald Laboratory 1 Ubbelohde Suspended-level Laboratory 1 Vapor-bath SUBpended-level Laboratory 2 Modified Ubbelohde Laboratory 3 Vapor-bath suspend ed-level (inspection)
34.8 34.78 34.85
5.31 5.328
34.87 34.95
5.325 5.32
...
...
Saybolt Equivalent from A. S. T. M. Eauations 100' F. 210' Fd (37.78' (98.89 C.) C.) 162 161.9
43.4 43.44
162.2
...
162.3 162.6
43.43 43.4
162
43.4
cleaning or recharging to obtain check determinations, and ( 5 ) the use of dual capillaries keeps all efflux times between 100 and 600 seconds. I n practice, about 40 oils can be inspected a t two temperatures in one day by one operator using two instruments. This includes check determinations or a total of 160 viscosities. ACCURACY.The degree of accuracy a t present is limited by the viscosity standards available. Checks on the same instrument to better than 0.1 per cent are attainable. SIMPLICITY. Viscosities a t 37.78' and 98.89" C. (100' and 210" F.) are sufficient for most practical purposes. The 54.44"C. (130" F.) Saybolt inspection is used chiefly on oils whose times of flow a t 100" F. would be unnecessarily long. With the vapor bath suspended-level instrument the heaviest bright stocks can be examined a t 100' F. as quickly and easily as light oils. SMALLSATVPLE.Only about 12 cc. of oil are required for a determination. is afforded at VISIBILITY* This is a decided improvement, especially a t 210' F., as in
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localities of low barometric pressure, where an oil bath must be used, with Ostwald viscometers, visibility is poor. FREEDON FROM ALIGKMENT ERRORS.The instrument has the same low sensitivity to alignment errors as the offset type O s t n d d of Gruneisen (6) and later of Willihngana, McCluer, Fenske, and McGrew (IO). DRAINAGEERRORS.Drainage errors are negligible on oils. They are of two types in Ostwald viscometers: Drainage in the lower bulb results from varying amounts of oil adhering to the walls and thereby altering the effective liquid head. In a given instrument, it is dependent upon the viscosity of the oil and the rate at which the oil is drawn up into the upper bulb. This error is completely avoided in the suspended-level type of instrument. Drainage in the up er bulb, unlike lower bulb drainage, affects the volume term in foiseuille's equation rather than the height. Hence, in certain instances these two errors may nearly offset each other. According to McCluer and Fenske ( 7 ) ,the drainage should be determined by the use of an auxiliary bulb of the same size and shape as the upper bulb. This can be done by a procedure of weighing the auxiliary bulb both dry and after a determination. However, in the case of similar liquids such as petroleum oils the possible error is negligible, as the volume remaining is proportional to the efflux time. Since in a given instrument the efflux time varies directly as the viscosity, the drainage is a constant which is taken care of if the instrument is calibrated on similar liquids. RAPIDOPERATION.Continuous operation is unnecessary, as the viscometer can be brought up to temperature in about 3 minutes. SAVINGIN SPACE. Each instrument requires about a square foot of desk space. COST. The instrument is relatively inexpensive, as no thermostat, relays, stirrers, rheostats, or thermoregulators are required. RETENTION OF ORIGINAL CALIBRATION.Two instruments in daily use for nearly a year have shown no detectable deviation from the original calibration.
Disadvantages FRAGILITY. The instrument, being made of glass requires care in handling. However, no breakages have been encountered so far even in the hands of nontechnical operators. NECESSITYFOR APPLYINGA FACTOR TO RESULT. It is necessary to apply a factor, determined for each instrument, to the results. However, by the use of precision capillary tubing and careful dimensioning, any predetermined factor may be obtained, thus allowing a standardization of the viscometer. I n the original Ubbelohde (8) viscometer it is the custom to make the factor a power of ten by suitable dimensioning. CLOGGINGOF CAPILLARY.I n the smaller capillary sizes, care is necessary to screen out all solids. A metal screen is used, as fibers from cloth and paper filters are troublesome if allowed to get into the tubes.
Literature Cited (1) British Engineering Standards Association, No. 188 (1929). (2) Davis, Lapeyrouse, a n d Dean, Oil Gas J., 30, 46, 92 (1932). (3) Dean, E. W., a n d Davis, G. H. B., Chem. & Met. Eng., 36, 618 (1929). (4) Ellis, Trans. Am. I n s t . Elec. Engrs., 40, 469 (1921). (5) Griineisen, Wiss. Abhandl. physik. tech. Reichsanstalt, 4, 166 (1905). (6) Larson, C. M., a n d Schwaderer, W. C., Nail. Petroleum News,24, 2, 26 (1932). (7) McCluer a n d Fenske, IND.E m . CHEM.,27, 82 (1935). (8) Ubbelohde, J . Inst. Petroleum Tech., 19, 376 (1933). (9) Ubbelohde, O d und Kohle, 11, 145 (1935). (10) Willihnganz, MoCluer, Fenske, a n d McGrew, IND. ENG.CHEM., 4 n a l . Ed., 6, 231 (1934).
RECEIVED June 3, 1935. Presented before the Division of Petroleum Chemistry at the 89th Meeting of the American Chemical Society, New York, N. Y., April 22 to 26, 1935.
