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Rheology is a branch of science dealing with the deformation and flow of materials which includes the subject of viscosity. Viscosity is a measurement of internal friction, or resistance, to flow by external forces. Viscosity is the ratio of shear force to shear rate expressed by:
The viscosity of many materials depends on shear rate and is classified accordingly. Viscosity is expressed as Absolute n, and Kinematic, v. Absolute viscosity is determined by measuring the shear force required to produce a given shear rate and is independent of the density of the liquid. Kinematic viscosity is measured by the time required for a given volume of liquid to flow through a capillary or restriction. It is related to flow caused by the hydrostatic head of the liquid and therefore strongly dependent on density.
The relationship between Absolute and Kinematic viscosity is given by the following, where p is density:
Rising Bubble Technique, Principle
The time required for an air bubble in liquid to rise is directly proportional to its viscosity. Bubble rise rates vary with the inside diameters of viscosity tubes; therefore, the tubes must be precision matched to a tolerance that will keep bubble rise rate variations below the level of visual detection. Variations in viscosity readings or comparisons are visually imperceptible from viscosity tubes with inside diameter tolerances closer than +/- 0.025mm. Any variations in readings are attributed to differences in viscosities.
Viscosity is determined by either measuring the time an air bubble takes to pass between two specifically spaced marks on viscosity tubes, or by comparison to the bubble rate rise of calibrated viscosity standards. The accuracy of the method is therefore a function of precision matching of tube bores and identical score heights, which are measured from the center outside bottom.
Other factors, such as uniformity of temperature and verticality of tubes, influence timed bubble travel and controlled laboratory techniques are essential for precise results. For example, a variation in temperature of 0.1°C will cause a 1% variation in the timed bubble travel, and a one radius departure from tube verticality may introduce a 10% error in timed bubble travel.
Procedure for Using Cargille Viscosity Tubes
A viscosity tube is filled to the 100mm level score with the material to be tested. A cork is inserted to the 108mm score which provides and fixes a uniform air bubble. The sample, and standard if used, is placed in a suitable rack and immersed in a water bath until a uniform temperature of 25°C (77°F) is attained. If viscosity determinations are performed by referral to comparison samples of known viscosities or precisely calibrated standards, then they too should be at the same temperature as the samples. Viscosity is determined by one of two methods:
The bubble rate rise of the samples is compared to samples of known viscosities or calibrated standards contained in tubes of the same dimensions. The rack containing the samples and standards is removed from the water bath, held vertically to stabilize the air bubbles and quickly inverted maintaining verticality. A comparison of air bubble speed of the samples to the standards permits estimation of viscosity; the faster the rate of bubble rise, the lower the viscosity. The user may prepare viscosity standards or employ Gardner Viscosity Standards.
Time-Method (ASTM D-1545)
Time-Test Tubes are required to determine viscosity by the Timed-Method. The length of time, in seconds, required for an air bubble to pass from the 27mm to 100mm score mark, a distance of 73mm, is approximately equal to the viscosity of the liquid in "Stoke" units; alternatively, viscosity may be expressed in "Bubble Seconds." It is important to note that the position of the air bubble in relation to the 27mm score should be similar at the end of the test on the 100mm score.
Viscosity Tube Selection Guide
Cargille Viscosity Tubes are divided into four Classes as determined by precision sorting in accordance with bore size, number of score marks and contemplated method of use for bubble rise testing. The numbers assigned refer to bore size, which is expressed in mm.
TIME-TEST VISCOSITY TUBES, 10.65mm
CLASS "G10.65" AND CLASS "V-10.75" TUBES
CLASS "V" VISCOSITY TUBES
NOTE: All score marks are measured from the center of the outside bottom. **Excluding 10.65 and 10.75mm sizes.
Viscosity Tube Specifications
Cargille Viscosity Tubes are manufactured to the closet possible tolerances and then precisely measured for final selection.
All Cargille Viscosity Tubes are permanently marked with two score marks, at 100mm and 108mm heights with the exception of Time-Test Viscosity Tubes, which have an additional score at 27mm. In compliance with ASTM D-1545, all score marks are measured from the center outside bottom of the tubes The score mark at 100mm represents the filling level and the 108mm score is used to fix the size of the air bubble in the tube by inserting a cork to this mark. The 27mm and 100mm scores on Time-Test Viscosity Tubes are used to time bubble rise. In instances where more than one viscosity tube size is used, in the same laboratory, an additional score is inscribed, on request, to differentiate between sizes.
Cargille Quality Assurance Service
Cargille Laboratories, Inc. maintains a master record for every Viscosity Tube customer. All production specifications related to the specific viscosity tubes supplied are maintained indefinitely. Each subsequent request for similar viscosity tubes is individually processed by referral to the firm's specifications record. This record keeping avoids any possibility of potential typographic error in requisitioning viscosity tubes. Cargille's unique Quality Assurance Service is automatically effected and is applicable to singular or multiple viscosity tube purchases.
Viscosity Tube Price Schedule
Viscosity Tube Accessories
CARGILLE "10" VISCOSITY TUBE HOLDER DISCONTINUED
VISCOSITY TUBE SAMPLE STORAGE BOX
ROUGH SORT TUBES
Accessories Price Schedule
All prices are subject to change without notice. Please contact us to confirm prices.