What Is Viscosity
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 Principle & Technique
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.
How To Use 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
Time-Test Viscosity Tubes are frequently referred to as ASTM D-1545 Tubes, which originated from the adopted standard method of test for viscosity as described in ASTM D-1545* issued by the American Society for Testing and Materials. Time-Test Tubes are scored in three places, 27mm, 100mm and 108mm. The score mark at 27mm is used for a 73mm timed bubble travel. Time-Test Viscosity Tubes are also subjected to additional testing procedures to ascertain the 10.65mm bore which is very critical for precision measuring.
Class “G10.65” And Class “V-10.75” Tubes
Class “G-10.65” and “V-10.75” Viscosity Tubes are used principally for direct comparisons to Gardner Viscosity Standards. Both Classes of tubes are inscribed at the 100mm and 108mm lengths. For bubble rate rise comparisons to Gardner Viscosity Standards, the following equivalency chart is provided:
Cargille Class “G-10.65” = Gardner Grade A Tubes
Cargille Class “V-10.75” = Gardner Grade B Tubes
Class “V” Viscosity Tubes
Class “V” Tubes are the least expensive of the viscosity tubes. Their principal utility is for comparison to other samples or standards of known viscosities as prepared by the user. Class “V” Tubes are the Cargille specific assigned sizes between 10.50 and 11.00mm, with the exception of the 10.65mm and Class “V-10.75″mm sizes. Within this range, a specific tube size will be assigned, recorded in the Cargille Master File for future reference on reorders to insure furnishing the identical tube size. The Class “V” Tubes are also inscribed at the 100mm and 108mm lengths.
|CLASS||SCORE MARKS||I.D. +/-0.025mm||METHOD OF TESTING|
|Time-Test, 10.65mm (ASTM D-1545)||108mm, 100mm, 27mm||10.65||Bubble time rise or bubble comparisons|
|Class ”G-10.65″||108mm, 100mm||10.65 (equivalent to Gardner A tubes)||Bubble comparisons to Gardner Standards; or to calibrated standards with identical tube dimensions|
|Class ”V-10.75″||108mm, 100mm||10.75 (equivalent to Gardner B tubes)||Bubble comparisons to Gardner Standards; or to calibrated standards with identical tube dimensions|
|Class “V”||108mm, 100mm||10.50 to 11.00**||Bubble comparisons to standards prepared by the user|
NOTE: All score marks are measured from the center of the outside bottom. **Excluding 10.65 and 10.75mm sizes.
Cargille’s Quality Assurance
Cargille Laboratories 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.