Instruments capillary viscometers

I = complex impedance, B = conductivity bridge, C = capillary viscometer, P = pycnometer or dilatometer, V = volumetric glassware, I = instrument, U = method unknown... 

Inside film heat transfer coefficient 496 INSINGER, T. H. 486, 492, 564 Institution of Chemic al Engineers 516 Instruments, capillary tube viscometer 196... 

A computer-controlled rheology laboratory has been constructed to study and optimize fluids used in hydraulic fracturing applications. Instruments consist of both pressurized capillary viscometers and concentric cylinder rotational viscometers. Computer control, data acquisition and analysis are accomplished by two Hewlett Packard 1000 computers. Custom software provides menu-driven programs for Instrument control, data retrieval and data analysis. 

Instruments are controlled by information contained 1n the experimental setup file. For each type of instrument (shear history simulator, rotational viscometer, reciprocating capillary viscometer), the hardware 1s controlled so that the parameters of shear rate, temperature and time comply with the desired test conditions. This involves controlling devices such as pumps, bath heaters, valves and variable-speed motors. The setup and control parameters are recorded in the experiment file along with the resulting measured data. If necessary, the experiment can easily be repeated. 

In this experiment, a Tubing Shear History Simulator was coupled with a Reciprocating Capillary Viscometer to simulate the above conditions. Results from the experiment are given in Tables I and II and Figure 3, and were retrieved directly from the project data base. Total Instrument use time for this experiment was 17 hr, of which 16.5 hr were completely unattended operation.. Data analysis, including plotting of figures, required less than five minutes. 

The screening capillary viscometer can be operated manually as well as automatically. If manual operation is desired, an interactive program is available to aid with data collection. Program-operator interaction takes place through terminal input and also with a push-button data collection indicator on the instrument itself. The immediate on-line analysis of results and the ease of data storage and retrieval are just some of the benefits realized by using this program 1n conjunction with manual operation of the instrument. 

There are two main types of viscometer rotary instruments and tubular, often capillary, viscometers. When dealing with non-Newtonian fluids, it is desirable to use a viscometer that subjects the whole of the sample to the same shear rate and two such devices, the cone and plate viscometer and the narrow gap coaxial cylinders viscometer, will be considered first. With other instruments, which impose a non-uniform shear rate, the proper analysis of the measurements is more complicated. 

It is important that we know at what Reynolds number our instrumental configurations give turbulent flow and work below this figure or we will think that shear thickening is occurring A figure of Re < 3000 to 10,000 is usually satisfactory for cone and plates or capillary viscometers, but values as low as 300 may be the maximum for some cup and bob units. 

Capillary viscometers are useful for measuring precise viscosities of a large number of fluids, ranging from dilute polymer solutions to polymer melts. Shear rates vary widely and depend on the instruments and the liquid being studied. The shear rate at the capillary wall for a Newtonian fluid may be calculated from equation 18, where Q is the volumetric flow rate and r the radius of the capillary the shear stress at the wall is rw = rApf2L. 

All glass capillary viscometers should be calibrated carefully (21). The standard method is to determine the efflux time of distilled water at 20°C. Unfortunately, because of its low viscosity, water can be used only to standardize small capillary instruments. However, a calibrated viscometer can be used to determine the viscosity of a higher viscosity liquid, such as a mineral oil. This oil can then be used to calibrate a viscometer with a larger capillary. Another method is to calibrate directly with two or more certified standard oils differing in viscosity by a factor of approximately five. Such oils are useful for calibrating virtually all types of viscometers. Because viscosity is temperature-dependent, particularly in the case of standard oils, temperature control must be extremely good for accurate calibration. 

Some orifice viscometers, such as the Shell dip cup and the European ISO cup, which resembles a Ford cup with a capillary, have long capillaries. These cups need smaller kinetic eneigy corrections and give better precision than the corresponding short-capillary viscometers. However, they are still not precision instruments, and should be used only for control purposes. 

Various methods are used to examine the viscosity characteristics of metallized gels. Two types that have received extensive application are the cone and plate viscometer and the capillary viscometer. Both instruments can measure rheological characteristics at high shear rates, and the former is useful for low shear rate measurements as well. 

Figure 3 shows a curve of the 13 vol. % 4 graphite slurry in water as determined with a capillary viscometer. This is the same material examined on the cone and plate unit. The apparent viscosity is 1.6 poise at a shear rate of 103 sec. 1 and decreases to a value of 0.39 poise at a shear rate of 104 sec."1, the viscosity data being corrected for the true wall shear rate. The flow curves obtained from both instruments agree quite closely. 

Solution Equation (16) shows that the velocity gradient is not uniform in a capillary viscometer any more than it is in a concentric-cylinder instrument. The rate of shear dvldr is directly proportional to the radial distance from the axis of the cylinder. At the wall it has its maximum value, which is proportional to Rc] at the center of the tube it equals zero. Some intermediate value, say, the average, might be used to characterize the gradient in a given instrument. This quantity will be different for capillaries of different radii. All of this is similar to the situation in concentric-cylinder viscometers. 

A variety of laboratory instruments have been used to measure the viscosity of polymer melts and solutions. The most common types are the coaxial cylinder, cone-and-plate, and capillary viscometers. Figure 11 -28 shows a typical flow curve for a thermoplastic melt of a moderate molecular weight polymer, along with representative shear rate ranges for cone-and-plate and capillary rheometers. The last viscometer type, which bears a superficial resemblance to the orifice in an extruder or injection molder, is the most widely used and will be the only type considered in this nonspecialized text. 

As stated in Chapter 1, for the determination of intrinsic viscosity, [ ], of a polymer, viscosity values of several dilute solutions, when the relative viscosities ( / s) of the dispersions are from about 1.2 to 2.0, are determined. To facilitate such measurements, the so called Ubbelohde glass capillary viscometer is used that has a large reservoir to permit several successive dilutions of a polymer solution (Figure 3-19). Because intrinsic viscosity measurement is important, the test procedure for using the Ubbelohde viscometer is outlined here in brief (Cannon Instrument Co., 1982). 

From that point, the necessity of continuously measuring viscosity, in addition to polymer concentration, became obvious. Several attempts were made to adapt existing viscometers as GPC detectors, but the problem of internal volume was critical. Ouano [2] published the first design of a single-capillary viscometer which was based on pressure measurement. Several similar designs [3-6] were pubfished and a commercially available instrument, the Waters Model 150CV (Waters Associates, Milford, MA, U.S.A.), based on a design described in Ref. 4, became commercially available. 

I = complex impedance, B = conductivity bridge, C = capillary viscometer, P = pycnometer or dilatometer, V = volumetric glassware, I = instrument, U = method unknown (not provided in the reference). a Conductivity at 29 K calculated from VTF parameters given in reference. b Binary composition of 42.0-58.0 mol % [(CH3)3S]Br-HBr. 

The most common viscosity test is the kinematic viscosity method (ASTM D445, IP-71, DIN 51566 and ISO 3104). Note that lubricant viscosity is discussed in detail in the next chapter. The kinematic viscosity is the product of the time of flow and the calibration factor of the instrument. The test determines the kinematic viscosity of liquid lubricants by measuring the time taken for a specific volume of the liquid to flow through a calibrated glass capillary viscometer under specified driving head (gravity) and temperature conditions. The test is usually performed at a lubricant temperature of 40°C and/or 100°C to standardize the results obtained and allow comparison among different users. 

There are two types of capillary viscometers. In gravitational instruments, gravity drives vertical flow through a capillary and a timer is used to measure the flow rate. For liquids with low vapor pressures, an open-ended viscometer is suitable this design has been well studied, and commercial instruments are available. Accuracy of 1% or better can be achieved. For more volatile liquids,... 

---