Capillary viscometer design

Capillary viscometers are simple and inexpensive. They are normally constructed from glass and resemble a U-tube with a capillary section between two bulbs. The initial design originated with Ostwald and is shown as part A in Figure 3.2-1. The Cannon-Fenske type, a popular modification of the Ostwald design that moves the bulbs into the same vertical axis, is shown as part B in Figure 3.2-1. 

Capillary viscometers that have this design are called Ostwald viscometers. There are many specific designs of Ostwald viscometers. The most frequently used are the Cannon-Fenske viscometer... 

ISO 1628-4 1999 Plastics - Determination of the viscosity of polymers in dilute solution using capillary viscometers - Part 4 Polycarbonate (PC) moulding and extrusion materials ISO 7391-1 1996 Plastics - Polycarbonate (PC) moulding and extrusion materials - Part 1 Designation system and basis for specifications ISO 7391-2 1996 Plastics - Polycarbonate (PC) moulding and extrusion materials - Part 2 Preparation of test specimens and determination of properties ISO 11963 1995 Plastics - Polycarbonate sheets - Types, dimensions and characteristics. 

The new viscometer design utilizes two sets of the capillary and pressure transducer assemblies like the one shown in Figure 1. 

We begin with a brief discussion of Newton s law of viscosity and follow this with a discussion of Newtonian flow (i.e., the flow of liquids that follow Newton s law) in a few standard configurations (e.g., cone-and-plate geometry, concentric cylinders, and capillaries) under certain specific boundary conditions. These configurations are commonly used in viscometers designed to measure viscosity of fluids. 

Equation (4.4) is an unwitting statement that the velocity (l/t) of a sol s planar flow is inversely proportional to t. A capillary viscometer is designed to maintain r, l, V, and t (1 atm) constant, so that ti, is direcdy proportional to tt. The generalized equation for a single measurement (single-point viscometry) is... 

Viscometers II and III in the table merely illustrate the different ranges of shearing stresses that can be reached. As a rule a smaller radius and shorter length of the capillary are used for high-gradient viscometers. With some simple calculations one can easily design a capillary viscometer which serves the particular purpose of any experimental work. The dimen-... 

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. 

The single-capillary viscometer (SCV) is represented in Fig. la. Its design is a direct extrapolation of classical viscometry measurement. It is composed of a small capillary, through which the solvent flows at a constant flow rate, and a differential pressure transducer (DPT), which measures the pressure drop across the capillary. SCV obeys PoiseuiUe s law and the pressure drop AP across the capillary depends on the geometry of the capillary, on flow rate Q, and on viscosity of the fluid 7j according to... 

The first multiple-capillary viscometer was designed by the Viscotek Company (Houston, TX, U.S.A.) [7]. It is represented in Fig. lb. It is composed of four identical capillaries, assembled as a bridge. Here, the difference is generated by the bridge itself as a differential viscometer. The central DPT provides a differential viscosity signal DP and the second, DPT, provides inlet pressure IP. The intrinsic viscosity [17] can be calculated by the formula... 

Another design was described by the Dupont Company [8]. It is represented in Fig. Ic. It is composed of two single-capillary viscometers. The first one (measure) is located as a normal SCV (Fig. la) and measures polymer plus flow. The second one (reference), identical to the first one, is located after a holdup reservoir which is connected after the refractometer and measures only the flow with pure solvent. It is just necessary to obtain the difference between the two signals to obtain the polymer signal with no flow contribution. 

The last differential viscometer design is the Waters Corporation detector [9], which is in the Alliance GPCV2000 high-temperature instrument. It is composed of three capillaries, two differential pressure transducers, and two holdup reservoirs it is represented in Fig. Id. The pressure transducers are connected flow-through this eliminates the need for frequent purges. This detector provides, at the same time, relative viscosity information and relative flow information. This design does not require a perfect matching of the capillaries. 

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,... 

Polymers that have been suggested for mobility control in oil reservoirs include polyacrylamides, hydroxy ethyl cellulose, and modified polysaccharides which are produced either by fermentation or by more conventional chemical processes. In this paper the solution properties of these polymers are presented and compared for tertiary oil recovery applications. Among the properties discussed are non-Newtonian character for different environmental conditions (electrolytes and temperature), filterability, and long term stability. The behavior of these water soluble polymers in solution can be correlated with the effective molecular size which can be measured by the intrinsic viscosity technique. A low-shear capillary viscometer with a high precision and a capability of covering low shear rates (such as 10 sec - - for a 10 cp fluid) has been designed to measure the viscosities. The measurement of viscosities at such slow flow conditions is necessitated... 

Table 4-1 shows some of the dimensions and operating characteristics of Cannon-Fenske viscometers. These particular instruments are designed for flow times in the range 200-1000 seconds. Capillary viscometer outflow time is governed partly by the acuity with which the transit of the... 

Capillary Viscometer An instrument used for the measurement of viscosity in which the rate of flow through a capillary under constant applied pressure difference is determined. This method is most suited to the determination of Newtonian viscosities. Various designs are used, such as the Ostwald and Ubbelohde types. 

As mentioned earlier, viscometric manipulations are simplified considerably by using a capillary instrument of the Ubbelohde type, as modified by Davis and Elliott, rather than one of the Ostwald or Fenske t3q)e. This viscometer has a side arm at the base of the capillary which breaks the liquid flow to form a suspended level, and also reduces kinetic-energy corrections. The latter are very important, but can be made negligible by careful viscometer design. Kinetic-energy corrections are inversely proportional to the flow time, and so the viscometer should be designed so that solvent flow-times of 150-200 seconds are achieved. Full details of viscometric techniques can be found elsewhere. ... 

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