Rotating disc tests

The rotating disc and rotating cylinder have been successfully applied in the laboratory to study the effect of flow on corrosion rates and are much easier to use than actual pipelines and other real geometries. The results of these tests can now be correlated to geometries likely to be found in pipes, pumps, bends, etc. in plant by use of dimensionless group analysis. There-... 

The use ot rotating discs to carrv itsi spaiiiicns has been extended to studies rtf protective coatings in what are considered to be accelerated tests o( such coatings tor service underwater... 

Kassner used a rotating disc, for which the hydrodynamic conditions are well defined, to study the dissolution kinetics of Type 304 stainless steel in liquid Bi-Sn eutectic. He established a temperature and velocity dependence of the dissolution rate that was consistent with liquid diffusion control with a transition to reaction control at 860 C when the speed of the disc was increased. The rotating disc technique has also been used to investigate the corrosion stability of both alloy and stainless steels in molten iron sulphide and a copper/65% calcium melt at 1220 C . The dissolution rate of the steels tested was two orders of magnitude higher in the molten sulphide than in the metal melt. 

Beside laminar flow created by e.g. a rotating disc electrode mrbulent flow provides a means of artificially enhanced transport. A consistent mathematical description and analytical treatment of this mode of transportation is not possible. Various approximations have been proposed and tested for correctness [84Barl], an experimental setup has been described [78Ber, 83Her, 831wa]. From comparisons of measured and calculated current density vs. electrode potential relationships exchange current densities are available. (Data obtained with this method are labelled TPF.)... 

The drug dissolution rate could be determined by dispersing the powder in a test medium under suitable agitation or by studying the dissolution for a constant surface area by using the rotating-disc method (Fig. 21.6). The latter method should be the technique of choice, except when studies of the effect of particle size are of... 

Fig. 7. Schematic representation of examples of test geometries used. Open systems channel, rotating disc (Hoyt 1972, 1986). Closed systems pipe flow, couette- or searle-systems (Kulicke 1986)...

The DIN method is given in ISO 464954. The principle of the machine is illustrated in Figure 11.5 a disc test piece in a suitable holder is traversed across a rotating drum covered with a sheet of the abradant, which is why it is also called the rotary drum abrader. In this way, there is a relatively large area of abradant, each part of which is passed over in turn by the test piece, so that wear of the abradant is uniform and relatively slow. In the standard method there is no provision for changing conditions from those specified, other than a lower force for soft rubbers, but it would be possible to use other abradant cloths or papers and to vary the force on the test piece. The degree of slip is 100% and it would be inconvenient to test in the presence of a lubricant. Although not versatile, the method is very convenient and rapid and well suited to quality control. 

The majority of experiments on ion-bombarded materials have been performed using the pin and disc method. Here a standard pin under a load rubs against a rotating disc of the test material. Other methods such as rotating rods or the analysis of the abrasion have been applied less to ion-bombarded samples. 

Figure 12.1 Six configurations of simple dissolution test apparatus (a) the Wruble method in which the dose form is rotated in a container of solvent (b) a simple beaker method (c) an apparatus in which the dosage form is in a cradle which oscillates vertically (d) the rotating disc method to determine the intrinsic dissolution characteristics of a compressed powder (e) method in which the dosage form is placed in a medium which is shaken and (f) a method in which the decreasing size of suspensions is measured on dissolution.

The main merits of the rotating disc apparatus are the well-defined hydrodynamic conditions and constant surface area. These reduce the risk of artefacts in dissolution rate determinations caused by non-ideal test conditions. Furthermore, it is possible to determine an intrinsic dissolution rate and to perform other mechanistic evaluations of the dissolution process. The main limitation of the method is that it is not suitable for drugs that form fragile or porous discs, since it is not possible to maintain a constant surface area. 

Fig. 5.10.2 Test concept for determination of the quadrature number of a rotating disc gyro. The section on the left lists all electrically measured properties. Sections to the right show the sequence of models and simulations employed to extract sensor parameters, including the quadrature number...

To illustrate the extraction of model parameters from functional parameter tests, we return to the example of the rotating disc gyro and the 5-step model outlined in Section 5.10.2.2. 

Rotating disc electrode spectrometer, 71 Rotrode filter spectroscopy, 71 Round robin test, 128... 

Coran, 1978). During this test, fully mixed but unvulcanized rubber is contained in a heated cavity. Imbedded in the rubber is a rotating disc. Viscosity is continuously measured (by the torque required to keep the rotor rotating at a constant rate) as a function of time. The temperature is selected to be characteristic of rather severe processing (extrusion, calendaring, etc.). 

In pilot and prodnction plants, the flow conditions are determined by the nature of the apparatus and process, bnt in laboratory tests, they have to be individnally chosen. To determine the effects of static on very gently moving media, it is snfflcient to stir the medinm with an agitator. If exposure of the material to flowing media is expected, special corrosion tests are essential for simulation, for example, circnlation tests with pipe or channel flow and nse of rotating discs or cylinders as specimens. 

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