Coating resistance calculation

In analyzing the results on a cathodically protected pipeline, the protection current density and coating resistances should be calculated for individual sections of the pipeline in addition to the on and off potentials, the pipe current, and the resistances at insulating points and between the casing and the pipeline. The results should be shown by potential plots to give a good summary [15] (see Fig. 3-20). 

The variation in the on and off potentials or the potential difference along the pipeline will usually indicate faults that prevent the attainment of complete cathodic protection. The protection current requirement of the pipeline may be estimated from experience if the age and type of pipeline is known (see Fig. 5-3). Figure 3-20 shows the variation in the on and off potentials of a 9-km pipeline section DN 800 with 10-mm wall thickness. At the end of the pipeline, at 31.84 km, an insulating unit is built in. The cathodic protection station is situated at 22.99 km. Between this and the end of the pipeline there are four pipe current measuring points. The applied protection current densities and coating resistances of individual pipeline sections are calculated from Eqs. (3-40) and (3-41). In the upper diagram the values of... 

The following terms apply to the specific coating resistance which is related to the surface, S r is the value calculated from the specific resistance of the coating material using Eq. (5-1) ... 

Surface resistivity. One side of the specimen is coated with a circle of silver paint surrounded by a ring of silver paint. The uncoated distance between the circle and the ring is an effective length on which surface resistivity is measured. The other surface of the specimen is fully coated with silver paint. Current and voltage are measured and surface resistivity calculated. If samples contain internal or external antistatics, the measurement is performed under a controlled atmosphere to eliminate the influence of temperature and relative humidity. Also, specimen conditioning is used to account for migration of the antistatic to the surface. The surface of specimen containing antistatics is not coated with silver paint, but electrodes are... 

AC impedance relies on the ability to monitor the behavior of a coating that absorbs water by modeling it as a capacitor. By measuring capacitance increase as a function of immersion time, the diffusion coefficient of water and the amount of saturated water within the coating film can be calculated. In addition, any severe under-film corrosion can be monitored by a drastic decrease in the coating resistance and charge transfer resistance. 

Using this equation, the approximate (T — T ) value required for a film of a thermoplastic copolymer to be dry-to-touch, ie, to have a viscosity of 10 mPa-s(=cP), can be estimated (3,4). The calculated (T — T ) for this viscosity is 54°C, which, for a film to be dry-to-touch at 25°C, corresponds to a T value of —29 "C. The calculated T necessary for block resistance at 1.4 kg/cm for two seconds and 25°C, ie, rj = 10 ° mPa(=cP), is 4°C. Because the universal constants in the WLF equation are only approximations, the T values are estimates of the T requited. However, if parameters such as the mass per area appHed for blocking were larger, the time longer, or the test temperature higher, the T of the coating would also have to be higher. 

It is possible to calculate a number of different kinds of "effective" crosslink densities. Bauer et al have used a quantity they termed the "elastically effective crosslink density " (Cel) correlate cure with solvent resistance and other physical properties of coatings (7-10). The correlation was basically empirical. Formally, the is a calculation of the number of functional groups attached to the infinite network for which there are at least two other paths out to the network on the given polymer or crosslinker. Thus, chains with only one or two paths to the infinite network are excluded. The following expression can be written for... 

The polymers were dissolved in methylisobutylketone (MIBK) and spin-coated on oxjdized silicon wafers (1100 X thick Si02 layers) to form 5000 A thick films. After a prebaking to improve adhesion to the substrate, the resist samples were irradiated 0 through the mask (A) using the Al K 152 emission line at 8.3 A as X-ray source. The electron beam gun was operated at a 300 W power and the source to sample distance was U.9 cm. Taking into account the absorption of the aluminum foil mask,the different X-ray fluxes available on the sample were calculated from the relation given by (9) ... 

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

First chemical test measurements have been conducted with the array chip. Figure 6.19 shows the results that have been obtained simultaneously from three microhotplates coated with different tin-dioxide-based materials at operation temperatures of 280 °C and 330 °C in humidified air (40% relative humidity at 22 °C). The first microhotplate (pHPl) is covered with a Pd-doped Sn02 layer (0.2wt% Pd), which is optimized for CO-detection, whereas the sensitive layer on microhotplate 3 contains 3 wt% Pd, which renders this material more responsive to CH4. The material on microhotplate 2 is pure tin oxide, which is known to be sensitive to NO2. Therefore, the electrodes on microhotplate 2 do not measure any significant response upon exposure to CO or methane. The digital register values can be converted to resistance values by taking into account the resistor bias currents [147,148]. The calculated baseline resistance of microhotplate 1 is approximately 47 kQ, that of hotplate 2 is 370 kQ and the material on hotplate 3 features a rather large resistance of nearly 1MQ. 

All the results were confirmed by investigations with both back- and front-side illumination [37]. In this study, which also describes theoretical aspects, a final resolution of 17 pm for an epi-structure was achieved, which consisted of a thin layer (3 pm, specific resistance of lOQcm) and a thick silicon substrate (0.38 pm, with low specific resistance of 0.005-0.02 Q cm). Instead of using a pattern grid on the front-side to study the smallest possible resolution, in this work the sensor chip was coated half with a metal layer. This forms on the covered side a metal-insulator-semiconductor (MIS) structure. The light pointer was moved from the metallised area to the uncovered area and the resolution was determined at the borderline by measuring the photocurrent that depends on the diffusion length of the carriers. Since, for the assumption that the diffusion length is the main decisive and critical parameter for the amount of carriers which could reach the metal-covered part of the semiconductor substrate from a specific distance, the calculation of the minimal resolution was experimentally observed. 

Another method presented in this paper is the indirect eb method when the C -lace of a LiNbOs ferroelectric is preliminary coated by a highly defective layer of the amorphous photo-resist material pmma. The thickness of this dielectric layer is large enough to protect the LiNb03 from penetration of high energy electrons into the bulk. In the presented calculations and simulation a very limited number of electrons penetrated into the LiNbOs crystal, so most of the injected electron charge remains trapped in the pmma layer. 

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