Potential drop test method

The Direct Current Potential Drop Method for Testing of Powder Metallurgical Parts. 

The Direct Current Potential Drop method (DCPD) has been evaluated for non destructive testing of uniaxially produced Powder Metallurgical (P/M) parts. The aim is to adapt DCPD to be functional as an ndt tool during production of parts. Defects can occur at different stages in the production cycle which means that DCPD has to be performed on components in different states and searching for different defects. 

Both AC and DC potential-drop methods are well-established techniques for monitoring subcritical crack growth. A combined AC/DC potential-drop measuring technique can, in some cases, help in obtaining more information from a single test, in particular for the onset of stable crack growth.172... 

Another important electroehemical corrosion test method is the measurement of current between an area with dominating cathodie reaction and an area with dominating anodic current. This is done by separating the two areas, using two specimens, and then connecting these specimens to each other via a currentmeasuring instrument. To avoid a potential drop in the instrument, a zero-resistance... 

The resistance of membranes can be measured by AC impedance methods [85,86], using the four-point-probe technique. The test membrane is placed in a cell consisting of two Pt-foil electrodes, spaced 3 cm apart, to feed the current to a sample of 3 x 1 cm and two platinum needles placed 1 cm apart, to measure the potential drop (see Fig. 4.3.26). The cell is placed in a vessel maintained at constant temperature by circulating water. The impedance measurements are then carried out at 1-10 kHz using a frequency-response analyzer (e.g., Solatron Model 1255HF frequency analyzer). After ensuring that there are no parasitic processes (from the phase angle measurements, which should be zero), one can measure the resistance directly. The membrane resistance can also be obtained directly from the real part of the impedance (see typical data in Fig. 4.3.27). 

The electrical potential drop method uses the electrical resistance of the specimen to measure the crack length. A constant electrical current is applied between two points of the specimen far away from the crack and the potential drop in the vicinity of the crack is measured. Comparison with a calibration curve allows calculation of the crack length. Obviously, the specimen has to be electrically isolated from the testing machine and the displacement transducer. 

Potential drop is measured across probes inserted into the soil. The resistivity is calculated using constants provided with the particular geometry of soil box being used. Due to the disturbance of the soil during sampling and possible drying out of the soil during shipment, this method of soil resistivity measurement is less likely to represent true, in-place soil resistivity than an actual field test. 

The sessile drop method has several drawbacks. Several days elapse between each displacement, and total test times exceeding one month are not uncommon. It can be difficult to determine that the interface has actually advanced across the face of the crystal. Displacement frequency and distance are variable and dependent upon the operator. Tests are conducted on pure mineral surfaces, usually quartz, which does not adequately model the heterogeneous rock surfaces in reservoirs. There is a need for a simple technique that gives reproducible data and can be used to characterize various mineral surfaces. The dynamic Wilhelmy plate technique has such a potential. This paper discusses the dynamic Wilhelmy plate apparatus used to study wetting properties of liquid/liquid/solid systems important to the oil industry. 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

As part of the optimisation programme, it is important to evaluate the ocular irritation potential of formulation prototypes. The Draize test, established in the 1940s, is the most widely used method for the identification of primary irritants. There have been modifications to the original test, but they all involve instilling a drop of the formulation into the conjunctival sac of one eye of an albino rabbit, the other eye acting as a control. The condition of both eyes is then evaluated after stipulated time periods and scored relative to the control eye. A high score indicates that the formulation is likely to be an irritant and would not be recommended for progression. 

Water penetration rates are usually calculated according to the gas laws from measurement of pressure decay upstream of the filler over the whole period of testing with the gas (air) volume above the fluid held constant. They are therefore subject to temperature variations. Although the principle of the water penetration test is sound, and the avoidance of the use of potentially adulterating solvents is attractive, the low rates of water penetration calculable from only very small pressure drops within test systems have raised doubts about the robustness of the method for routine application in its contribution to the decision-making process. 

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