Contact separation current, measures

Table 24.1 Results of Contact Separation Current Measurements ...

Electrical connections for resistivity measurements usually include separate current and voltage leads and the resistance is measured by determining voltage changes with a constant current source across the sample. This technique avoids spurious voltages at the contact points of a two or three contact connection but introduces a consideration of current path separate from the voltage measurement that can lead to faulty measurements. 

Completing a circuit and measuring the current that flows on contact or separation can quantify the transfer of electrons [2]. A typical result on contact electrification is shown in Figure 24.3. The time constants associated with the current peaks can be adjusted by inserting a series resistance in the measurement circuit. In real time, the equilibration of surface state electrons occurs instantaneously. In these experiments, the plasma polymer of tetrafluoroethylene (TFE) was deposited on two different substrate, nylon film and polished brass, and the contact and separation currents were measured with a (uncoated) brass probe... 

Separation of the cathodic and anodic components of the net current (measured at the end of forward and backward pulses) in square-wave voltammetries (SQWVs) provided only anodic components for PTA Y electrodes immersed into BU4NPI ),/ MeCN, as depicted in Figure 8.15. In contrast, SQWVs display well-developed anodic and cathodic components for PTA Y electrodes in contact with LiClO4/ MeCN. This feature, indicative of reversible electron transfer processes, was found to be more pronounced on decreasing square-wave frequency. 

When low resistance values (< 10kf2) are being measured, series resistance errors may arise from the leads connecting the electrodes to the "resistance meter" or from the contact resistances between the electrodes and the test material. Both may be eliminated by using "four-electrode" measuring systems where the electrodes and leads used for current and potential connection are completely separate. Obviously, the meter must be one designed for four-terminal measurement and will employ the "fixed current.measured potential" system. 

This simplification is only permissible with very low-resistance contact and if no other current flows in the pipeline. Currents flowing in the pipeline outside the measured span must be measured separately and taken account of in the calculation. This follows also indirectly when locating a contact with an unknown line. Figure 3-21 shows the necessary pipe current measurements on either side of the supposed contact point which, with U = t/C = IR, gives the following calculation of the fault distance [28] ... 

The four-point probe method can be used to measure the in-plane electronic conductivity of the GDM, as shown in Figure 4.10. Two copper strips are placed near the ends of the GDM, and two copper wires separated a distance of L are placed befween the two strips. The strips and the wires are pressed against the GDM to form good contacts. A current passes from one sfrip fhrough the GDM along the in-plane direction to the other strip, and the voltage drop between the two wires is measured. Then the in-plane conductivity is... 

The simplest way to measure the resistivity of a solid material is to cut a bar-shaped sample, and to make separate current contacts on the short edges and voltage contacts on the surface, as shown in Fig. 13.16. The use of four terminals ensures that the measured voltage does not include the voltage drop due to the current contacts. The resistivity is given by ... 

Calculating the resistance of each current-carrying component separately is a very cumbersome and lengthy procedure, in addition to being not very accurate due to the large number of approximations. Some of the joints and components may still have been omitted from these calculations. The easier and more often recommended procedure is to measure the resistance between the extreme ends of each feeder in its ON condition by an Ohm-meter. This resistance will also include the contact resistance of each terminal and joint. 

Figure 11 -7 shows the basic circuit diagram for a tank with two domes. The protection current flows via the two interconnected openers of the cover grounding switch to the cathode connection. If one of the covers is opened, the protection current circuit is broken and the tank grounded via the closing contact. The unconnected cable connection of the tank is without current and can be used for measuring potential. By this method, only one tank at a time is separated from the protection system while the other parts of the installations are still supplied with protection current. 

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

No attempt will be made in this section to consider all the separate measurement clauses to be found in current test method standards. Until the ISO standard for the measurement of dimensions has become established long enough for all test methods to have been revised and reference it (if that happens), each test method will have its own procedure and there will not be universal agreement on detail. The essentials are to distinguish between a non-contact measurement and one applying a specified pressure, in the latter case to use the correct standard pressure, and to measure within the accuracy limits specified. 

In this recently developed device shown in Fig. 5.25 [30], two immiscible liquid streams converge in a micro-channel and then separate. A well-defined flow has been demonstrated, with no entrainment of the opposed flows, for flowrates from 10 4 to 1.0 cm3 s 1. The highest flow rate corresponds to solution contact times around 1 ms, corresponding to a kL of the order of 0.1 cm s 1. Thus, the confluence microreactor has the potential to measure liquid-liquid interfacial rate constants much higher than those accessible by current methods, and interesting applications are anticipated. The device itself has been patented [31]. 

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