Conductivity probes

The use of a normal conducting probe offers the advantage to apply a mobile head, that is able to work on complex topologies. It is suitable for handhold and mechanized inspection as well. 

Nonintrusive Instrumentation. Essential to quantitatively enlarging fundamental descriptions of flow patterns and flow regimes are localized nonintmsive measurements. Early investigators used time-averaged pressure traverses for holdups, and pilot tubes for velocity measurements. In the 1990s investigators use laser-Doppler and hot film anemometers, conductivity probes, and optical fibers to capture time-averaged turbulent fluctuations (39). 

It turns out that in low-viscosity blending the acdual result does depend upon the measuring technique used to measure blend time. Two common techniques, wliich do not exhaust the possibilities in reported studies, are to use an acid-base indicator and inject an acid or base into the system that will result in a color change. One can also put a dye into the tank and measure the time for color to arrive at uniformity. Another system is to put in a conductivity probe and injecl a salt or other electrolyte into the system. With any given impeller type at constant power, the circulation time will increase with the D/T ratio of the impeller. Figure 18-18 shows that both circulation time and blend time decrease as D/T increases. The same is true for impeller speed. As impeller speed is increased with any impeller, blend time and circulation time are decreased (Fig. 18-19). 

Water-level controls continuously monitor the level of water in a steam boiler in order to control the flow of feedwater into the boiler and to protect against a low water condition which may expose the heating surfaces with consequent damage. The controls may be either float operated or conductivity probes. 

Make-up water. A semi-sealed section within the main tank will feed water from the treatment plant to the main tank either through a separate make-up tank or. In either case, the water level will be the same in both sections, excluding the ullage left for condense returns. Control of the make-up water level may be by float valve, float switches or conductivity probes. These methods allow water to flow through the treatment plant, although conductivity probes or switches permit a positive flow and avoid the risk of slippage. 

A specially built conductivity probe was used to investigate the gas holdup in a novel internal-loop airlift reactor. The gas holdup generally increases with increasing solid holdup due to increased flow resistance. A model based on energy balance was developed that can be used to predict the average gas holdup in this novel interal-loop airlift reactor. 

Electrical conductance probes These can be either flush probes or wire probes. Flush probes are imbedded in a nonconducting wall, with one electrode connected to a voltage source and the second through a precision resistor to ground (Telles and Dukler, 1970 Chu and Dukler, 1974). Wire probes use closely spaced, nearly parallel conducting wires of small diameter, which are positioned normal to the flow (Brown et al., 1978). 

Wold DV, Haag R, Rampi MA, Frisbie CD (2002) Distance dependence of electron tunneling through self-assembled monolayers measured by conducting probe atomic force microscopy unsaturated versus saturated molecular junctions. J Phys Chem B 106 2813-2816... 

Bouwmans (1992 see also Bouwmans et al., 1997) used a particle tracking technique in a RANS flow field to estimate trajectories of neutral and buoyant additions, to construct Poincare sections of additions crossing specific horizontal cross-sectional planes, to predict probabilities of surfacing for buoyant additions, and to mimic the temporal response of conductivity probes. 

The conductivity probe is fragile. Use caution when setting this up in the ring stand. 

Plug the conductivity probe into the adapter cable in channel 1 of the CBL unit. 

Using the 100-mL graduated cylinder, measure 70 mL of distilled water into the 150-mL beaker. Raise the beaker until the conductivity probe is in the water. After the conductivity meter stabilizes, press TRIGGER on the CBL unit. 

Use the conductivity probe to monitor the conductivity of the sodium chloride solution. Record the conductivity in Data Table 2. 

Direct measurement of concentration fluctuations for liquid flow in packed and fluidized beds have been made by Hanratty et al. (H4), Prausnitz and Wilhelm (P12), Cairns (Cl), and Cairns and Prausnitz (C4). Detailed descriptions of electrical conductivity probes used for measurement of these fluctuations have been given by Prausnitz and Wilhelm (Pll) and Lamb et al. (LI). 

Loiacono MJ, Granstrom EL, Frisbie CD (1998) Investigation of charge transport in thin, doped sexithiophene crystals by conducting probe atomic force microscopy. J Phys Chem B 102 1679-1688... 

Since the deviation of device characteristics may come from the local heterogeneity of metal/molecule interfaces, the local characterization of the molecular structure and interfaces is necessary to learn how to alter processes and materials in order to achieve high yield, stable process condition, and low deviations of device characteristics. A few researchers have proposed the use of the scanning probe microscopic technique [conducting probe AFM, surface potential... 

Ring-seals of all kinds tend to be mechanically weak and sensitive to thermal shock. Tungsten-through-glass seals and pockets for thermocouples or conductivity probes are the most frequent instances of ring-seals on vacuum equipment, and these should never be subjected to extreme cold. 

A simple conductivity probe is shown in Fig. 3.20 (KPG). This probe can be used for both rough and precise measurements, but when used for precise measurements of low conductances it may give rise to capacitance effects. To avoid large capacitances, the leads to the conductivity cell should be situated as far apart as possible (see, e.g. Fig. 3.15), and they must be screened. 

Fig. 3.20. Common conductivity probe for vacuum work. A couplings to shielded cables from conductivity meter 5 a B. 19 or B.24 socket fitting onto cone on observation/reaction vessel C graded-seal soda glass to borosilicate glass D shielded leads spot-welded to thick Pt wires E sealed through the soda glass probe and held together by the lead glass bead F.

Fig. 3.24. The Krummenacher reactor. Most of the parts are explained in the text. To start a reaction, the retainer L is withdrawn magnetically so that H drops and breaks P which contained the initiator. The thermocouple C and the conductivity probe (not shown) were linked to the same recorder N. All the taps were all-metal BiPl valves (see Sections 2.2.4.3 and 2.4).

Fig. 3.27. The Pask-Plesch reaction calorimeter, approximately to scale. A phials of reagents, B phial magazine, C cold finger (not essential), D phial breaker, E vessel of calorimeter, F heater, Gj Pt wires of the conductivity probe, Gj terminals, H vacuum jacket, thermometer probe, terminals from thermometer probe and the compensating leads, K tap for evacuation of pseudo-Dewar space or admitting air, M magnetic pusher, T main tube,...

Fig. 9 a Schematic illustration of the measurement with a conducting-probe AFM. b Relationship between resistance and DNA length for poly(G)-poly(C) (dark marks) for poly(A)-poly(T) (empty marks). The exponential fitting plots of the data are also shown, c Typical I-V curves of poly(dG)-poly(dC), the linear ohmic behaviors on I lOO nm at the repeat measurement of five samples, d Rectifying curves of poly(dG)-poly(dC) at L=100 nm (from [68], with permission Copyright 2000 by the American Institute of Physics)... 

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