Metal resistivities

If the temperature range of interest is large, say 1 to 400 K, then diode thermometers are recommended. Diodes have other advantages compared to resistance thermometers. By contrast, diode thermometers are veiy much smaller and faster. Bv selection of diodes all from the same melt, they may be made interchangeable. That is, one diode has the same cahbration cui ve as another, which is not always the case with either semiconductor or metallic-resistance thermometers. It is well known, however, that diode thermometers may rectify an ac field, and thus may impose a dc noise on the diode output. Adequate shielding is required. 

The.se arc similar to stator resistance starters, as discussed in Section 4.2..3 and can be used in the rotor circuit to control the rotor side resistance. Figure 5.10 shows the smooth variation of resistance by electrolytic vaporization compared to a conventional metallic resistance variation. I hc self-variable resistance of electrolyte is equivalent to almost three or four steps of a metallic resistance and makes such starters economical. Normally one step is sufficient for motors up to 160 h.p. (For spccd-torquc... 

Thermistors are temperature-dependent resistances, normally constructed from metal oxides. The resistance change with temperature is high compared with the metallic resistances, and is usually negative the resistance decreases with temperature increase. The temperature characteristics are highly nonlinear. Such thermistors, having a negative temperature coefficient, are called NTC thermistors. Some thermistors have a positive temperature coefficient (PTC), but they are not in common use for temperature measurement. 

The measurement range of a thermistor is dependent on the probe type, typically -100 to +300 °C. The stability is not as good as that of metallic resistances. Thermistors are not standardized like some of the metallic probes. The thermistor has the advantage of a high change of the resistance with temperature. A very wide variety of sizes and shapes and a low price makes them attractive in relation to the metrological performance. 

Ocelli, M. L., Metal-Resistant Fluid Cracking Catalyst Thirty Years Of Research, ACS Symposium Series, No. 52, Washington, DC., 1990, p. 343. 

The values of /"fh and ffc (the film resistances for the hot and cold fluids, respectively) can be calculated from the Dittus-Boelter equations previously described and the wall metal resistance / from the average metal thickness and thermal conductivity. The fouling resistances of the hot and cold fluids /"dh and are often based on experience, but a more detailed discussion of this will be presented later in this chapter. 

Plants handling aqua regia Aqua regia is used extensively in the extraction and refining of the precious metals, and tantalum, as one of the few metals resistant to this medium, is used for dissolution/evaporation pans, reactor lids and all immersed ancillary equipment. 

Naz N, HK Young, N Ahmed, GM Gadd (2005) Cadmium accumulation and DNA homology with metal resistance genes in sulfate-reducing bacteria. Appl Environ Microbiol 71 4610-4618. 

Nies DH (2003) Efflux-mediated heavy metal resistance in prokaryotes. FEMS Micobiol Rev 27 313-339. 

Sarret G, L Avoscan, M Carriere, R Collins, N Geoffroy, F Carrot, 1 Coves, B Gouget (2005) Chemical forms of selenium in the metal-resistant bacterium Ralstonia metallireducens CH34 exposed to selenite and selenate. AppZ Environ Microbiol 71 231-2337. 

Silver S, LT Phung (1996) Bacterial heavy metal resistance new surprizes. Annu Rev Microbiol 50 753-789. 

In microbes without a permeability barrier, or when the barrier fails, a mechanism must be in place to export metals from the cytoplasm. These active transport systems involve energy-dependent, membrane-bound efflux pumps that can be encoded by either chromosomal- or plasmid-borne genes. Active transport is the most well-studied metal resistance mechanism. Some of these include the ars operon for exporting arsenic from E. coli, the cad system for exporting cadmium from Staphylococcus aureus, and the cop operon for removing excess copper from Enterococcus hiraeP i9A0... 

Some metals can be converted to a less toxic form through enzyme detoxification. The most well-described example of this mechanism is the mercury resistance system, which occurs in S. aureus,43 Bacillus sp.,44 E. coli,45 Streptomyces lividans,46 and Thiobacillus ferrooxidans 47 The mer operon in these bacteria includes two different metal resistance mechanisms.48 MerA employs an enzyme detoxification approach as it encodes a mercury reductase, which converts the divalent mercury cation into elemental mercury 49 Elemental mercury is more stable and less toxic than the divalent cation. Other genes in the operon encode membrane proteins that are involved in the active transport of elemental mercury out of the cell.50 52... 

Genome-Based Approaches to Investigate Microbial Metal Resistance Mechanisms... 

Nies, D.H., Microbial heavy-metal resistance, Appl Microbiol Biotechnol, 51 (6), 730-750, 1999. 

Silver, S., Plasmid-determined metal resistance mechanisms Range and overview, Plasmid, 27 (1), 1-3, 1992. 

Rouch, D.A., Lee, B.T.O., and Morby, A.P., Understanding cellular responses to toxic agents A model for mechanism-choice in bacterial metal resistance, J Ind Microbiol and Biotechnol, 14 (2), 132-141, 1995. 

Nies, D.H. and Silver, S., Ion efflux systems involved in bacterial metal resistances, J Ind Microbiol... 

Hu, N. and Zhao, B., Key genes involved in heavy-metal resistance in Pseudomonasputida CD2, FEMS Microbiol Lett, 267 (1), 17-22, 2007. 

Mergeay, M., Towards an understanding of the genetics of bacterial metal resistance, Trends Biotechnol, 9 (1), 17-24, 1991. 

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