Mounting, electrodes

Mounting electrodes in a bioreactor is costly, and there is an additional contamination risk for sensitive cell cultures. Some other sensors of prac ticai importance are those for dissolved oxygen and for dissolved carbon dioxide. The analysis of gas exiting from a bioreactor with an infrared unit that detects carbon dioxide or a paramagnetic unit that detects oxygen (after carbon dioxide removal) has been replaced by mass spec trophotometry. Gas chromatographic procedures coupled with a mass spectrophotometer will detect 1 the volatile components. 

Each photoelectrode was retested after mounting and current voltage plots obtained. All photoelectrodes decreased in performance and one became essentially inoperative. This is attributed to excessive handling during mounting which resulted in abrasion and deterioration of the deposit. The maximum power output of the individual mounted electrodes in the assembly varied from 5.TO mW to 0.13 mW. The maximum power obtained from the assembled converter was 27 mW. Under short circuit conditions the maximum power at 90.2 mA and 100 mV was 9 0 mW. The light intensity incident at the photocell was 92.5 mW/cm. ... 

The capillary plasma reactor consists of a Pyrex glass body and mounted electrodes which are not in direct contact with the gas flow in order to eliminate the influence of the cathode and anode region on CO2 decomposition. Analysis of downscaling effects on the plasma chemistry and discharge characteristics showed that the carbon dioxide conversion rate is mainly determined by electron impact dissociation and gas-phase reverse reactions in the capillary microreactor. The extremely high CO2 conversion rate was attributed to an increased current density rather than to surface reactions or an increased electric field. 

Figure 4 Primary current distribution rules applied to the case of a flush mounted electrode with an insulating edge of various angles.

Contact detectors are somewhat easier to miniaturize. There are generally two subtypes those with twin axially mounted electrodes and those with twin or quadruple radially mounted electrodes. The former can be operated in DC mode or AC mode. In the DC mode, the detector signal directly originates from the field strength between the electrodes and, given the current, is inversely proportional to the detector cell resistance. In the AC mode, both axially and radially mounted electrodes form part of a closed primary circuit of an isolation transformer, the output of which is also inversely proportional to the cell conductivity. Alternatively, the output can be linearized with respect to the conductivity. 

After extensive activation both designs achieved good performance 4>eo 5.5eV and Vb at 5A/cm < 2eV. (see Figure 3 below for some JV curves). However given the complexity and expense assoeiated with variable-spacing research converters, a new test vehicle was designed to allow faster iterations Built from demountable standard UHV components, it featured a 7cm stainless steel cube to which various flange-mounted electrodes and instrumented ports could be attached, hence its name - the "cube" converter. 

Figure 1639 Electrical resistance monitoring with surface-mounted electrodes, (a) Surface-mounted electrodes pattern (b) schematic of delamination and matrixcracking phenomena in relation to the electrode placement [92].

Surface resistivity is a measure of the ability to resist the surface current. It is the resistance when a direct voltage is applied between two surfaces mounted electrodes of unit width and unit spacing. The value is expressed in ohms. 

---