Electrometers controls

Figure 1. Kinetics of K efflux from Chlorella under Og stress. External K measured with cation-specific electrode (Beckman Instruments, Fullerton, model 39137) with 10" cells/ml suspended in a 10 mM Tris-Cl, ImM CaCU, pH 9 solution (10 ml, total volume). Control efflux (0—0) is linear after 20 min (denoted as 0 time here). The addition of Og ( — ) for 30 min and continuous Og (O—O), 26 fimoles/liter air flow (25 cc/min) are shown. The electrode output is amplified by an electrometer coupled to an antilog converter (12,13).

The direct-current (DC) volt-ampere (V-A) characteristics were measured with a microcomputer-controlled Keithley 617 electrometer in the temperature range 10 to 60 °C. From linear part of V-A dependences the resistance ffl - U/I was determined [32], where U is the voltage on the needle electrodes and I is the current passing through the sample. 

Figure 6.27. Digital electrometer for processor-based gas chromatograph. Digital feedback loop provides autoranging, eliminating the need for balance control s. ... 

A test system, controlled by personal computer (PC), was developed to evaluate the performance of the sensors. A schematic of this system is shown in Figure 3. The signals from the sensors were amplified by a multi-channel electrometer and acquired by a 16 bit analog to digital data acquisition board at a resolution of 0.0145 mV/bit. The test fixture provided the electrical and fluid interface to the sensor substrate. It contained channels which directed the sample, reference and calibrator solutions over the sensors. These channels combined down stream of the sensors to form the liquid junction as shown in Figure 1. Contact probes were used to make electrical connection to the substrate. Fluids were drawn through the test fixture by a peristaltic pump driven by a stepper motor and flow of the different fluids was controlled by the pinch valves. 

Chromatography. Gas chromatographic analysis (Tracor, Model 220) was performed with a 1.8 m 4 mm i.d. "U" column packed with 1.5% SP-2250/1.95% SP-2401 coated on Supelcoport 100/120 support. The oven temperature was 220°C, and the inlet and detector temperatures were 250 and 350°C, respectively. The gas flow was 60 mL/min with 95 5 argon methane. Detection was accomplished with a Ni° electron capture detector controlled by a linearized electrometer (Tracor). Output signals were processed electronically (Varian CDS-401). 

Fig- 4.25 Components used to impose and monitor conditions providing cathodic protection by an impressed external current. Note Power supply may be either a galvanostat or a potentiostat. In the latter, the electrometer provides feedbackto the potentiostatto control to constant potential. Electrometer provides check to show that the metal is at the protection potential. 

It is not obvious why (13.1.31) is called an electrocapillary equation. The name is a historic artifact derived from the early application of this equation to the interpretation of measurements of surface tension at mercury-electrolyte interfaces (1-4, 6-8). The earliest measurements of this sort were carried out by Lippmann, who invented a device called a capillary electrometer for the purpose (9). Its principle involves null balance. The downward pressure created by a mercury column is controlled so that the mercury-solution interface, which is confined to a capillary, does not move. In this balanced condition, the upward force exerted by the surface tension exactly equals the downward mechanical force. Because the method relies on null detection, it is capable of great precision. Elaborated approaches are still used. These instruments yield electrocapillary curves, which are simply plots of surface tension versus potential. 

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