Electrodeless conductivity

The use of chemical analysis to monitor the quality of the raw materials or finished products of industrial processes goes back a long way.313,314 Indeed, some techniques owe their development to the need of industry for rapid analytical techniques. However, analytical methods are now often intimately bound up with the production itself, and supply much of the information required for the control and regulation of the process.315 A good example of a continuous monitoring technique that can be used in process control is that of electrodeless conductivity measurement its history has been described.316 A history of early industrial pH measurement and control systems has been given.317... 

Conductimeters used for process analysis are as simple as those using a Wheatstone bridge as a detector or as sophisticated as electrodeless conductivity systems. In addition to those described above, they are employed for a... 

Figure 8 Representation of an electrodeless conductivity measuring circuit. (From Light TS and Ewing GW (1990) Measurement of electrolytic conductance. In Ewing GW (ed.) Analytical Instmmentation Handbook, pp. 641-658. New York Dekker.)...

The electrodeless conductivity technique is used mainly in the chemical process industries for continuous monitoring where its freedom from maintenance is superior to contacting techniques. The smallest electrodeless probe is 3.6 cm diameter,... 

Figure 10 Digital electrodeless conductivity instrument used for online process measurements. (Courtesy of The Foxboro Company, Foxborough, USA.)...

Numerous applications of electrodeless conductivity sensors have been published for the chemical, pulp and paper, aluminum, mining, and food industries. Similar instrumentation has been used for in situ measurements of the salinity of seawater. An instrument for continuous analysis of oleum in the range of 100-102% equivalent sulfuric acid, with an accuracy of 0.01%, is illustrated in Figure 11. A historical review has been published by Light (see Further Reading). 

Figure 11 A continuous analyzer for oleum using an electrodeless conductivity sensor. (From Shavtr R and Light TS (1982) Online analysis of oleum using electrodeless conductivity. ISA Transactions 21 63.)...

Light TS (1989) Electrodeless conductivity. In Stock JT and Oma MV (eds.) Electrochemistry, Past and Present, pp. 429—441. ACS Symposium Series 390. Washington, DC American Chemical Society. 

Light TS, McHale EJ, and Fletcher KS (1989) Electrodeless conductivity. Talanta 36 235-241. 

An electrodeless conductivity instrument can measure the depleted acid concentration with reasonable accuracy in the range of 65-80%. Above 80% H2SO4, inflections in the curve of conductivity vs concentration make the measurement less reliable. 

A specific gravity meter uses a pneumatic detector and the nitric acid concentration meter is an electrodeless conductivity meter. Eight-inch-long, one-lnch-diameter BF, detectors and incline Nal gamma monitprs are used routinely as process and waste stream and Unk monitors. 

A second method of measuring the conductance without the use of contacting electrodes has become popular, especially in the chemical process industries. Usually referred to singly as "electrodeless conductivity", it has also been called "inductive" or "magnetic" conductivity. This method is the subject of this paper and is described below. 

Although instruments for electrodeless conductivity measurement have been commercially available since the 1950 s for process industry applications, literature review of this subject is lacking. This paper will review and discuss the history and applications of electrodeless conductivity. 

Single representation of an electrodeless conductivity measuring circuit. 

Limitations. The electrodeless conductivity technique using low-frecjuency inductive cells is available for analysis and control in the chemical process industries and in other continuous monitoring applications. Although its stability, freedom from maintenance, and accuracy, are superior to contacting conductivity techniques, lack of bench models of this type has hindered its laboratory use and application to date. 

One of the reasons that electrodeless conductivity is not favored as a laboratory tool is due to the size of the probes and the sample size requirement. The smallest electrodeless probe is about 3.6 cm in diameter and has an equivalent cell constant of 2.5 cm". It requires a minimum solution volume of several hundred milliliters to ensure a complete solution loop without wall effects which distort the apparent cell constant. For a large probe of 8.9 cm diameter, the cell constant is 0.45 cm and an effective solution volume of several liters may be needed. For the lower conductance ranges, which recjuire a smaller cell constant, the diameter of the probe and measuring container must increase. 

Sperry patent illustrating electrodeless conductivity cells externally mounted to the solution being tested (24). 

The earliest application of electrodeless conductivity measurements appears to have been for measurement of salinity at various ocean depths (1, 11, 45, ). Other early uses have also included determination of the equivalent conductances of salts at high concentrations (3, 34, 53) and the monitoring of nitric... 

Electrodeless conductivity is a technicjue for measuring the conductance of a solution using the electrical inductance principle at low frequencies. This method does not use contacting electrodes thereby eliminating maintenance and other errors due to surface effects created by coatings and fouling. The measurement enjoys widespread acceptance in the chemical process industries. Its history and a review of the literature have been presented here. 

E.M. Ormrod, G.T.W. Lombaard, S.L. An Electrodeless Conductivity Meter of Improved Sensitivity and... 

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