(a) Discussion of Applications. Electrical conductivity of a solution is a measure of all ions present. Pure water is a very poor conductor. The conductivity of a water solution is, in practice, almost exclusively due to ions other than the hydrogen (H+) and hydroxyl (OH ) ions. Figure 4.55 shows the conductivity of certain electrolytes as a function of their concentration in water.

Most practical applications of electrical-conductivity measurements fall into one of the following categories

1. Concentration in simple water solutions. Common examples are sodium chloride, sodium hydroxide, and sulfuric acid In such cases the concentration—conductivity curve must be known in advance, or the system must be experimentally calibrated.

2. Boiler steam quality detection. The exact nature of the electrolyte is usually less important than its magnitude. Nuclear installations require extremely pure feedwater

3. Measuring the extent of a reaction. Reactions such as precipitation, neutralization, and washing soluble electro­lyte from insoluble materials can be monitored by con­ductivity measurements. These procedures require calibra­tion or a comparison between conductivities of streams before and after the reaction.

4. Detecting contaminations. Leaks in heat exchanger with the resultant contamination. Any sudden change in conductivity of the heat-exchange medium is taken as leakage. Salt-water contamination of freshwater can be detected as well as breaks in condenser tubes.

(b) Measurement Methods. Measuring Circuits The a-c Wheatstone bridge circuit (Fig. 4.5 3) is the most widely used technique. It is sensitive, stable, and accurate. An ohmmeter circuit (Fig. 4.56) can also be used. The current is a function of cell resistance, the system is sensitive to

Fig. 4 56—Conductivity-measuring system using a simple ohmmeter circuit. (From D M. Considine, Process Instru­ments and Controls Handbook, p 6 163, McGraw Hill Book Company, Inc, New York, 1957 )

voltage variations In addition to these circuits, an a-c crossed-coil electrodynamometer can be used in a conduc­tivity-measuring circuit. One of the two crossed moving coils responds to the current flow in the conductivity cell circuit, the other responds to the source voltage. It is a relatively simple technique, but it is not as accurate or sensitive as the Wheatstone bridge.

Conductivity Cells. The first criterion m selecting a conductivity cell is that the cell constant must be such that the resistance of the solution under test falls within the limits of the cell range. When high electrolytic resistance is being measured, as in the determination of steam purity, a capacitative impedance in series with the cell has a negligible effect on bridge readings, on the other hand, capacitative impedance in parallel impairs the sharpness of bridge balance. Impedance varies inversely with frequency. Therefore, low bridge frequencies are desirable when measuring high resistance. A relatively low cell constant, such as К = 0.1, has large electrodes close together and is suitable for measuring high resistance systems. Spreading the plates apart and constricting the electrolyte cross section increases the cell constant.

The mechanical features of conductivity cells are illustrated in Fig. 4.57. There are four basic types: dip cells, designed for dipping or immersing in open vessels; screw-in cells, designed for permanent installation in pipelines and tanks; insertion cells with removal devices, designed to permit removal of the element without closing down the line in which they are installed; and flow cells, glass or plastic with internal electrodes close to the wall to offer little resistance to the flowing medium. (In small sizes, the flow-cell tubes are connected to the system with rubber or plastic tubing; in large sizes standard pipe flanges are used.)

Temperature, flow velocity, and presence of solids have significant effects on conductivity-cell performance. Tem­
perature should be held as nearly constant as possible. The conductivity of most solutions increases about 2.5% for each 1 C rise in temperature. The flow velocity should be sufficient to ensure circulation of liquid between the electrodes. Entrained solids and high velocity increase the scouring effect. Low velocity can result in the accumulation of solids and the plugging of the cell chamber.

Chemical considerations are important. Strong electro­lytes, such as hydrochloric acid, can slowly dissolve platinum electrodes. Tantalum or graphite electrodes should be used. Hydrofluoric acid measurement requires cells of tetrafluoroethylene and platinum. Conductivity measurements of condensed steam or demineralized water

can be made with borosilicate glass, dense ceramic, and most plastics. Types 304 and 316 stainless steel, nickel, gold, and platinum structural parts and electrodes are suitable. The use of fluxes in the fabrication of electrodes should be avoided so that there will be no subsequent contamination by leaching of electrolyte material. Avoid contamination by eliminating such items as pipe-joint compounds and dopes. Thoroughly wash items after chemi­cal cleansing or replatimzation.

Temperature compensation is particularly important. As shown in Fig. 4.58, the conductivity of sodium chloride solutions is temperature dependent. At uniform concentra­tion the conductivity increases about 2.5%/C. The most common means of temperature correction is the inclusion in one of the bridge arms of an adjustable resistor calibrated in temperature units. The calibration is based on an average temperature coefficient of conductance. This technique is used where variations in temperature are small. A knob is set to correspond to the temperature reading at the conductivity cell. It is best to avoid frequent knob settings by maintaining a constant sample temperature external to the cell by the use of throttling valves. Automatic tempera­ture-compensation methods can be used. A second conduc­tivity cell dipping into an isolated sample forms the variable-resistance arm of the bridge. Changes in tempera­ture will affect both the thermal cell and the measuring cell to the same extent, canceling out the temperature effect. Other automatic temperature compensators include bi­metallic strip electrodes, expanding or contracting metallic bellows coupled to the variable resistor, a rising mercury column in a special thermometer to shunt the standard arm of the bridge, a resistance thermometer that automatically adjusts the standard arm resistance, and thermistors of high negative temperature coefficient.

(c) Sources of Error. Errors m conductivity measure­ments may be attributable to

1. Insufficient circulation. Sluggish response is a symptom.

2. Contaminated cell. Sluggish response to great concen­tration changes.

3. Need of electrode revitalization. Characterized by broad null point or stepwise change in recorder.

4. Electrical leakage in conductivity cell, characterized by erratic results.

5. Leaching of electrolytes. Characterized by drift toward higher conductance.

6. Temperature errors. Characterized by drifting when concentration is known to be constant

7. Reference temperature. Characterized by inability to obtain check reading from two different instrument sys­tems even though each bridge and cell checks out against data.

8. Bridge calibration. Bridge will not check fixed resistor values. Check resistors in bridge circuit.

9. Change of cell constant. Characterized by inability to obtain correct instrument reading in known solution.