Order up better measurements

Second-order instruments can provide improved accuracy and reliability for electrochemical and other sensors.

By Larry Berger

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The reliability of the measurement is greatly enhanced by providing a separate diagnostic feature that constantly monitors the deterioration of the liquid junction half-cell and displays the results directly on the instrument or transmits this information to a remote location, as desired. The system shows the degradation as it occurs in the front, or process, chamber of the dual-chambered liquid junction. Monitoring this cell allows detection of contamination/dilution of the reference fluid before it can compromise the measurement made in the primary chamber.

The figure at the top of this article depicts the additional chamber isolating the reference half-cell from the process. It shows the potentials e1 (of the reference half-cell) and e3 (of the measurement half-cell) in their controlled and predictable environments. As the equation indicates, when these potentials are equal, they cancel and don’;t factor in the measurement. The liquid junction acts as an ionic bridge between the reference-controlled environment and the medium that is being measured.

But it is a fluid bridge that allows some interchange of material between the reference chamber and the process; this interchange eventually contaminates or dilutes the material in the outer chamber (illustrated by the color gradient). Contamination/dilution of this chamber compromises the primary reservoir that houses the actual reference half-cell.
The system monitors the outer chamber for contamination, displays the level and alerts the user before any serious error occurs in the measurement.

 
 
Figure 2. The height of the solid bar in the display indicates the deterioration of the sensor. The bar starts to flash when reaching the limit, but before measurement accuracy suffers.

As shown in Figure 2, changes in the outer reference chamber appear as a solid bar on the upper right-hand side of the display. This makes it easy to follow the progressive contamination of the electrode (indicated by the darker color advancing through the chambers), without having to remove the sensor from its monitor/controlling function. Upon reaching full height, which indicates that a predetermined limit has been reached, the bar begins to flash, signaling the need to service the electrode. At this point, changes in the outer chambers have not yet affected the measurement, but portend deterioration that, if unchecked, could lead to erroneous readings, as shown in the last frame.

The potential of second-order instrumentation clearly goes beyond electrochemical sensors. The experience gained with these sensors should alert users to the concept and its benefits and should spur greater consideration of the approach. 

Larry Berger is president of Electro-Chemical Devices Inc., Yorba Linda, Calif.

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