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May
26

Improving pH and Conductivity Performance and Maintenance by Wireless

Wireless technology offers the opportunity to select the best electrode for an application. Spare wireless transmitters can be used to test electrodes in lab solutions set to mimic the concentrations and temperatures of the process. The process lab offers a controlled environment to vary the process conditions of interest and keep other conditions constant. The first through fourth variables of smart wireless transmitters are readily available for historization and analysis in the distributed control system (DCS). The slope, offset, and resistance of the electrodes can be studied. The accuracy and response time can be measured over an extended period of time to determine the performance and life of various electrode designs. For example, new high temperature glass formulations increase the life expectancy and prevent the deterioration of electrode response time from premature aging of the glass electrode.

Process temperature changes the dissociation constants and mobility of the ions and thus the actual solution pH and conductivity. For complex mixtures, the best way to determine the effect of process temperature is to vary the process sample temperature over the expected operating range. The change in the water dissociation constant (pKw) with temperature causes a change in solution pH that increases as the pH approaches the pKw (Figure 3). Smart wireless transmitters have the ability to configure solution temperature compensation. However, lab meters often cannot provide more than the standard temperature compensation for the electrode per the Nernst equation. Wireless transmitters could be used instead of lab meters for standardization with grab samples, the preferred method of calibration. Standardization is safer and more effective since it does not require removal of the process electrodes eliminating the associated risk of personnel exposure and damage to the glass, and disruption of the reference junction equilibrium.

The portability of wireless conductivity and pH measurements offer the opportunity to find the best location by plant trials. If several process connections are provided, the connection that offers the most representative measurement with the least noise and deadtime can be found. Noise is minimized by finding the location with the best mixing and least bubbles. Deadtime is minimized by reducing the transportation delay and increasing the velocity at the electrode, decreasing the electrode lag and coatings.

For more information checkout the paper to be presented at the ISA Water & Wastewater and Automatic Controls (WWAC) Symposium June 22 in Saint Louis Wireless-Measurement-and-Control-Paper. The paper also presents test results for wireless pH and temperature control of a bioreactor at Broadley-James Corporation and the development of inferential sensors for CO2 capture at the University of Texas Pickle Research Campus.