Measuring pH and Conductivity Wirelessly

Update and bump: Greg’s article is now live on the ISA.org InTech website. Below is the original Dec. 15, 2009 post.

I received an advanced copy of an article ModelingAndControl.com‘s Greg McMillan has recently completed, Exceptional Opportunities for Smart Wireless pH and Conductivity Measurements. In the article, he summarizes these opportunities:

…for inferential measurements, solution temperature correction, efficient calibration, noise minimization, and predictive maintenance by taking the advantage of smart features and wireless communication.

On inferential measurement, Greg notes that the connectivity, intelligence and portability of wireless conductivity and pH measurements increase the possibilities for successful inferential measurement creation. He writes:

The availability of the primary process variable (pH), and the auxiliary variables (milliVolts, temperature, reference impedance, glass impedance, and RTD resistance)… for a smart wireless pH transmitter, facilitates the monitoring of sensor performance besides developing relationships for solution temperature compensation, solvent concentration, and CO2 loading.

Inferential models developed within the automation system neural network algorithms use conductivity, pH, and temperature inputs to better predict solvent concentration and CO2 loading.

With respect to temperature compensation, Greg observes that the standard temperature compensation in pH measurement as defined by Nernst equation does not account for actual solution pH changes with changing temperature. Additional solution temperature compensation in smart pH transmitters is beneficial for many applications. Greg shares:

Lab tests where the pH and temperature of the sample are varied to cover the operating range are required to quantify the effect of weak acid and base dissociation constants on solution pH. Smart wireless pH transmitters allow the user to develop, document, and integrate the solution temperature compensation results from lab tests.

Most automation engineers have faced issues with electromagnetic interference (EMI) causing noise on their process measurements. For pH measurements, spikes can be caused by ground loops or the operation of motors and variable speed drives. Wiring to the instrumentation can act as an antenna for this noise. Wireless devices avoid the EMI issues that wiring induces.

Many pH applications are difficult, due to electrode coating, plugging, and aging that can occur in days or weeks. Wireless lab and field pH and conductivity measurements in a lab process sample:

…creates interesting opportunities for predictive maintenance on when to clean or replace electrodes.

The technology team envisions how these smart pH and conductivity measurements could be enhanced with:

…a model for a particular sensor and run a simplified principal component analysis (PCA) within the transmitter to detect a failure.

The article shares specific examples of the team’s work with the University of Texas and their absorber for CO2 capture and distillation column for solvent recovery. I’ll update this post when I know when and where the article will be published. Until then, I hope this gives a brief sample of some of the innovations occurring on the pH and conductivity measurement and control fronts.

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