Chemical Processing - July 2008 - (Page 35)

-100 Silver/Silver chloride reference + Ionic cloud Potassium chloride + + - Reference junction Reference junction ++ - + + - KCI electrolyte + H+ - Figure 3. Ionic bridge between measurement electrode and reference junction completes the circuit. Aging of the pH sensors. A new probe when calibrated will have 0 mV offset at pH 7.0, and a 59.3 mV/pH slope. As the probe ages, both the offset and slope will change. The altered slope may translate into sluggish response to changes in pH. This will call for frequent calibration. Figure 6 explains the aging process of pH sensors and how it impacts the accuracy of pH measurement. Galvanic interference — the ground loop. A static buildup in the process fluid may produce an electrical potential that affects the mV output of the sensor. This mismatch in potential starts at the pH electrode’s measuring tip and travels through the cable into the pH transmitter’s input circuitry. A ground loop can cause the following problems: • an offset from the actual pH reading; • a drift of the actual pH reading. This could be an increasing or decreasing drift and can start and stop at any time; and • saturation of the signal to a high or low level, or to a level anywhere in between. You can confirm galvanic interference by placing the sensor in a plastic or glass beaker of process sample and then grounding the solution to the process piping. If the pH changes by more than ±0.1 pH, a ground loop is present. Calibration and storage. pH sensors can only read accurately when correctly calibrated. Proper calibration demands known concentrations of buffers and stable temperature and humidity conditions — ideally, conducted in a lab environment by a trained technician. As already noted, don’t store the sensors in DI water. Instead, use 3 molar KCl (saturated KCl solution) at room temperature. Hypoionic processes. In water purification processes such as reverse osmosis (demineralization) or distillation, water becomes extremely low in ionic strength. Measuring pH on such processes becomes vary challenging due to Meet your plant’s performance objectives with Chesterton® high reliability sealing Depend on the sealing experts at Chesterton to design a detailed plant-wide sealing program that meets the sealing priorities for your facility. Our knowledgeable and experienced specialists and application engineers can help you meet goals for emissions control, increased Mean Time Between Repair, and reductions in total maintenance costs. We offer a comprehensive line of seals, from high technology gas barrier models to cost-saving cassette designs, all backed by Chesterton’s engineering expertise. Our unique combination of innovative seals and services can be tailored for maximum impact to meet your plant’s performance objectives for high reliability sealing. Please visit our website at www.chesterton.com/ms to find your local service office. © A.W. Chesterton Company, 2008. All rights reserved. 21789 http://www.chesterton.com/ms http://www.chesterton.com/ms

Table of Contents Feed for the Digital Edition of Chemical Processing - July 2008

Chemical Processing - July 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Who’s a Big Hit? Succeed with Condensate Control Take Some Basic Steps with pH Measurements Steam Projects Provide Fast Payback Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds/Ad-Lits Ad Index End Point

Chemical Processing - July 2008

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