Basics of industrial process measurements 2

Prologue: The articles here covers basics of our industrial instrumentation and control and though relevant information may be primary information in general to most of our esteemed professional colleagues. But these are the basics on which our automation and instrumentation control structure expands upwards. That's the reason it is discussed here to strengthen the base for those who don't have clear idea or for jogging the memory of more professional friends, taking them back in time when we learned these basics. However your continued feedback is important to improve the newsletter articles, and to add missing details where some people find it worth mentioning. Thank you all who are reading the series of articles and providing your valued feedback. One thing I would like to request is that please use the comments section for your input, rather than approaching individually so the concerns are shared among fellow subscribers.

In our last article, we have seen measurement basics for density, which is a prime measuring parameter in the industry after four major parameters which are flow, temperature, pressure and level. Additionally we have to measure the pH and conductivity in many industrial applications on line. So let's see what are these parameters and how these are measured in industry.

Have you ever done a litmus test. Though in English it is used literary to ascertain a true statement, but in actual litmus paper is one of the early method to check for the acidity or alkalinity of any liquid. The change in color of blue litmus paper to red indicates the liquid or solution is acidic. Similarly a red litmus changes its color to blue in an alkaline liquid or solution which will determine if the solution or liquid is acid or alkaline and of how much degree. Since it is not feasible for on line monitoring, different process is used.

pH or potential of hydrogen, as it stands in its full form is the measurement of how acidic or alkaline a fluid is. It has a scale of zero to fourteen. (0 - 14) Zero is the most acidic and 14 is the most alkaline. The neutral fluid (like pure water) which is neither acidic or alkaline (basic) has a pH value of seven, which falls in the middle of this range. Because the physical insignificance of hydrogen Ion presence, the definition of pH may vary for different measurement methods. One more thing worth mentioning is that the value of pH is not a linear relationship. Rather it increases or decreases logarithmically. that is when a pH scale decreases by 1, the fluid acidity increases ten times and vice versa.

In most of the measuring pH meters, a porous glass electrode is used. The method employs comparing hydrogen activity in a solution, comparing it with a reference solution and the resultant potential difference of the two electrodes is converted in the measurement scale to indicate pH. Earlier reference electrodes used mercury-mercurous chloride (calomel), but silver/silver chloride electrodes are used.

When these are immersed in a solution together, which ensures both electrode remain at same temperature, the Hydrogen ions create an electromotive force, which is compared with the fixed amount of electromotive force produced by the reference electrode. The resultant EMF or electromotive force will determine if the solution is alkaline or acidic, based on the Hydrogen activity in the solution.

Both analog and digitally displayed methods of pH measurements show the reading. In some processes it is a continuous reading being displayed whereas it measures spot readings when a portable meter is used. In laboratories also, these meters form an important part of its setup where pH measurement of different chemicals is measured. In industry, boiler feed water, water treatment plants, and many more industrial processes, pH plays a crucial role and should be contained within a specific range to avoid fluid properties going off-spec. Here continuous sample monitoring is carried out t ascertain that the product remains within the set range.

Another important parameter which is measured in industrial instrumentation is the conductivity. The term conductivity here refers to electrical conductivity. So we must not confuse with thermal conductivity here. Why measuring the conductivity is important?

Because it helps us in determining how much a liquid contains dissolved particles. For example pure water will conduct less current, but when salts or other impurities are added, the conductivity increases and it helps in determining how much percentage of dissolve solids are present in a particular solution. Thus a conductivity measurement will determine how conductive a solution or liquid is?

The basic principle is to immerse electrodes in the solution, and an enclosed cell is used where the two plates are fixed at a certain distance inside. The amount of current passing across the electrodes, is directly proportional to the amount of dissolved ions, or particles. The geometry of cells, potential difference applied across the electrodes, surface area of the electrodes plays an important role to determine the conductivity of any fluid.

Conductivity measurement is done in industry, in laboratory or on spot samples as well as a continuous monitoring parameter in water treatment, waste water and boiler feed water processes mostly. RO plants, and filtering plants are using these instruments also.

We have seen here some of the variables which are measured, monitored and controlled in various industries. Some we will be taking up in our other articles.

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