What Is the Most Useful Measurement?

Compiled by Mike Henley 



(Editor’s note: This column is based on recent discussions in the LinkedIn Ultrapure Water Group. This column seeks to accurately reflect comments from each contributor. On occasion, there may be the need to edit contributor comments for clarity or length. Readers are invited to join the Ultrapure Water Group and to participate in discussions. An important purpose of the group is to provide a forum for practical examination of issues facing endusers of high-purity water.)


Useful Measurement?

Mike: “For controlling a high-purity water system, what is the most useful measurement for overall operation? Why?”

Mark: “Resistivity, total organic carbon (TOC), particle counts, silica, ionics, and trace metals can be useful.”

Mike: “Why these particular measurements?”

David: “For continuous in-line, real-time monitoring of a high-purity water system, conductivity/resistivity provides a relatively sensitive and modest-cost measure of water purity. It responds to all ionic impurities with sensitivity to the low-ppb range and is generally useful throughout the treatment system to monitor the various stages of reverse osmosis (RO) and deionization (DI). However, it cannot be used to identify the particular contaminants without knowledge of the system. There are various off-line technologies that can provide greater specificity and sensitivity but at much higher cost and delay in obtaining results.

In addition to conductivity/resistivity for ionic species, on-line TOC measurements can be useful to detect organic contaminants, many of which are not ionic and conductive. Various technologies are used for this measurement, depending on the range of TOC and the background of the sample.”

Mark: “You really need to first consider the UPW application, be it power generation for steam (where sulfate and silica are important) versus pharmaceuticals where endotoxins are important, versus microelectronics where boron, silica, trace metals, and particles are very important. Guidelines especially for semiconductor fabs can be accessed at: As is true for semiconductor manufacturing, other industries also have their own specific specs.

Mohamed: “For controlling and monitoring the UPW specification and limits, the most common parameters to be analyzed are resistivity rather than conductivity for accurate measurement of UPW. Turbidity, total suspended solids (TSS), TOC, sodium, silica, metal traces, and iron measurements can also be useful, depending on the application.

In-line, real time monitoring of UPW is time saving and minimizes cost.

As mentioned earlier, off-line measurements that can give a specific measurement but they will be time consuming, where sometime a rapid analysis is required for continuous monitoring and controlling of the system in power plants, semiconductors, and pharmaceuticals.

Cross checking with on-line analysis is also required for more confirmation and minimization of measurement errors. Examples would include ICP mass spectroscopy for measurement of cations such as iron, copper, lead, and ion chromatography for measurement traces of anions such chloride, phosphate, and sulfate.”

Nissan: “Useful versus Compendial. David is correct that the most useful measurement is conductivity as this gives you an overall perspective of the material content and electrolytic and ionic content of the water. It also defines the purity classification of the water. However, conductivity/resistivity is not the only measurement for Compendial compliance. These are the reasons for TOC microbial and endotoxin in pharmaceutical Water for Injection (WFI). Overall, conductivity is the most useful measurement as it denotes the measurement of all ionic species in the water.”

Training Needs

Mike: “What are training needs in the UPW community? Here are some examples: RO fundamentals? Microbial control in high-purity systems? Ion exchange and electrodeionization (EDI) basics? Troubleshooting?”

Mark: “Trace 10-nm particle counting and identification.”

Nikhilesh: “It is a difficult question to map one’s training needs. If one is associated with UPW at a senior level, he needs to know a great deal of every aspect of the treatment technologies, plus have a good knowledge of the market. On the shop floor side, the individual should have good basic knowledge of the job he is doing.”

Mike: “From the standpoint of the technicians (shop floor personnel), what are areas you would identify as important to attain a grasp of the overall water system operations? As an example, RO operation and control?

Nikhilesh: “I would feel following are important to know for a RO technician: water chemistry basics, osmosis and osmotic pressure, RO membrane material, membrane construction, membrane modules, thin-film composite membrane construction, cross-flow mode water path, RO purifier details, recovery and rejection criteria, factors affecting RO performance, pressure, temperature, and recovery. Other important areas include membrane fouling, including biofouling, membrane cleaning, and membrane preservation. The technician should have some basic knowledge of microbiology.”

Mohamed: “This is not a simple issue. UPW combines different technologies (chemical treatment, filtration processes, ion exchange, electrodialysis, EDI, and water chemistry). I think it would require many types of training.

For RO, first, there can be a need for understanding water chemistry and filtration processes. I recommend starting with water chemistry, pretreatment (chemical, physical and biological treatment), filtration processes, and types of filtration (dead-end filtration and cross-flow filtration). Other aspects would include RO operation and maintenance, basic components of an RO, RO troubleshooting, membrane chemistry, membrane fouling and scaling, membrane treatment, membrane chemical cleaning, factors affecting RO membrane performance, optimization of membrane performance, RO operating parameters such as recovery, flux and salt passage, power consumption and minimizing energy use, operational costs, post treatment, and the chemistry of drinking water.”

Why Polish UPW?

Mike: “What is the value of "polishing" UPW? Is it necessary?”

K.V.: “Yes, polishing is essential to maintain the UPW quality. UPW is very pure and aggressive. When it comes into contact with various materials of construction it can get easily contaminated with inorganics, organics, gaseous, and microbial impurities.”

Mike: “Is UPW polishing more critical in certain uses? Or, should it be done by all UPW users?”

Nikhilesh: “It is certainly important in semiconductor industry applications. High-purity water with a conductivity of 0.06 microsiemens per centimeter (μS/cm) or lower is used in the cleaning processes of electronic component production lines.”

What are your thoughts? You may join this or other discussions at the Ultrapure Water group. Simply go to After logging in, or setting up an account, go to the “Interests” section and search for Ultrapure Water under groups. You may also click on the “LinkedIn” button on our website. We welcome new members.


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