-------------------------Monitor------------------------ The Newsletter for PC-Based Data Acquisition and Control Issue 92 www.windmill.co.uk March 2006 --------------------ISSN 1472-0221---------------------- Welcome to Issue 92 of Monitor. This month we give tips on using your PC to measure pH. Should you wish to cancel your subscription to Monitor you can do so at http://www.windmill.co.uk/newsletter.html CONTENTS ======== * Windmill News: Data Acquisition Glossary Updated * How to Use a Computer to Measure pH * DAQ News Roundup ________________________________________________________ ________________________________________________________ Windmill News: Data Acquisition Glossary Updated ________________________________________________________ Thanks to readers pointing out terms we had missed, we have been able to further expand our glossary of data acquisition and control terms. With links to further information and in-depth articles. http://www.windmill.co.uk/glossary.html ________________________________________________________ ________________________________________________________ How to Use a Computer to Measure pH ________________________________________________________ pH is measured in many industries, including nearly all handling water. In fabric dyeing the permanence of the colour and the speed of the process depend on pH. pH determines product quality in paper mills. A change in pH in a lake or river is an early indicator of pollution. And the correct pH is essential to ensure proper beer production. What is pH? =========== pH indicates the acidity of a solution. The term "pH" was first used by W. M. Clark in 1920. Previously the Danish biochemist Sorenson had used the term PH - H being a subscript of P. The H stands for hydrogen. (An acid forms hydrogen ions in solution.) It's unclear where the p came from. It's often said that the p refers to the German word protenz, which apparently means power. This makes sense because the pH scale is a logarithmic one, so the p would refer to the power or exponent of 10. However, in that case why was the German word for hydrogen not used? Jens Norby published a paper in 2000 arguing that the little p was simply a constant (Trends in Biochemical Sciences 25 (1) (2000) pp. 36-37.) The pH scales ranges from 0 to 14. It is logarithmic, so each interval is 10 times more acidic or alkaline than the last. For example, pH 6 is ten times more acidic than pH 5, and one hundred times more acidic than pH 4. pH Sensors ========== Various methods of measuring pH are available, but the most common one used in laboratory and industry is the glass electrode method. In this, the pH of a known reference solution is compared to the pH being measured. Two electrodes are used: a glass measurement electrode and a reference electrode. The measurement electrode comprises a glass bulb attached to a glass stem. The bulb is a pH sensitive membrane filled with a conducting buffer solution. A silver wire is enclosed in the glass. The difference in pH between the solutions inside and outside the thin glass membrane creates an electrochemical force (voltage) proportional to the difference in pH. This is passed via the silver wire. The reference electrode has a stable potential and also features a silver wire, enabling a complete circuit to be made and the voltage generated by the glass electrode to be measured. The measurement and reference electrode may be individual and separate, or may be combined into one probe. (There may also be a temperature compensating electrode.) Individual electrodes are less practical than a combined probe, but may be more precise. The probe may be connected to a pH meter that displays the current pH reading. How to Get pH Measurements into a Computer ========================================== - pH Meter - First let's take the case of a pH meter - a measuring device which displays the pH of the sample. This will often have an RS232 or USB interface. This means that you can connect it to the serial COM or USB port on your computer. You will also need some software to collect the data from the pH meter. Software like the Windmill COMIML program which reads data from the PC's RS232 COM port (https://www.windmillsoft.com/daqshop/rs232-modbus.html). The Windmill software regularly collects the pH readings, every 5 seconds say, and saves them on the computer's hard disk. Windmill can also show live charts of the data, pass data in real-time to other programs like Excel and collect pH data alongside other data such as temperature or flow rate. - pH Electrodes - What if you are not using a pH meter but instead wish to connect the pH electrodes to the PC? To do this you need a data acquisition (DAQ) device. This device may be a card that you plug into the PC. More commonly, it is a unit sitting between the electrodes and the computer, connected to one of the PC's communication ports: USB, RS232, Ethernet or RS485 for example. You connect the electrodes to the DAQ device. The device then regularly takes pH readings and passes them to the computer. pH electrodes have a very high output impedance and you cannot just connect them to a normal voltage input on your data acquisition unit. You will need instead to choose a DAQ unit that will amplify the signal to the appropriate level. For example, our small, portable, Microlink 751 unit - which plugs into the PC's USB port - has its own pH conditioning unit. The larger Microlink 3000 system comprises a frame of modules. Each module is dedicated to a specific task: voltage input, counting, current output and so on. It has a special module dedicated to pH signal conditioning that you slot into the frame (http://www.microlink.co.uk/3800.html). You connect the electrodes to this module. You then connect the pH module to the normal voltage measuring module and take pH readings at the same time as other measurements. DAQ devices designed for pH measurement need input protected sockets to which you connect the electrodes. As pH readings change relatively slowly, if possible you should choose an integrating analogue-to-digital converter as this reduces noise interference. These typically offer maximum sampling speeds of around 40 readings per second. Further Reading =============== Trouble Shooting pH Measurements... http://www.windmill.co.uk/monitor122.html More on Windmill Software... http://www.windmill.co.uk/windmill7.html http://www.windmill.co.uk/https://www.windmillsoft.com/daqshop/rs232-modbus.html Example pH Monitoring Applications... http://www.windmill.co.uk/fish.html http://www.windmill.co.uk/oil.html pH Data Acquisition Hardware... https://www.windmillsoft.com/daqshop/pH-measurement.html https://www.windmillsoft.com/daqshop/pH-data-logger.html http://www.microlink.co.uk/3800.html ________________________________________________________ ________________________________________________________ DAQ News Roundup ________________________________________________________ Welcome to our roundup of the latest findings in data acquisition and control. If you would like to receive news updates as they happen then grab our RSS newsfeed at https://www.windmillsoft.com/monitor.xml. Read http://www.windmill.co.uk/newsfeed.php for information how. EU-funded sensor network goes a step beyond RFID An EU-funded project is going a step beyond existing radio frequency identification (RFID) systems by developing a sensor network that will allow items to communicate more information about their surroundings. The technology can be used with goods or equipment to make them "smart". Items such as cartons of foods will be able to warn operators when the storage limit in a warehouse is reached, if a leak occurs or if one is placed in the wrong location. Source: CEE-foodindustry.com http://www.cee-foodindustry.com/ Internet Standards Gives Birth to a New Industry in Building Automation Systems The adoption of Internet Standards in the Building Automation Systems (BAS) market is giving birth to a new industry focusing on information management and analysis. According to the ARC Advisory Group, the worldwide market for BAS is expected to grow at nearly 5 percent over the next five years. The adoption of Internet communication standards in the BAS market is further extending the concept of smart buildings by allowing companies to create a single repository for all facilities data so that it can be easily retrieved and shared between all applications and all organizations within an enterprise. Source: ARC Advisory Group http://www.arcweb.com/ IEC Try to Solve Environmental Regulation Muddle As regulations about the environment increase, so life becomes more difficult for the electrical and electronics industries because the amount of paperwork increases in the form of green procurement surveys or supply chain questionnaires. The regulations are often a list of what one may not do and too much of this, with varying criteria, creates a real Tower of Babel. How to resolve the problem? IEC (International Electrotechnical Commission) thinks the answer is a Material Declaration Standard used worldwide and has started work on this. Source: IEC http://www.iec.ch/news_centre/ ________________________________________________________ ________________________________________________________ * Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see http://www.windmill.co.uk We are happy for you to copy and distribute this newsletter, and use extracts from it on your own web site or publication, providing the above notice is included and a link back to our website is in place. An archive of previous issues is at http://www.windmill.co.uk/newsletter.html and an index of articles at http://www.windmill.co.uk/newsletter.html Windmill Software Ltd, PO Box 58, North District Office, Manchester, M8 8QR, UK Telephone: +44 (0)161 833 2782 Facsimile: +44 (0)161 833 2190 E-mail: [email protected] http://www.windmill.co.uk/ https://www.windmillsoft.com/
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