About the author:
Marianne R. Metzger is accounting specialist for Accent Control Systems, Inc. Metzger is a member of the Editorial Advisory Board of Water Quality Products. She can be reached at 610.630.7640, ext. 145, or by e-mail at [email protected].
Marianne R. Metzger
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When installing water treatment devices, a little testing can go a long way. I can’t emphasize this enough because knowing the quality of the water to start with will help not only size the equipment but also help prevent future problems and identify all the contaminants for proper treatment. Most water treatment professionals will perform some tests on site to get a general idea of the water quality, such as conductivity, total dissolved solids (TDS) and oxidation-reduction potential (ORP). It is important for pH and chlorine analysis to be done on site to get the most accurate reading, so these tests are commonly run by water treatment professionals. In this article, I will discuss the various applications for testing, what the results tell us, and how to properly maintain your testing equipment to ensure a high level of accuracy.
Conductivity & TDS
Conductivity is run extensively because it is a quick and inexpensive way to determine the ionic strength of the water. Conductivity is a measurement of a substance’s ability to conduct electricity. Pure water tends to have lower conductivity than water that contains high salts, minerals, acids or bases. A high conductivity may result in the need for additional analysis for the common culprits such as chlorides, silica, sodium, phosphates and hardness minerals. Temperature can greatly affect a conductivity reading, so you can expect an error of 2 to 5% per 1°C. Many meters on the market contain temperature sensors, which automatically correct for temperature.
Conductivity is often used to estimate the TDS gravimetric method, which is time consuming and costly. Conductivity is measured in millisiemens per centimeter or microsiemens per centimeter, while TDS is measured in parts per million or milligrams per liter. A simple calculation is used to convert conductivity into TDS; however, the calculation will vary based on the calibration solution used. Many meters automatically convert microsiemens into parts per million. Calibration is important to ensuring a high level of accuracy. Calibration involves using a solution with a known concentration for comparison. Meters should be calibrated based on how often they are used. A good rule of thumb for TDS and conductivity meters is once a month for meters used on a daily basis.
ORP
ORP measures the potential for oxidation. Oxidation reduction refers to the gaining and losing of electrons and is helpful in determining what chemical reactions can occur. This becomes extremely important to ensure proper sanitation. Disinfectants such as chlorine, bromine and ozone are powerful oxidizers, so monitoring the ORP helps to determine when these oxidizers are used up. In order to protect public health, monitoring ORP and pH helps in maintaining an adequate level of disinfectants, not only in drinking water but also public pools and water parks. ORP is measured in units of milivolts. ORP does not require instrument adjustment as much as it needs electrode maintenance. To clean the electrode, simply dip the top of the electrode in a dilute (1:100) acidic solution for about two minutes; then rinse with clean water.
pH
Finally, pH is one of the most important levels to monitor because it has an impact on almost all water treatment applications. It is also important to note that in order to get an accurate pH reading, the testing should be done on site immediately after sample collection because it changes as soon as it is exposed to carbon dioxide. If you must send a sample to a laboratory to meet regulatory requirements, you should have the sample to the laboratory as quickly as possible, typically within 24 hours, and the sample needs to be maintained within 2 to 6°C. These meters are much more sensitive, so they need to be calibrated at least daily, possibly more depending on usage patterns. Buffering solutions are used in calibrating pH meters. These solutions are designed to drive an acidic or alkaline solution to a certain pH level and maintain that level. You need to calibrate your meter to a range that is near what you expect in order to get the most accurate reading, so you will need to become familar with the pH levels in your area.
Saving Time & Money
Using onsite testing equipment will help you save time by giving you a valuable analysis on the spot. It is extremely important to properly maintain your testing meters with an appropriate cleaning and calibrating schedule, as inaccurate test results can result in costly sizing errors. Your supplier or the manufacturer should be able to give you advice on what meters will meet your needs based on your applications and specifications, and how to care for them to ensure a high level of accuracy.
