Parameters for Water Quality Field Testing
Test instruments can be a magic wand for water quality professionals
Water treatment professionals rely on many pieces of information in order to make good decisions on the proper course of action for their customer’s water quality issues. The most important clues for a water treatment professional’s decision-making process come from accurate water quality analysis information. Guesswork and second-hand opinion just make for more work over the long run.
Testing results of critical water quality parameters are not always reliable. Parameters such as pH, oxidation-reduction potential (ORP) temperature and chlorine can become unstable in a very short time. These parameters are better suited for field testing rather than waiting for lab time. Accurate field-testing is also critical for spot checks on the water treatment system’s health to ensure the customer’s system is “tuned” properly.
Field testing can be a very difficult, time consuming process, which until recently, required expensive, bulky equipment with a seemingly endless number of pre-test setup steps. Handy, simple-to-use testing and measurement devices are a real plus. Also, testing devices that can perform multiple functions, reducing the amount of gear you have to lug around is a real plus. In the last few years, big testing devices have given way to small, pocket-size instruments.
There are many factors to consider when selecting portable test and measurement equipment. One of the most important factors is the accuracy of the equipment. Good quality equipment from a reliable manufacturer is a must. Ease of use, compact size and rugged performance are highly desirable as well. Conversely, complicated, finicky and bulky devices are undesirable. Besides, why buy a portable laboratory when you just need a couple of parameters, reliably reported in a simple format?
In addition to shrinking in size, the price of test and measurement devices has come way down. These pocket sized, “stick” style test instruments are a welcome change to expensive portable test labs, especially for the person looking for three or four parameters.
What should you look for in a portable testing and measurement device? Let’s look at some of the most common test parameters, best taken in the field for accuracy and used quite frequently for diagnostic and system “tune-up” service calls. The parameters are pH, ORP, chlorine and TDS.
Testing for pH
The term pH is by scientific definition, the reciprocal of the hydrogen ion concentration, or simply put, the level of acid or caustic (base), of any given solution. Water is considered neutral if it registers 7.0 on the pH scale. Each degree reported up from 7.0 to 14.0 relates to the caustic nature of the water and each degree down from 7.0 down to 0.0 is describes the acidity of the water. Each unit of measurement is 10 times that of the previous number, so if the pH is 4.0, that is 1,000 times more acidic than the neutral pH of 7.0. The pH of water affects and is affected by many aspects of the water chemistry. Suffice it to say that the correct pH is very critical to understanding the type of water you will be treating as well as dictating the type of treatment required to meet the customers’ specifications.
Stable pH is affected by both time duration and temperature, making it one of those parameters that are more accurately obtained by field test. When choosing a pH tester, look for a pH meter that automatically temperature-corrects the pH value it reports. pH measurement devices also have a reputation of being difficult to work with as well as being inaccurate if not handled properly. The most common problem is the short life span of reference probes, primarily due to the reference junction drying out. The highest quality pH “stick” devices have “flat surface” probes, which can better deal with a variety of measurement requirements.
Oxidation reduction potential (ORP), also know as REDOX, is a useful measurement for monitoring and controlling chemical reactions. These testers measure the oxidizing or reducing nature of a sample. This gives an overall indication of how “reactive” the sample is, and can tell the technician if enough of the agent of choice is present in the sample. Examples of where ORP readings are useful include ozone or chlorine control and chromate reduction or cyanide destruction. An ORP measuring electrode is identical to a pH measuring electrodes except a noble metal is used in place of the pH glass as the measuring element. Noble metals or alloys, such as gold or platinum, have a high resistance to corrosion and oxidation. Nobles are used in the construction of thin-film circuits, metal-film resistors and other metal-film devices. Nobel metals are used in ORP devices because they will not enter into the chemical reaction-taking place. Other noble metals such as gold can be used, but platinum is the most common.
Two methods are recognized by the EPA for mandated testing on wastewater discharge, DPD and the Ion Selective Electrode (ISE) method. Because of the fragility of the sample conditions, these tests are most accurate when done on site.
While both methods are easy to run, DPD is a colorimetric method, and is affected by turbidity and color in the sample, scratches on the sample vial, and stray sunlight. The ISE method is unaffected by these issues, making it an accurate as well as convenient method to make these measurements in the field.
Total dissolved solids
The most common “stick” style testing meter is the total dissolved solids (TDS) conductivity monitor. TDS is a critical parameter for many industries, including fresh, waste, cooling and boiler water treatment. The value of a reliable TDS reporting cannot be understated.
TDS can be recorded in many forms depending on the water treatment application. The most common reporting forms are micrograms, mircosiemens, parts per million, parts per thousand, milligrams per liter and density equal to one gram in a liter.
When selecting a TDS/conductivity meter, look for one with automatic temperature compensation. Errors of more than 2% per degree from the reference temperature will be encountered if temperature is ignored. It is also convenient is to choose a meter with a wide measurement range, as some manufacturers offer only single range meters in this class of product, requiring the technician to have multiple meters to cover the expected range of measurement.
While many manufacturers offer products to accomplish some or all of the above measurements, the Extech Instruments ExStik line of pocket meters offer a unique combination of features and convenience for measuring all of the above parameters, as well as conductivity, salinity and dissolved oxygen. Many features included are those normally found only on more expensive portable meters. Six different ExStik products are now available. Also offered are several kits, combining multiple measurement parameters in a single stick meter with interchangeable probes, making this a very economical way to make high quality measurements.