PRB Articles


Electronic Testing in Water Analysis

Electronic-testing devices for water analysis seem to be everywhere these days. You can’t pick up a trade publication without running into an advertisement or two. Some pool professionals are using them and love them.

Others have purchased sophisticated pool and spa meters, and just allow them to collect dust in a box. So how do you know if an electronic-testing tool is right for you? After all, aren’t all the electronic testers faster, easier to use and more precise? The answer is yes … and no. Unfortunately, there is no one electronic tester that measures 25 parameters, costs less than $50, and has no limitations. Maybe someday such an instrument will be available, but for now let’s take a look at what is available, and what to consider if you are planning to make the switch.

Before discussing all of the instruments available to measure the quality of pool and spa water, let’s talk about some of the important factors to consider when purchasing an electronic-testing tool.

Cost

There is no doubt every organization is watching its dollars closely in the current economy. When evaluating a new electronic-testing solution for water-analysis needs, know that the upfront cost will be considerably more than a couple of bottles of reagent, or a new reagent test kit, for that matter. The price of single-parameter meters start at around $75; multi-parameter instruments will set you back around $150 or more, with some instruments costing close to $1,000. And those are just the portable instruments. If you are considering continuous online monitoring, the cost increases. For the purpose of this article, I will deal with the portable testing variety.

When considering the investment, also look at the cost of additional reagents, including calibration standards. It is often helpful to calculate the costs on a per-test basis, which will allow you to make a comparison to other testing methods or a competitive unit. For example, a single unit may be capable of measuring five different parameters. Each parameter may only require one reagent. In comparison, a liquid test used to test the same five parameters likely will require more than one reagent per test (some require as many as three). Therefore, the cost of operating the electronic meter may be better than you might expect by only comparing the costs of the reagents.

Ease Of Use

How easily does the instrument appear to operate? Review the activation and technique required to complete a test, and consider how error may be introduced. This is an especially important question if you are performing many tests in a day, or you share the testing responsibility with someone who may have difficulty using the instrument regularly. Also, consider the speed at which the testing is completed. Does it take more or less time than your existing method, and what does that mean to you? Many manufacturers will have operating instructions online. They may even have the complete manual available for free download. It can be helpful to review this documentation prior to purchasing a new unit.

Accuracy/Precision

This is always a big factor when looking to upgrade to electronic testing. It might seem that because you are using electronic tests, you are able to obtain more accurate results. That may not always be the case. Fortunately, this should be easy to investigate. Most manufacturers provide this information in some form. You should be able to determine the expected accuracy (how close to the actual value) and precision (repeatability) by reviewing the sales materials for a particular unit. If this information is not available, you may want to ask, “Why not?”

Calibration

Does the unit require periodic calibration and, if so, how difficult is it to perform? Know how often it is recommended for a particular unit. As noted above, you may also want to know the cost of calibration standards in advance so you are not surprised when you have to spend $40 or more on the appropriate standards. Some units may require calibration from the manufacturer. If this is the case, you will need to plan to send the meter away during a downtime or off-season, or perhaps you’ll acquire a backup instrument for the times that the meter is away. You may also want to ask the manufacturer how long the typical calibration will take, and if there is any associated cost.

Limitations

Like accuracy and precision information, the limitations of any instrument should be readily available. Pay careful attention to the operational ranges of the instrument to ensure they meet your testing needs. Also, take note of any interfering substances that may affect test results. It would be unwise to purchase an instrument that is not recommended for use when hardness levels of water exceed 500 ppm if the hardness in your area is at least that high straight from the tap. Manufacturers may not be as forthcoming with the interference information, but you should be able to find it nonetheless. For example, the pH and temperature limitations for Oxidation-Reduction Potential (ORP) are well-documented, and plenty of information is available on this topic.

The Instruments

Taking all of these factors into consideration, you can make a wise decision about the meter that best fits your testing needs. Regardless of the instrument you choose, the real benefit is the elimination of the “guesswork” typically required. For example, many users find comparing a reacted color to a color standard difficult, if not impossible. Instruments may be ideal for those with poor color acuity or some form of color blindness.

Sensors/Electrodes

For some of the parameters regularly measured in pool and spa water, an analysis can be performed with the simple push of a button. For example, pH can be measured by a portable pH electrode partially submerged into the pool or spa and the appropriate button pushed to activate the reading. Unfortunately, this technology does not allow for testing of all important parameters in pool and spa water. Typically, these sensors will only measure pH, total dissolved solids (TDS), salt, ORP and temperature.

The latest in sensor/electrode technology allows some or all of these parameters to be measured on one unit at a price that is not a major investment. Several manufacturers now offer systems capable of measuring pH, TDS, salt, ORP and temperature in seconds, all with the same instrument. The least-expensive of these instruments typically sells for around $350.

ORP and pH are measured in much the same way by these types of systems. Voltage is generated between a reference electrode and a measuring electrode with pool water between. A change in the current equals a change in the measured value. Even though there are two electrodes, these are often contained inside a single unit, giving it the appearance of only one probe. It is important to point out that ORP does not replace regular monitoring of free available chlorine. Regulations require testing free chlorine, even in systems fit with ORP-monitoring ability.

Conductivity results are used to approximate TDS and salt. Conductivity is the measure of the water’s ability to conduct an electrical current. A reference solution with known concentrations is used as a calibration standard. The unit then assumes the water “make-up” is similar to that of the standard, and measures its ability to conduct an electrical current, which is converted into a salt or TDS reading, depending on the setting and calibration. Unfortunately, this is really just an approximation, as conductivity is not a direct measurement of TDS or salts. However, it is a fast and easy method that can provide a close approximation.

· Advantages--Electrode systems provide near-instant results for the parameters they are capable of measuring. Often, several parameters can be measured by the same unit by switching modes. No additional reagents are needed for regular testing. The results also can be highly accurate and precise, depending on the instrument. These instruments are typically easy to use and operate with little or no training required. This technology is also suitable for continuous online monitoring.

· Disadvantages--The electrodes require careful handling and cleaning/rinsing with distilled or deionized water after each use. It is also important to carefully follow the manufacturer’s recommendation for storage of the electrodes. These systems also require periodic calibration, and it may be difficult to tell when they are not reading accurately.

Colorimetric Tests

This instrument is often referred to as a photometer or colorimeter. Either way, the basic technology is the measurement of light intensity at defined wavelengths as it passes through a reacted sample. A calculation based on a set calibration curve allows the measured value to be converted to the appropriate value of the measured parameter.

This technology continues to improve as reagent technology improves. As effective as these systems can be, they are reliant on the reagents (liquid, tablets or powder pillows) for obtaining accurate measurement. Therefore, as reagents are improved and use-life is extended, colorimeters become more accurate and reliable. Additionally, the cost of these systems has dropped in recent months as low-cost optics and internal components become available. Some multi-parameter colorimeters can be found for less than $150.

· Advantages--Most significant pool and spa parameters can be measured with this technology, and several are available in combinations with as many as 25 tests on one unit. These instruments provide a high degree of accuracy and precision. They will typically meet all regulatory requirements for testing. Most parameters require only a single reagent for testing.

· Disadvantages--Reagents are required for testing, which adds cost and handling concerns. Additionally, these systems may take a longer time to complete tests due to the mixing and testing completion times.

Reflectance Meters

Reflectance testing is the newest technology in the pool and spa market. This system utilizes test strips instead of reagents to measure the intended water parameters. A test strip is reacted and placed on the clear channel where light is reflected off the reacted test pads. The reflected value is then read by an optical reader that allows for a colorimetric measurement. This measurement is then converted by complex algorithm to calculate concentrations of the measured parameters. This technology has been used in the past in the medical industry for measuring blood-glucose levels.

· Advantages--A quick and easy test for a few critical parameters at a time. For example, a three-way test for free chlorine, pH and alkalinity can be completed in 20 seconds. The cost of replacement reagents is low, as test strips are generally inexpensive. The upfront cost of these systems is also inexpensive compared to other electronic testing equipment.

· Disadvantages--The technology is dependent on test-strip results in order to calculate the water analysis. Test strips will yield slightly more variation--therefore, less precision--than other comparable methods. Not all parameters are currently available.

When selecting an electronic testing instrument, there is much to consider. Now you are more knowledgeable about selecting an appropriate tester as well as learning some of the advantages and disadvantages of the specific methods. Keep these in mind and you will find the unit that is right for you.

Joe Sweazy is Technical Sales and Services Manager for HACH Company/ETS Business Unit, manufacturer of AquaChek and other water-quality products in use around the world. He has published more than a dozen articles on pool and spa water chemistry, and has presented numerous seminars at conferences of the Association for Pool and Spa Professionals (APSP) and at the World Aquatic Health Conference. He may be reached at jsweazy@hach.com

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