By Joe Sweazy & Drew Chuppe

If, like many water treatment professionals, you’ve expanded your operations to include pool and spa services, we’ll wager you’ve discovered there are peculiarities to maintaining these facilities that you didn’t expect. Your employees out there in the field—experienced though they may be in terms of water softeners and the like—may not know how to deal with an irate customer whose hair has turned green after swimming in their backyard pool!

Make copies of this handy field guide and be sure to give one to everyone treating pools and spas—they’ll be glad you did.

Testing can be tricky
Have you ever tested swimming pool water with two different test kits and gotten two completely different results? Which one do you trust? You might expect that two accurate test kits would give two accurate results and that those results would be the same, since the water did not change in the two minutes between tests. Usually that will be true, but not always. There are certain situations you need to be aware of, wherein two different test kits may give you two different test results.

Free chlorine
Liquid and tablet test kits use a chemical called DPD, short for diethyl-p-phenylenediamine, as the indicator to measure free chlorine. If you are testing for free chlorine using a DPD test kit, be aware that high levels of combined chlorine, or chloramines, can cause false positives with this test. Low to moderate levels of combined chlorine is neutralized in most DPD kits.

But occasionally, levels of combined chlorine will build up to such an extent that the test result will show that there is some level of free chlorine when in fact there is none present. Usually a strong, foul chloramine odor will confirm the presence of a high level of combined chlorine. Utilize another testing method, as some test strips use an indicator called syringaldazine to detect free chlorine. Chloramines do not react with syringal-dazine. Therefore, the liquid or tablet DPD kit may show there is some significant level of free chlorine present when the test strips show that there is no free chlorine in the water.

OTO, which is short for orthotolidine, is a common liquid indicator for determining the amount of chlorine in the water. This test reagent reacts with the amount of chlorine in the water, changing the color of the sample to yellow. The darker the yellow color becomes, the higher the concentration of chlorine. It is a quick and inexpensive way to measure chlorine. However, this indicator is only capable of measuring total chlorine. Remember, that is the sum of both free and combined chlorine. Some or all of the chlorine measured by OTO may be free chlorine; some or all of the chlorine may be combined chlorine. It is important to know when you have combined chlorine present so you may treat the pool or spa water appropriately. Measurement of chlorine with OTO will challenge you to accurately determine when and how to shock the water with an oxidizer to eliminate impurities.

For example, you may open a pool in the springtime and measure the chlorine using OTO. It appears that there is plenty of chlorine in the pool, when in fact all of the chlorine present is in the combined form and cannot sanitize. In reality, at most pool openings a large dose of free chlorine is needed to oxidize the wastes.

Like chlorine, bromine is present in swimming pool water in two forms, free and combined bromine. However, combined bromine is an effective sanitizer and does not give off such a foul odor, therefore we often use the term bromine referring to all forms of the sanitizer. Many test kits on the market will measure total bromine, while others will measure only free bromine. Be sure you know what your test kits are measuring. Test kits that measure only free chlorine and also measure bromine are only reading free bromine. Most test kits will report on the label or somewhere in the kit if it is measuring total bromine. If not, chances are you are only reading free bromine. Obviously, a test kit that measures total bromine will have a tendency to read higher than a kit that measures only free bromine since it is measuring all forms of the sanitizer. So two different bromine tests may give significantly different bromine readings.

It is best to use a test kit specifically labeled for bromine testing, not a combination of chlorine and bromine. A total bromine test is optimized to test all forms of bromine that would make up the sanitizer content. Tests that measure both chlorine and bromine are optimized for testing chlorine. In addition, some tests measure free chlorine and bromine. Doing this, testing free chlorine and multiplying by the factor of 2.25 (because bromine is 2.25 times heavier) will only give you a free bromine reading. This type of test does not take combined bromine into consideration, which as mentioned earlier, is an effective sanitizer.

Monopersulfate is a non-chlorine chemical that is commonly used to shock treat swimming pool water. It is a powerful oxidizer which causes a DPD test for free chlorine or bromine to react to show a positive reading. A small amount of monopersulfate residual will show an indication that there is a free chlorine or bromine residual present when there is none, or show a higher residual than there actually is. Some test kit manufacturers have now successfully neutralized monopersulfate in this reaction. Once again, this should be stated somewhere in the kit, probably in the manual, that monopersulfate is not an interference.

Additionally, test strips for free chlorine that utilize the syringaldazine indicator do not have this interference. So monopersulfate will potentially cause some kits to read higher free chlorine or bromine than the actual level. Also, monopersulfate does dissipate relatively quickly so waiting for a couple of hours after shocking with monopersulfate will generally guarantee that this interference will not be a problem.

Monopersulfate can eliminate wastes in a pool or spa without the unpleasant side effects of chlorine. While monopersulfate cannot effectively sanitize (kill bacteria) in a pool or spa, you can use it to shock the water. This allows bathers to return to the water sooner—typically less than 30 minutes after you add the monopersulfate shock to the water. Monopersulfate can eliminate impurities through oxidation and conserve the chlorine residual for killing bacteria. Best of all, monopersulfate does not leave behind any irritating or unpleasant by-products when it oxidizes wastes.

Because of the unpleasant odor and irritation associated with chlorine, mineral purification systems and other alternate methods of sanitization have been growing in popularity. These systems use dissolved minerals like copper (to kill algae) and silver (to kill bacteria). While these minerals can keep your pool or spa safe from pathogenic organisms, minerals are incapable of oxidation. So a mono-persulfate residual is maintained in the mineral purification systems to oxidize wastes and debris. (In the presence of high levels of chlorine or bromine, mono-persulfate dissipates very quickly. In mineral purification systems, where the level of chlorine or bromine is quite low, the monopersulfate residual level will stay in place much longer.) The combination of minerals to sanitize and monopersulfate to oxidize provides protection similar to that of chlorine.

Cyanuric acid
Cyanuric acid combines with chlorine to protect it from the UV rays of the sun, but releases it on demand when it is needed to sanitize the water. Cyanuric acid, Triazine-2,4,6-Triol in scientific terms, is an acid with a pH of approximately 4.0. Therefore, cyanuric acid may also shift the pH of the water downward when added directly to a pool. If cyanuric acid is present in the water in sufficient levels, less chlorine degradation occurs. Keeping chlorine in the water longer will help to protect the swimmers in the pool. An ideal level of cyanuric acid, 30 to 50 ppm, should be maintained to prevent rapid chlorine loss. Some chlorine compounds have been developed that already contain an amount of cyanuric acid. If you are using dichlor or trichlor as the primary sanitizer, cyanuric acid is being introduced along with the chlorine. Usually, no additional cyanuric acid is needed when using a stabilized chlorine compound. However, cyanuric acid levels may build up with the continued use of one of these sanitizers. When cyanuric acid levels are high, it will reduce chlorine efficiency and contribute to high total dissolved solids. Local health authorities often require swimming pools to be maintained under 100 ppm. Cyanuric acid levels in pools should not exceed 150 ppm. On the other hand, low cyanuric acid levels (less than 30 ppm) indicate that chlorine will dissipate very quickly when exposed to sunlight.

There are two methods for testing cyanuric acid that are common to the pool and spa industry: test strips and turbidity tests. With the test strips, a patented chemistry allows for the direct measurement of cyanuric acid using a color reaction proportional to the concentration. This color reaction takes place on a test pad that is compared to a color chart to analyze the level present. A recent improvement enables users to measure up to 300 ppm directly (without the need to dilute a pool water sample). In addition, greater color distinction allows for increased accuracy and ease of use. This is a dip-and-read test that is completed in 30 seconds.

The turbidity test is often referred to as a disappearing dot test. This utilizes a chemical reaction that creates turbidity proportional to the concentration of cyanuric acid. The more turbid the water, the higher the concentration of cyanuric acid. This turbid water is added one drop at a time to a comparator with a mark (usually a dot) on the bottom. A user adds drops until the mark disappears from view. The turbidity of the water dictates the amount that will be necessary to make the mark disappear. The amount of water in the comparator tube when the mark disappears corresponds to a value marked on the tube to indicate the cyanuric acid level. If the level measures 100 ppm or greater, a dilution must be performed to determine how much higher the actual level is. This involves adding fresh water to the sample and running the test again.

Back to that irate pool owner with the green hair: metals are to blame. Copper and iron are the two most common problems. Most metal compounds enter the pool or hot tub with the source water; once inside, free chlorine is a powerful oxidizing agent. Test water frequently if you are using a copper-based algaecide or a mineral purification system and keep copper below 0.2 ppm to avoid staining and scale.

Total dissolved solids (TDS) can be introduced into the pool through a variety of sources. It will build up with every chemical that is added to your pool or spa. For instance, for every pound of dry chemical added, the TDS level can jump approximately eight ppm in a 15,000-gallon pool. That same pound of dry chemical in a 500-gallon spa can increase the TDS by approximately 250 ppm. Liquid chemicals will also contribute to the total dissolved solids. One gallon of liquid chlorine, for example, would increase the TDS in a 15,000-gallon swimming pool by approximately 17 ppm. TDS buildup occurs as water ages and chemicals are added.

It is a good idea to test the TDS at least once a month in pools, even more frequently in spas and most importantly, whenever there is a problem. For proper swimming pool and spa maintenance, the National Spa and Pool Institute (NSPI) suggests the following operational guideline levels for TDS: 300 ppm—minimum; 1,000-2,000 ppm—ideal; 3,000 ppm—maximum; or 1,500 ppm maximum at pool startup. Pools with salt generators can generally maintain a slightly higher level. The primary component of TDS in these pools is sodium chloride, which is very soluble.

There are three ways to measure TDS levels. A portable conductivity meter is one common method; it measures conductivity of certain species in a sample and then uses factors to compute guideline TDS ppm values. These instruments require periodic calibration to maintain accuracy.

The second TDS testing method is a liquid drop test. A reagent is added to the solution until a color change is observed. Each drop of reagent is counted, then multiplied by a factor to calculate the TDS value.

The third alternative is an inexpensive, dip-and-read test strip that has been recently introduced. You simply compare the reacted color of the strip to a color chart to determine the TDS level. It is simple enough that anyone can use it and it does not require calibration.

Another factor to keep in mind if comparing test results is the precision factor. Most liquid/tablet kits allow for very precise readings for certain parameters. Total alkalinity and hardness, for example, are measured in increments of 10 ppm. This allows the user to determine within 10 ppm what the actual result is. Other test kits and test strips may not offer such precision. Although likely to give accurate results, these test kits may not allow you to determine within 10 ppm of the actual result. Because these tests are colorimetric (you use colors to determine the concentration) and open to interpretation, users may determine results slightly higher or lower than the actual concentration indicated, causing a further discrepancy between the two test results. So keep in mind that interpretation may play an important role in getting similar test results with two different kits.

Technique is also important to every test kit; this is more essential for some kits than it is for others. Generally, liquid/tablet kits require more precise user techniques to obtain very precise results. Test strips are usually much less technique dependant, but offer less precision. Additionally, directions are very important. Many inaccurate test results occur when individuals do not follow directions, or follow the wrong directions! Many test kits look very similar and may in fact use the same set of directions. However, do not assume that because they look similar, the directions are the same. This can be a significant source of variation. Make sure you know and follow the directions for each test kit, as directions change from time to time and from manufacturer to manufacturer.

Higher water temperatures
Pools usually operate between 76-86°F (25-30°C), while spas fall in a range of 96-104°F (36-40°C). This difference in temperature changes the water chemistry in important ways. Chemical reactions take place much faster in spas than in pools. For every 10°C (18°F) increase the chemical reactions proceed twice as fast. For instance, a spa at 102°F allows chemical reactions to happen in half the time of a pool at 84°F. Any chemical adjustments occur more quickly. The water comes to equilibrium sooner and water treatment can be completed in a shorter period of time.

The water in a hot tub also evaporates at a high rate due to higher water temperature, rapid water circulation and aeration. As the water evaporates, the hot tub owner adds make-up (fill) water to refresh the system. Any water that evaporates is pure; basically pure water leaves behind everything else: the TDS.

Higher temperatures in spa water will cause most chemicals to dissolve faster than in lower temperatures, except calcium carbonate. This form of hardness works in the opposite way: it actually is more insoluble in hot water. Therefore, calcium carbonate scale is more likely to occur in hot spa water. The hot water in spas also makes people sweat. The average bather sweats a pint (about half a liter) in just 20 minutes. And the power jets in a spa scrub off dirt and dead skin very quickly. All this means that the filter and chemical sanitizer in a spa have to process a high percentage of waste. Consequently, paying close attention to the sanitizer level is critical in a spa. A spa makes a perfect incubator for bacteria if not cared for properly. Hot water promotes the growth of most types of bacteria. Susceptible bathers can acquire serious illnesses if water is chemically imbalanced.

Smaller volume
Clearly, spas have a much smaller volume of water than pools. This glaringly obvious difference leads to some other differences that may not be as obvious. Spas experience a much heavier bather load because they are so much smaller. While two people in a spa might feel cozy, being in a pool where the swimmers are elbow-to-elbow is not. A common load in a spa would have one person in 100 to 400 gallons of water. Pools tend to have at least ten times that amount of water for every swimmer. Consider this: two bathers in a 400-gallon spa are roughly equivalent to 150 people in a 30,000-gallon pool.

This significant bather load can decrease the sanitizer levels very quickly. As a result, many places set regulatory limits on the number of bathers in spas. A common standard for public spas is one bather per one square meter of surface area.

Because of the lower water volume in a spa, chemicals need to be measured precisely and the water tested more often. Misjudging the required dosage can drastically alter the chemistry in a small volume of water. For this reason there are chemicals specifically designed and labeled for treating spas. These lower the risk of adding too much or not enough of a particular chemical.

The water in a hot tub or spa should turn over every 25 to 45 minutes when the system is running with the proper filtration, due to the smaller volume and increased wastes. The water in a pool might turn over in 6 to 12 hours. The faster turnover rate can take a toll on the spa’s filtration system, particularly if the chemicals are not in proper balance. As a result, maintenance on the filter will need to be performed more frequently.

A hot tub or spa, with its small water volume and higher water temperature, uses up the sanitizer residual very quickly. This is why you need to maintain a higher level of sanitizer in hot tub or spa water than you do in your pool. The NSPI (APSP) standards reflect this fact, requiring sanitizer levels to be higher in spas and hot tubs.

Last, the spa’s water volume means that even small additions of sanitizer and other chemicals can have an immediate effect on the pH of the water. If the wrong amount of a chemical is added to a pool, there is a little time before the chemical circulates throughout the entire system. In the case of a spa, that reaction time is lost. To avoid equipment damage the sanitizer should be measured carefully and the pH tested frequently to avoid equipment damage.

Pool and spa services can give a quick boost to your revenue stream—knowing how to keep them optimized for your customers’ enjoyment will mean long term success.

About the authors
Joe Sweazy is Technical Sales and Services Manager for HACH Company/ETS, manufacturer of AquaChek, the world’s #1 brand of pool and spa test strips and other water quality products that are in use around the world, simplifying analysis with reliable, accurate results. He has published more than a dozen articles on pool and spa water chemistry and has presented numerous seminars at conferences of the National Spa and Pool Institute (APSP). He can be reached by email at [email protected]

Drew Chuppe is the Manager of Global Marketing for HACH Company/ETS. He and Sweazy are co-authors of the AquaChek Dealer Training Program, a free, Internet-based training course on pool and spa water chemistry. For more information, visit the website, He can be reached by email at [email protected]

For more information about the AquaChek and AquaTrend products, call toll-free at 1-888-AquaChek (1-888-278-2243) or log onto



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