Important pool water tests every service pro should be using

by arslan_ahmed | November 18, 2022 7:00 am

[1]By Terry Arko

Why is it important to routinely test pool water? The answer is simple: the state of water in a pool can be unpredictable and chaotic when it is not properly managed. The water itself is an unruly element. This liquid comes with a whole list of demands from the very day the tap is turned on. When water is added to a constructed hole in the ground, it will enter with either the need to consume or unpack. Immediate action is required to control the potential chaos coming from the very existence of water.

Calcium (Ca) is the primary ingredient water has a hankering for. When water is hungry due to lack of sufficient calcium, it goes on the hunt to find it. In a plaster or aggregate pool, it will seek its food in the material of the surface. Plaster and other cementitious surfaces have plenty of calcium. To keep the integrity of the surface, a minimum amount of a calcium additive, such as calcium chloride (CaCl₂), must be added. The minimum calcium to start must be 150 parts per million (ppm). To ensure this, a calcium hardness test is required.

Before adding any water to a newly surfaced pool, the source should be tested for calcium. One should never wait until the pool is filled before testing for calcium levels as it could lead to aggressive fill water taking calcium from the surface while the pool is being filled. There are startup systems which allow owners to add calcium as their pool fills to prevent any of it from being taken from the surface.

The ideal level of calcium for plaster pools is 350 ppm. For vinyl liner and fibreglass, it is 250 ppm. If the pool is in a hard water area, the source should be tested first as well. Hard water contains high amounts of calcium. This will unpack what it cannot hold in the form of a calcium carbonate (CaCO₃) scale, this makes it even more important to perform a calcium hardness test of source water before filling a pool.

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Next comes water balance, and the two important tests for it are total alkalinity (TA) and pH. Service professionals address TA first because it is the buffer for pH. Primarily, it acts as a buffer to prevent pH from decreasing while also slowing its increase. The TA test is a titrimetric test which uses differing reagents. An initial reagent is added, which appears as a green colour, then a titrant reagent is added one drop at a time until the colour changes to red. The drops used are counted as a calibration number, usually up to 10, to determine the results.

To obtain a true carbonate alkalinity reading, any amount of cyanuric acid (CYA) must be determined and considered as one-third of the TA reading. For example, a CYA reading of 60 ppm should be divided by three. The resulting number is then subtracted from the TA test, and the result is the true carbonate alkalinity. For example, if a TA test results in 120 ppm and the CYA comes out as 60 ppm, then one-third of 60 ppm is 20 ppm. Subtracting the TA of 120 ppm minus CYA 20 ppm determines a carbonate alkalinity of 100 ppm.

A pH test is a measurement of the presence or lack of hydrogen in water. Hydrogen lowers the pH, and it comes from the addition of muriatic acid—also known as hydrochloric acid (HCl). It is important to note the “H,” which stands for hydrogen. Every time acid is added, hydrogen is increasing, which means the pH is decreasing. Lack of hydrogen causes the pH to increase.

One way to decrease hydrogen is by aerating the water. Forcing air into the pool water causes carbon dioxide (CO₂) to off gas and leave rapidly. When this happens, more hydrogen is consumed, and the pH goes up.

The most common pH test is performed by using phenol red reagent and comparing the colour to a standard from 8.0, down to 7.0. This is a fairly accurate test; however, it is impossible to determine if pH is below 7.0 or above 8.0. Further, false readings can occur if chlorine levels are greater than 10 ppm, and there may be a false high pH purple colour. Some technicians add one drop of sodium thiosulfate (Na₂S₂O₃) if the chlorine test high. A digital pH tester may prove to be a better investment as it will measure exact pH readings and colour comparison is not necessary.

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Today’s modern pools tend to run high on pH due to the increased aeration from negative edge, waterfalls, fountains, and various other water turbulence-prone features. The ideal pH level to achieve is 7.5. Minimizing the time of running features which cause aeration and increasing acid can help as well.

It is important to maintain pH within pool standards. Some effects of allowing a high pH are:

• Lack of disinfection
• Scale formation and metal staining
• Cloudy water
• Low oxidation-reduction potential (ORP)

Contrastingly, low pH can lead to:

• Corrosion
• Etching of plaster
• Staining
• Damage to vinyl liner and fibreglass

TA and pH are two crucial tests that must be performed routinely. For the best results, service professionals should use a digital photometer to test. When using a reagent test, it is very important to ensure the reagents are fresh and the test block is kept clean and intact.

There are many sorts of chlorine present in pool water at any given time. When it comes to the efficacious treatment of the water, there are three main tests:

• Total chlorine (TC)
• Free chlorine (FC), which should range between 2 to 4 ppm
• Combined chlorine (CC), which should range between 0 to 0.2 ppm

Total chlorine (TC) test

TC is a measurement of both FC and CC. The test can indicate the presence of chlorine in the water, but it cannot tell how much FC or CC is present. Relying on a TC test when trying to maintain water quality is ineffective. An orthotolidine (OTO) test method for determining whether the water in the pool is safe or not is ineffective as well. OTO only indicates if there is chlorine present—it cannot determine what form of chlorine is most prevalent. Therefore, it is important to have a way to test for both TC and FC.

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Free chlorine (FC) test

FC is the test experts focus on the most. It consists of two main constituents, hypochlorous acid (HOCl) and hypochlorite ion (OCl-). When chlorine is added to pool water, it creates HOCl. This molecule can kill 99 per cent germs and algae. Contact with water causes some HOCl to disassociate into hydrogen (H+) and OCl-, the latter is an ion with only one per cent ability to kill germs and algae.

Together HOCl and OCl- make up FC. The standard recommendation for FC is 2 to 4 ppm. The FC test measures both HOCl and OCl-; however, the test cannot tell the percentage of either in the water. Since the primary purpose for chlorine is to keep the water safe from germs and algae, the highest per cent of FC should be HOCl.

The most effective way to produce the highest per cent of HOCl is by maintaining the pH and temperature of the water. The lower the pH and water temperature, the more HOCl is produced, which is good. The higher the pH and water temperature, the less HOCl and more OCl- is produced, which is bad.

At a pH of 7.5 and water temperature at 20 C (68 F), 55 per cent of HOCl is produced. At a pH of 8.0 and water temperature at 30 C (86 F), HOCl goes down to 24 per cent. Water temperature cannot always be controlled in the summer, which is good enough reason to run the pH slightly lower in the hot season. Pool owners can also produce 55 per cent HOCl in water with a temperature of 30 C (86 F), by adjusting pH down to 7.4.

Combined chlorine (CC) test

CC is another way of saying chloramines. This chlorine is combined with contaminants in the water to form a nitrogen-bound chlorine. It is a poor disinfectant for pools and leads to irritating chlorine gas odours, particularly in indoor pools. Therefore, it is vital to test both TC and FC. By doing so, the level of CC in the water is determined. Subtracting FC from TC gives the number for CC in the pool. The ideal level of CC in pools is zero; the maximum acceptable level is 0.4 ppm. A more preferred level is no more than 0.2 ppm.

One test method used to measure both FC and CC down to 0.2 ppm is a FAS-DPD test. There is also a photometric test for determining TC, FC, and CC. For the safety of swimmers, and to keep algae from growing, it is vital to know what type of chlorine is dominant in the pool water.

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Total Dissolved Solids (TDS) test

TDS is a test which is often overlooked but is key for the overall quality of pool water. The test is a measurement of all byproducts which have accumulated during the life of the water. Water is a great absorber and holder of micron-sized particulates, such as calcium, salts, phosphates, metals, and various other tiny solids. However, water becomes oversaturated from high TDS. When the level of solids reaches the point where water can no longer absorb, it starts doing the opposite. This will occur by persistently cloudy water, scale formation, and decreased sanitizer efficiency. Testing for TDS routinely, or at least seasonally, is one of the most vital tests to perform.

While most standards do not give the maximum amount of TDS, it is recommended it should not exceed 1500 ppm over the start-up water. Should this occur, the efficiency of chlorine is reduced by 50 per cent. This means it will take 50 per cent more chlorine to properly sanitize the pool water. TDS tests are usually performed with electronic or digital meters, which are a worthwhile investment for any pool service provider.

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Some other tests that may prove beneficial in the management of pool water are nitrates, phosphates, and metal tests for copper or iron. Both nitrates and phosphates are two key nutrients for algae. Testing and managing of these key nutrients in pool water is vital to keep algae blooms from appearing at times when sanitizer levels may drift low. Tests which reveal the presence of metal are paramount, especially at start-up time, and the presence of any metal such as copper or iron should ideally be addressed with the use of a metal chelator or sequestering agent before sanitizers are introduced into the pool water. If metals are not removed or sequestered, staining can occur when sanitizers are added to the water or as a result of the pH drifting up or down.

All in all, it is crucial to be proactive in managing pools, and proper testing of both the pool and source water cannot be overstated.

Author

Terry Arko is a product training and content manager for HASA Pool Inc., a manufacturer and distributor of pool and spa water treatment products in Saugus, Calif. He has more than 40 years’ experience in the pool and spa/hot tub industry, working in service, repair, retail sales, chemical manufacturing, technical service, commercial sales, and product development. He has written more than 100 published articles on water chemistry and has been an instructor of water chemistry courses for more than 25 years. Arko serves as a voting member on the board of the Recreational Water Quality Committee (RWQC). He is a commercial pool operator (CPO) course instructor, a teacher of the pool chemistry certified residential course for the Pool Chemistry Training Institute (PCTI), a California Pool and Spa Association (CPSA) board member, and a member of Pool & Spa Marketing’s editorial advisory committee. He can be reached via email at terryarko@hasapool.com.

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