Water balance in pools and hot tubs is a relatively simple concept. Take the readings from the most commonly tested parameters (pH, total alkalinity [TA], calcium hardness [CH], water temperature, and total dissolved solids [TDS]) and then use these values in a mathematical formula that produces a number. This number defines whether the water is corrosive, scale forming, or in ‘balance’ (i.e. neither corrosive nor scale forming).
Although there are many formulas available (called an Index), the most used and trusted one in the pool and hot tub industry is the Langelier Saturation Index (LSI). Here is a basic review of each parameter and how they interrelate.
Potential of hydrogen (pH)
A measurement of the acidic or basic nature of water, pH is measured on a scale from zero to 14. A pH of seven is considered neutral, whereas anything less is increasingly more acidic, while anything greater is increasingly more basic. Many may remember this from high school chemistry.
The liquid indicator called phenol red is generally used to measure pH; however, it can also be measured using a meter. The recommended pH range for pools and hot tubs is 7.4 to 7.6.
Total alkalinity comprises many different entities (e.g. carbonates, bicarbonates, hydroxides, etc.) and is defined as the buffering capacity of water to prevent wide pH swings. In other words, it helps control pH. A drop test, where the sample color goes from green to red, is typically used to test this water parameter.
Total alkalinity is a measurement of all forms, including the carbonate and cyanurate alkalinity present in a sample. Since water balance calculations only use the carbonate alkalinity portion, a correction should be applied to compensate for the cyanurate portion. This can be done by taking one-third of the cyanuric acid value and subtracting it from the tested total alkalinity value. The resulting number is called ‘carbonate’ alkalinity, which should be used for all treatment purposes and water balance calculations. The ideal range for total alkalinity in pools and hot tubs is 80 to 120 parts per million (ppm).
Calcium hardness in pool and hot tub water is a critical measurement since too much of it can lead to cloudy water and possible scaling, while too little can cause the water to ‘balance’ itself by seeking out calcium from grout, plaster, etc.
Similar to total alkalinity, calcium hardness is usually tested with a drop test where the sample color goes from red to blue. The recommended calcium hardness range is 200 to 400 ppm for pools and 150 to 250 ppm for hot tubs.
Water temperature contributes only slightly to water balance; however, it can affect test results if the sample temperature is greater than 29 C (85 F). Essentially, at these higher temperatures, the reagents used in the test can be cooked, which will result in a false reading. It is always best to let warmer water cool down before performing any tests.
Total dissolved solids
Probably the most misunderstood water balance parameter, total dissolved solids, is simply the accumulation of all the solubilized, inert material that has entered the water through treatment chemicals, bathers, and pretty much anything added to the water for whatever reason.
Total dissolved solids are not an issue unless the reading is 1500 ppm greater than the start-up level. It can take years (even decades in some cases) for TDS to become a treatable problem. This parameter is usually determined by taking an electronic measurement, although there are drop tests (and test strips) available. Due to the length of time, it can take for total dissolved solids to change, tests only need to be performed a few times a year.
Finally, this is where the LSI comes into play. The LSI uses all five parameters to determine if the water is balanced.
LSI = pH + TF + logCH + logALK – CONSTANT
Where: LSI = Langelier Saturation Index
pH = measured pH
TF = temperature factor
CH = measured calcium hardness
ALK = measured alkalinity minus cyanurate alkalinity
Constant = 12.1 if total dissolved solids are less than 1000 ppm and 12.2 if they are greater than 1000 ppm
When water is perfectly balanced, the LSI equals zero. If the LSI is +0.5 or greater, the water could become cloudy or scaling can occur. An LSI value of -0.3 or less may cause the water to become corrosive.
To avoid maintenance professionals from having to perform complicated mathematical equations in their head, or memorize log tables, most reliable test kit manufacturers have easy-to-use devices or even mobile phone apps that can do these calculations.
Common sense goes a long way in assuring a maintenance professional’s testing techniques will produce accurate, reliable, and consistent results.
Common sense action #1: Start the swim season with fresh reagents, and replace missing instructions and any broken, faded or stained equipment
Reagents are perishables. Just like a head of lettuce or pills in a medicine cabinet, the chemicals in a test kit (or on a test strip) will degrade over time, even under optimum conditions. The process of deterioration speeds up though when storage conditions are not ideal. Hot and cold temperature extremes, as well as prolonged exposure to air, sunlight, humidity, and moisture, the useful life of water treatment reagents will diminish. A maintenance professional cannot get accurate readings with deteriorated reagents, stained test cells, or faded color standards.
Common sense action #2: When replacing reagents or labware, do not interchange products from different manufacturers or even within one manufacturer’s product line
Each manufacturer makes its reagents in different concentrations. Their color standards are developed for specific reagent concentrations, and the view depths of test cells are highly specific. (For instance, this author’s company produces four different phenol red solutions for pH testing and a different type of color comparator is used with each.)
The exception to this rule is DPD (N, N Diethyl-1,4 phenylenediamine sulfate) liquid and tablet test kits, which can be interchanged in most color-matching chlorine/bromine tests.
Common sense action #3: Read the manufacturer’s instructions carefully before beginning a test
No matter how many times a maintenance professional has performed a test it is always wise to read the instructions, as manufacturers, in some cases, may change a procedure.
Take test strips for example. Some manufacturers instruct users to dip the strip quickly, while others direct the user to swirl the pads a specified number of times or swish the strip back and forth. If a maintenance technician dips when they should swish, the color development on the test strip will become compromised.
Further, if changes are made to the length of time a technician must wait when observing between test readings, and they are unaware, then the results obtained will be unreliable.
Common sense action #4: Use the specified sample volume and amount of reagent(s), and mix as directed
If a recipe calls for a teaspoon of salt and a tablespoon is added instead, the food will be inedible. Instructions for water tests are recipes, too. When a chemist develops a test procedure, everything is carefully calibrated to work together to give an accurate reading―the sample size, the amount of reagent dispensed, the way in which the chemicals are combined with the water sample, and the timing between each step of the procedure.
If the instructions say, crush the tablet and then stir until it is completely dissolved, the test must be performed in this manner or the reaction will be incomplete. If a technician is supposed to swirl after each drop and, instead, shakes the sample, they could unwittingly alter the pH and alkalinity.
The bottom line is, failure to follow the recipe exactly will result in an inaccurate reading, which could possibly lead to under- or overdosing the chemical treatment…or maybe not even being aware any treatment is needed.
Common sense action #5: Work with a water sample that is representative of conditions in the pool as a whole
If a pollster wants to learn if a certain region supports longer school days, but then only asks students, the results will be misleading. If one only asks teachers—another special interest group—the results will still be misleading. In fact, the pollster will follow a strict protocol to make certain their report reflects the opinions of a wide swath of the public.
Water sampling is like polling in that a technician does not want to base their assessment on atypical answers. For instance, water chemistry at the surface of the pool is atypical—it is interacting with the air’s chemistry, evaporation is taking place, and it is where oils and debris float. Water chemistry at a return line, make-up water inlet, or chemical feeders is atypical—treatment chemical concentrations are different in these locations than in the pool at large. Water drawn from corner locations (i.e. dead spots) may not have experienced the same mixing action as open areas.
Therefore, to obtain a representative sample, maintenance technicians should draw water samples from mid-pool away from return lines and from at least elbow depth. The water sample should be tested immediately so its characteristics do not have time to change.
Common sense action #6: Conduct color-matching tests in natural daylight or with the aid of a daylight simulator
Anyone who has ever walked out of the house wearing two pieces of clothing that looked like they matched, but then once outside noticed they did not has experienced the phenomenon known as metamerism.
Different light sources make for different colors. If a test is designed to be read in daylight (after all, pools are mostly outdoors), artificial lighting (i.e. incandescent and fluorescent) will affect one’s color perception. Sunglasses can skew a person’s color perception as well.
The best possible light is outside, facing north, with the sun behind the technician. A technician should never face the sun, as it will adversely affect their ability to match colors.
Common sense action #7: Practice good testing technique and safety
First, store all reagents and/or test kits away from children. When gathering water samples, it is wise to avoid using glass containers since they could shatter. To prevent staining and cross-contamination of tests, remember to rinse out the sample container and the tests cells thoroughly after every test.
Reagent bottles should also be kept tightly capped and never interchanged. Finally, maintenance technicians must remember to wipe up any spills promptly.
To sum it up, to protect a client’s pool and/or hot tub investment, it is not only essential to maintain balanced water, but it is also important for maintenance technicians to be able to perform a wide range of tests precisely. By following the guidelines presented in this article, a maintenance professional will be able to obtain accurate water test results that are required for a healthy pool and/or hot tub environment for their clients.
This article was written by Wayne Ivusich and originally appeared on Pool & Spa Marketing [link].