water balance page


Water balance is not such a complicated exercise. It is simply the relationships of different chemical parameters to each other. Your water is constantly changing. Anything and everything directly and indirectly affects water balance - from sunlight, wind and rain to the oil, dirt and cosmetics which may enter the water.

You will likely not change the water in your pool for many years. Continuous filtration and disinfection remove contaminants which keep the water enjoyable, but this is not water balance. A pool that is "balanced" has proper levels of pH, Total Alkalinity and Calcium Hardness. It may also be defined as water that is neither corrosive or scaling. This concept is derived from the fact that water will dissolve and "hold" minerals until it becomes saturated and cannot hold any more water in solution. When water is considerably less than saturated it is said to be in a corrosive or aggressive condition. When water is over saturated, and can no longer hold the minerals in solution; this is known as a scaling condition. So then, balanced water is that which is neither over or under saturated. The cliché that "water seeks its own level" certainly applies here. Water which is under saturated will attempt to saturate itself by dissolving everything in contact with it in order to build up its content. Water which is over saturated will attempt to throw off some of its content by precipitating minerals out of solution in the form of scale.

How do we know when our water is over or under saturated? First of all, we use a good test kit (with fresh testing reagents) to measure the chemical parameters of pH, alkalinity and calcium hardness.

pH:
pH is a measure of how acidic or basic the water is. pH is a logarithmic scale from 0-14, with 7 being neutral. Below 7, a substance is defined as being acidic, while levels above 7 are said to be basic or alkaline. Everything that enters your pool has a pH value. Heard of acid rain? This is rainfall with a very low pH. The human eye, at a pH value of 7.35, is just slightly basic. This is coincidentally, in range with proper pH levels for your pool. To have pH in balance, we adjust the water with additions of pH increasers (bases) or pH decreasers (acids) to achieve the range of 7.2 - 7.8. If your testing (recommended daily) of the water shows a pH value below 7.2, the water is in an corrosive (acidic) condition, and we need to add a base to bring the pH into a more basic range and prevent corrosion. Conversely, if the pH is above 7.8, we are in a scaling (basic) condition and must add an acid to bring down the pH to prevent the formation of scale.

Total Alkalinity:
A close cousin of pH, the level of alkalinity in the water is a measurement of all the carbonates, bicarbonates, hydroxides and other alkaline substances found in the pool water. pH is alkaline dependent; that is, alkalinity is defined as the ability of the water to resist changes in pH. Also known as the buffering capacity of the water, alkalinity keeps the pH from "bouncing" all over the place. Low alkalinity is raised by the addition of a base (just like pH); sodium bicarbonate is commonly used. High levels of alkalinity are lowered by the addition of an acid (again, just like pH). Experts recommend "pooling" the acid in a small area of low current for a greater effect on alkalinity. That is, adding an acid will lower both pH and alkalinity. Walking the acid around the pool, in a highly distributed manner is said to have a greater effect lowering the pH than the alkalinity. Pooling the acid has the opposite effect. A very important component of water balance, alkalinity should be maintained in the 80-120 ppm range for gunite and concrete pools, and 125-170 ppm for painted, vinyl and fiberglass pools. Levels should be tested weekly.

Calcium Hardness:
When we speak of scale, we are talking about Calcium Carbonate, which has come out of solution and deposited itself on surfaces. It is a combination of carbonate ions, a part of Total Alkalinity and Calcium, a part of the Calcium Hardness level. The test for Calcium Hardness is a measure of how hard or soft the water is. Hard water can have high levels of calcium and magnesium. If these levels are too high, the water becomes saturated and will throw off excess particles out of solution, which then seek to deposit themselves on almost any surface inside the pool. This is calcium carbonate scale, a whitish, crystallized rough spot. If the levels are too low, the water is under saturated. The water becomes aggressive as it attempts to obtain the calcium it needs. Such soft water will actually corrode surfaces inside the pool which contain calcium and other minerals to maintain its hardness demand. If your Calcium Hardness levels are too high, you can use TSP to lower the levels, or a product called Hydroquest. It can also be accomplished by dilution (adding water to the pool which has a lower calcium hardness content). Levels which are too low require the addition of calcium chloride. Recommended range for calcium hardness is 200 - 400 ppm. Levels should be tested weekly.

The Saturation Index:
Also called the Langelier Index, this chemical equation or formula is used to diagnose the water balance in the pool. The formula is SI = pH+TF+CF+AF-12.1. To calculate the Saturation Index, test the water for pH, temperature, calcium hardness and total alkalinity. Refer to a chart for assigned values for your temperature, hardness and alkalinity readings and add these to your pH value. Subtract 12.1, which is the constant value assigned to Total Dissolved Solids, and a resultant number will be produced. A result between -0.3 and +0.5 is said to indicate balanced water. Results outside of these parameters require adjustment to one or more chemical components to achieve balance. This formula is not foolproof, however. Some readings for pH, calcium and alkalinity which, taken individually would be considered to be well beyond recommendations, can combine within the formula to produce "balanced water", when it just ain't so. Regardless, the SI can be used to pinpoint potential water balance problems.



Information provided courtesy of my friends at

 Poolcenter.com