As a part of regular maintenance, you shock a pool periodically to remove organic compounds, remove chloramines and free up the available chlorine to allow it to sanitise the pool. Two of the major indicators that a pool needs a shock treatment are a strong chlorine smell and swimmers complaining about eye irritation. These factors mean the pool needs more, not less chlorine, because chloramines produce the smell and irritation. They also bind up the free chlorine and keep it from performing its sanitising function.
Testing with a DPD Test Kit
Use a DPD (diethyl phenylene diamine) test kit to determine the free chlorine and total chlorine (free chlorine plus undesirable chloramines) in the pool. Use tab number one for free chlorine. Record the reading, and use tab number three to measure total chlorine. Record that reading also. If the total chlorine reading is higher than the free chlorine reading, the difference is the level of combined chlorine you need to remove. If the readings are the same, there is no combined chlorine in the pool.
Achieving Break point Chlorination
To remove chloramines and make free chlorine available in the water, you have to add 10 times the amount of free chlorine in the pool. For example, if the free chlorine level is 1.0 parts per million (ppm), you have to add 10 ppm of chlorine to shock the pool and remove the waste ammonia and other organic contaminants. This is known as break point chlorination. The result of achieving break point chlorination is called oxidation.
Calculating the Amount of Chlorine Needed to Shock a Pool
To determine the amount of chlorine needed to shock a pool, you need to know how many gallons of water are in the pool. If you don't already know, use the calculator in the Resource section to determine your pool volume. Water weighs 3.78kg. per gallon. The ppm calculation is a weight to weight measurement, so you need to know the total weight of your pool water.
To determine the total weight of chlorine needed to reach 10 times the amount of free chlorine in your pool (1.0 ppm in the example above), use the following example for a 10,000-gallon pool:
10,000 gallons X 378kg. = 378kg. Divide 1,000,000 by total pounds to determine how many ppm 0.454kg. of free chlorine will raise the free chlorine level. 1,000,000/83,400 = 11.99 (round to 12). One lb. of available chlorine will raise the free chlorine level in a 10,000-gallon pool to 12 ppm. To determine how much available chlorine you need to raise the level to 10 ppm, divide 10 by 12. 10/12 = .83. It will take .37.6kg. (about 4/5 lb.) of available chlorine to raise a 10,000-gallon pool to 10 ppm.
Using Different Types of Chlorine to Shock the Pool
Liquid chlorine contains almost exactly 0.454kg. of available chlorine per pound, so 4/5 gallon will raise the free chlorine level to 10 ppm.
For dry chlorine, you need to know the percentage of available chlorine per pound. The label on the container will list this percentage. For example, Cal-hypo contains 65 per cent available chlorine per pound. Therefore, it will take 0.581kg. of Cal-hypo (divide .83 by .65 = 1.28) to raise a 10,000 gallon pool from 1.0 ppm free chlorine level to 10 ppm free chlorine level.