Write an article based on this "Measure the free chlorine in the pool. Check the total chlorine in the pool. Subtract the free chlorine from the total chlorine to find the combined chlorine. Multiply the combined chlorine by 10 to find the breakpoint level. Figure out the desired change amount by subtracting the free chlorine level from the breakpoint level. Divide the volume of your pool by 10,000 gallons. Multiply the pool volume by the amount of product needed to raise the chlorine level 1 ppm."
Dip the test tube from a DPD kit down to a depth of 1.5 feet (0.46 m) to fill it up. Make sure the water reaches the fill line. Place a DPD #1 free chlorine tablet into the water in the tube. With the lid on, shake the tube until the tablet dissolves, then check the color of the water against the chart.  You can find these kits online or at pool supply stores; they measure how much chlorine is in the pool. They give the value in parts per million (ppm). Some kits may use drops instead of tablets. Always read the instructions for your kit. Write down the number for the free chlorine in your pool. Open the test tube. Using the same water already in the tube, add a DPD #3 total chlorine tablet. Put the lid on and shake the tube until the tablet dissolves. Compare the color to the given chart, which will give the total chlorine in ppm. Write down the total chlorine for your pool. Combined chlorines are the result of chlorine mixing with body waste like skin, oil, and urine. When the chlorine mixes like this, it's not as effective at disinfecting the water. Free chlorine is much more effective. For instance, if your total chlorine is 2.5 ppm and your free chlorine is 1.2 ppm, your combined chlorine is 2.5 - 1.2 = 1.3 ppm. The breakpoint level is when enough free chlorine is in the pool to break down the combined chlorines in your water. If you don't add enough, all the free chlorine will just convert to combined chlorines. If your combined chlorine level was 1.3, multiply that by 10 to equal 13, your breakpoint level. The desired change amount tells you how much you want the water to change in ppm. That will guide how much chlorine you add to the pool. For instance, the breakpoint level in the example is 13. Subtract the free chlorine level of 1.2 from 13 to get 11.8 ppm, the desired change amount. Typically, shock chemicals give an amount it takes to change the ppm level by 1 in 10,000 gallons (38,000 L). Therefore, divide the volume of your pool by 10,000 gallons (38,000 L).  For instance, if your pool is 60,000 gallons (230,000 L), divide by 10,000 gallons (38,000 L) to get 6. Always check your product for the correct amount. Check the back of the bottle again. It will tell you how much you need of the product to raise the chlorine level by 1 ppm in 10,000 gallons (38,000 L). For instance, you may need 2 ounces (57 g) of the product to raise the pool by 1 ppm. Multiply that number by the pool volume number you found in the last step and the desired change amount.  In the example, that would be 2 ounces (57 g) x 6 x 11.8 ppm = 141.6 ounces (4,010 g). Divide that number by 16 to get pounds: 141.6/16 = 8.85 pounds.