Tests both FC and TC. You have to subtract the FC from the TC to get your CC. This test will turn different shades of red when chlorine is present and uses a color comparator that you match up the shade of red with. Many men have problems with this test because they are unable to differentiate the shades of red! This test can bleach out when chlorine levels go above 10 ppm and will turn colorless. This can lead you to believe that there is NO chlorine in your pool when it is actually very high. You can use OTO to verify if chlorine is present since it will not bleach out at high chlorine levels.