YSI 900 [250900Y] Free and Total Chlorine Colorimeter. The YSI 900 Chlorine Colorimeter is a rugged, waterproof, single parameter instrument for the measurement of Total chlorine or Free chlorine. Whether you need to measure chlorine in wastewater, chlorine in groundwater, or in pools, this chlorine tester is waterproof with an easy to read display and will provide readings in minutes. The instrument features a real time clock, automatic data logging as well as the ability to export data to a PC or printer using the 900 Series Data Hub for infrared data transfer.