Hanna Marine Alkalinity Checker HI755 The HANNA HI 755 Checker HC bridges the gap between simple chemical test kits and professional instrumentation. Chemical test kits are not very accurate and have low resolution while professional instrumentation can cost hundreds of dollars and can be time consuming to calibrate and maintain. The HANNA HI 755 Checker HC is accurate and affordable. The Checker HC features an accuracy of 5 ppm (mg/L) 5% of reading and uses the colorimetric method.