The HI96721 meter measures total iron (Fe) content in water samples in the 0.00 - 5.00 milligrams per liter (ppm) range. The reagent contains both a reducing and a complexing agent: the first converts all but very most resistant forms of iron present in the sample to the ferrous or soluble state; the second reacts with the ferrous iron to form the characteristic orange-colored complex. This meter uses an exclusive positive-locking system to ensure that the cuvette is in the same position every time it is placed into the measurement cell. It is designed to fit a cuvette with a larger neck making it easier to add both sample and reagents. The cuvette is made from special optical glass to obtain best results. 0.01 milligrams per liter (ppm) resolution. +/-0.04 milligrams per liter +/-2 percent of reading at 25 degree C accuracy. Tungsten lamp. Silicon photocell with narrow band interference filter at 525nm light detector. Adaptation of the USEPA method 315B and Standard Method 3500-Fe B. 0 To 50 degree C (32 to 122 degree F); RH maximum 95 percent non-condensing environment. 9V Battery. Auto-shut off: after ten minutes of non-use in measurement mode; after one hour of non-use in calibration mode; with last reading reminder. Measures 7.6" length by 4.1" width by 2.7" height.