Hach-Lange 2100 AN IS Laboratory Turbidimeter Basic User Manual - Manual (Page 19)

When automatic ranging is selected, the display will flash all 0s when the
measurement is less than the range of the instrument (i.e., less than
20 FAU) or a negative value. Calibrate the instrument.
• When measuring absorbance or transmittance, set the zero reference
point again.
• When measuring absorbance, make sure that the reading is positive.
To measure samples with negative absorbance, set the analytical
zero using the sample with the greatest absorbance and read the
sample with the least absorbance. Record the reading as negative
absorbance.
English 19