How are turbidimeters calibrated?
How to calibrate a turbidimeter
Hach turbidimeters are calibrated by placing one or more prepared primary standards into the instrument and then completing a series of keystrokes on the instrument keypad. Step-by-step instructions are included in each instrument manual. The primary standard for turbidity is formazin. Formazin is a polymer solution that can be synthesized in the laboratory or purchased from Hach as a 4000-NTU standard. The 4000 NTU standard must be diluted to make several lower NTU standards immediately before calibration. Hach Stablcal® Standards are stabilized formazin standards that are recognized by the USEPA as primary formazin standards. They are widely used for turbidimeter calibration due to their convenience and ease of use. Stablcal® standards come in varying NTU concentrations and do not require dilution or any preparation other than mixing. All standards, except for the dilution water (<.1 NTU), must be gently inverted immediately before calibration to ensure that the formazin polymer is suspended. Wait a minute or two to allow entrained gases to escape the solution. Never agitate or invert the dilution water standard.