Confirm the NTU calibration point for the standard used, according to manufacturer's instructions. Discard used standard. Measure the turbidity of this suspension, making certain that it is within the accuracy specification of the instrument with regard to the true value. Repeat the calibration procedure if the measurement is not within the specification. Spectrophotometers are inaccurate for absolute turbidity values, and the instrument sensitivity is unrated.
Spectrophotometers commonly have a stored program for turbidity that has been factory-calibrated.
This can be verified but not adjusted. Check the instrument output against that of a different instrument every few weeks while the instrument is in use. Check the relative accuracy of the turbidity measurement before leaving for the field by inserting Formazin standards covering the FTU range needed. Use freshly prepared standards. Be accurate in your dilution of the stock suspension.