There could be several reasons for this.
1. There is a gentle drop in the voltage response of the Turbidity Sensor for the first several minutes that it is plugged in. If it is calibrated right when it is plugged in, then some time passes before a fairly clear sample is tested, the voltage may have dropped enough to produce a negative value. If this is suspected to be the case, the best solution is to let the sensor warm up for several minutes and recalibrate.
2. The distilled water used for the 0 NTU calibration point was not perfectly clear.
3. Fingerprints, lint, or dirt on the cuvette.
4. The white triangles on the cuvette and Turbidity Sensor were not lined up properly for either the calibration points or the sample reading.
5. The sample cuvette is not filled to the proper volume. The bottom of the meniscus should be at the top of the white line.