Necessity and Cycle of Calibration
Regular calibration is crucial for ensuring the accuracy of turbidimeter measurements. The calibration cycle should be determined based on the instrument's usage frequency and environmental conditions. For some online turbidimeters, calibration every 3 months is recommended. Instruments that have not been calibrated for an extended period (e.g., more than 3 months) will experience a gradual decrease in accuracy due to electronic component drift.
Preparation Before Calibration
The preparation of zero-turbidity water requires filtering distilled water using a microporous membrane with a pore size no larger than 0.2 μm. Filtering should be repeated at least twice. The resulting filtrate is the zero-turbidity water for calibration. The initial filtrate of approximately 200 mL should be discarded. Ultrapure water or pure water prepared in the laboratory using professional water purification equipment can also be used as zero-turbidity water, but pure water must be filtered according to requirements. Standard substances are typically national secondary standard substances-formazan standard solution-diluted to obtain the corresponding calibration series. Valid standard substances should be used.
Calibration Methods and Procedures
Calibration methods mainly include point calibration, two-point calibration, and multi-point calibration. For example, the Hach 1720E low-range turbidity analyzer can be calibrated using either a two-point calibration method (0 and 20 NTU) or a single-point calibration within the 1-40 NTU range. The Hach TU5300 sc/TU5400 sc online turbidity analyzer also supports single-point calibration within the 0–40 NTU range. Specific calibration procedures typically include zeroing and range calibration. For zeroing, fill a clean sample bottle with zero-turbidity water, place it in the instrument, and after the displayed value stabilizes, perform the zeroing operation to achieve a value of 0.00 NTU. For range calibration, use a series of standard turbidity solutions, input the standard values, and perform calibration after the display stabilizes.
Key Points for Daily Use
Before measurement, remove air bubbles from the sample and shake well to avoid measurement errors caused by air bubbles or sediment. Test samples as soon as possible to prevent temperature changes or sedimentation from affecting the results. Cuvettes must be clean and free of scratches. Avoid touching the optical path surface when handling them. A thin layer of silicone oil can be applied to the outer wall of the sample vial to reduce scattering interference caused by minor scratches. The sample vial should be thoroughly cleaned after each test. Select an appropriate instrument range based on the estimated turbidity range of the sample, avoiding using a large range to measure low turbidity or a small range to measure high turbidity. For colored liquids, use an instrument with color compensation to eliminate color interference. The instrument should be placed in a well-ventilated and dry place, avoiding direct sunlight and strong electromagnetic interference. Drastic fluctuations in ambient temperature can also affect the stability of electronic components.
Daily Maintenance and Care
Regularly clean the optical window and cuvettes to avoid contamination from dust, water stains, or sample residue. Some online turbidimeters have automatic cleaning devices, which can significantly reduce maintenance workload. Promptly check and replace consumable parts, such as aging light sources. Some advanced models are equipped with the Prognosys pre-diagnostic system, which can predict equipment maintenance and alert you to potential malfunctions. Regularly back up the measurement data stored in the instrument and export the data using USB interfaces or other functions for easy traceability and analysis.
