Protocol for Calibrating Turbidimeters
To effectively and accurately measure the clarity of water and provide an important “window” through which you can reliably judge the effectiveness of the treatment process, a properly calibrated turbidimeter is essential.
Turbidimeters need to be calibrated on a quarterly basis or as often as recommended by the manufacturer. Check the owner’s manual and follow the manufacturer’s instructions for calibration and record the calibration readings in a logbook.
For online, continuous recording turbidimeters, refer to the technical guidance document from the Drinking Water Branch listed below:
These guidelines became effective September 2000 and should be incorporated into your Standard Operating Procedures (SOP) manual.
TWO WAYS TO CHECK TUBIDIMETER ACCURACY
1. Calibration by comparison to a known standard. Use this method for quarterly checks.
2. Calibration by comparison to another turbidimeter. Use this method on a monthly basis to verify the performance of online reporting meters. Verification checks assume that the meter being used for comparison has, itself, been calibrated to a known standard. Ultimately, the validity of any turbidity reading depends upon the accuracy of the standard used to calibrate the turbidimeter.
TURBIDITY STANDARDS BACKGROUND
A turbidity standard is a liquid sample or other material having a defined and reproducible turbidity value. There are two types of turbidity calibration standards:
- Primary standards are used to either directly calibrate a meter or to calibrate a secondary standard. Primary standards are the “gold standard” for turbidity. All calibrations must eventually be traceable to a primary standard. The most common acceptable form of primary standard is formazin. The other is a commercially manufactured liquid suspension of styrene divinyl benzene polymer beads (SDB). Either is acceptable because it will give repeatable results. Formazin may be easily produced in the lab or may be purchased in various prepared forms. While a 4,000 NTU stock solution may be good for a year, more dilute solutions used in calibration generally must be discarded after one week. SDB solutions are more costly but remain stable almost indefinitely.
- Secondary standards include common examples such as liquid latex, SDB or Gelex solutions in a sealed sample container. They may also include glass rods or plates, plastic cylinders and mirror devices tailored for use in a specific manufacturer’s device. These standards are convenient, easy to use, cheap and reliable for daily calibration checks. However, to meet monitoring requirements, every secondary standard must first be calibrated to a primary standard. Also, since the secondary standard is likely to degrade with time, it is important to recheck its value against a primary standard every four months.
CALIBRATION TIPS
1. Allow the turbidimeter to warm up for at least 30 minutes prior to use.
2. Always use clean glassware and handle cuvettes so there are no fingerprints in the area where light passes through the sample.
3. Use alcohol and a lint-free cloth to clean the outside of the cuvettes.
4. Once the cuvette becomes scratched or pitted, replace with a new cuvette.
5. Rinse cuvette with about 0.5 ml of the standard to be used. Discard. Repeat with a second rinse to ensure that only the standard is present in the cuvette.
6. DO NOT pour used primary standard back into the bottle. This contaminates the contents.
7. Allow the standard to reach room temperature before using to alleviate fogging of the cuvette.
8. If problems are encountered with calibration, allow the sample to sit loosely capped with a clean cap until all air bubbles have settled out or gently tap sides of cuvette to release air bubbles, then take another reading.
If you have questions about calibration, using turbidimeters or your Standard Operating Procedure (SOP) manual, speak with technical ssistance staff listed below:
- Julie Roney, supervisor of Technical Assistance, phone 502-564-3410
- Jim Hamon, Frankfort, phone 502-564-3410
- Eric Sutton, Frankfort, phone 502-564-3410
- Crystal Davis, Columbia Regional Office, phone 270-384-4734
- Don DeKoster, Louisville Regional Office, phone 502-429-7122
- Jackie Logsdon, Madisonville Regional Office, phone 270-824-7529
