Do not remove air bubbles by letting sample stand for a period time because during standing, turbidity-causing particulates ma settle and sample temperature may change. Both of these conditior alter sample turbidity, resulting in a nonrepresentative measure ment.
Condensation may occur on the outside surface of a sample cell when a cold sample is being measured in a warm, human environment. This interferes with turbidity measurement Remove all moisture from the outside of the sample cell befor placing the cell in the instrument. If fogging recurs, let sample warm slightly by letting it stand at room temperature or partially immersing it in a warm water bath for a short time Make sure sample are again well mixed.
b. Nephelometer calibration : Follow the manufacture is erating instruction Run at least one standard in each instrume range to be used. Make certain the nephelometer gives stab reading in all sensitivity ranges used. Follow techniques
c. measurement of turbidity : Gently agitate sample. Wait until air bubbles disappear and pour sample into cell. When possible,pour well-mixed sample into cell and immerse it in an ultrasonic bath for 1 to 2 s or apply vacuum degassing,causing complete bubble release. Read turbidity directly from instrument display.
d. calibration of continuous turbidity monitors: calibrate continuous turbidity monitors for low turbidities by determining turbidity of the water flowing out of them, using a laboratory-model nephelometer, or calibrate the instruments according to appropriate secondary standard