31
SECTION 2,
continued
2.3.6
Measuring Overrange Samples
Nephelometric turbidity measurement depends on detection of light
scattered from particles suspended in the liquid. If the turbidity is very
high, a significant amount of light is blocked or absorbed by the
particles and only a small amount of light reaches the detector.
This results in a negative interference – the measured turbidity is
lower than the actual turbidity. This condition is called “going blind”.
A multidetector ratioing instrument, such as the Hach 2100P
Turbidimeter, minimizes this effect and extends the instrument range.
Highly turbid samples may also be diluted, but this should be avoided
when possible since it may alter the characteristics of the suspended
particles and produce erroneous results.
Light absorbing particles such as activated carbon and highly colored
samples may also cause an instrument to “go blind”. Dilution may not
correct for these interferences. A ratioing instrument will correct for the
presence of light absorbing particles and color.
2.3.7
Condensation (fogging)
Condensation may occur on the outside of the sample cell
when measuring a cold sample in a warm, humid environment.
Condensation interferes with turbidity measurement, so all moisture
must be thoroughly wiped off the sample cell before measurement.
If fogging recurs, let the sample warm slightly by standing at room
temperature or immersing it in a warm bath for a short period. After
warming, mix the sample thoroughly before measurement. Allowing
samples to warm can alter sample turbidity, so it is best to avoid
warming samples before measurement when possible.
2.3.8
Calibration
Turbidimeters must be properly calibrated with a primary standard.
Hach recommends formazin or StablCal Stabilized Formazin for
calibration. For U.S. Environmental Protection Agency (USEPA)
reporting, calibrate at least as often as required by the appropriate
regulatory agencies. The frequency of calibration depends on
environmental conditions (humidity, temperature) and use. If necessary,
calibrate more frequently.
