LaMotte 2020i Turbidimeter - Manual (Page 7)

compared with the intensity passing through a turbidity-free sample at 180° from the
light source. This method is good for highly turbid samples. The most common
instrument for measuring scatter light in a water sample is a nephelometer. A
nephelometer measures light scattered at 90° to the light beam. Light scattered at
other angles may also be measured, but the 90° angle defines a nephelometric
measurement. The light source for nephelometric measurements can be one of two
types to meet EPA or ISO specifications. The EPA specifies a tungsten lamp with a color
temperature of 2,200–3,000 K. The units of measurement for the EPA method are
nephelometric turbidity units (NTU). The ISO specifies a light emitting diode (LED) with
a wavelength of 860 nm and a spectral bandwidth less than or equal to 60 nm. The
units of measurement for the ISO method are formazin nephelometric units (FNU).
The 2020e meets the EPA specification and the 2020i meets the ISO specification.
The nephelometric method is most useful for low turbidity.
The 2020 is a nephelometer that is capable of measuring turbidity by both the
attenuation method and the nephelometric method. It uses a detector placed at 180°
to the light source for the attenuation method. It uses a detector placed at 90° to the
light source for the Nephelometric method. The 2020 also has a third detector that
monitors the intensity of the light source. It uses this detector to improve instrumental
stability and minimize calibration drift. The 2020 also has a signal averaging option to
improve the stability of readings on low turbidity samples.
The 2020 has two different turbidity calibrations, formazin and Japan Standard. The
formazin calibration is the EPA and ISO approved method of calibrating
nephelometers. This calibration can be used with user prepared formazin standards or
commercially purchased formazin standards. LaMotte Company approved AMCO
TM
standards labeled for use with the 2020 can also be used with the formazin
calibration. Stablcal
â standards below 50 NTU should not be used to calibrate the
2020.
The Japan Standard calibration is a calibration for a Japanese Water Works standard.
It is based on Japanese formulated polystyrene turbidity standards. This calibration
should only be use to meet Japanese Water Works requirements. The Japanese
polystyrene standards can only be purchased in Japan. Formazin, AMCO and
Stablcal
â standards cannot be used with this calibration.
Taking Turbidity Water Samples
Clean plastic or glass containers may be used for turbidity samples. Ideally, samples
should be tested soon after collection and at the same temperature as when collected.
SAMPLE DILUTION TECHNIQUES
If a test result is out of the range of the meter, it must be diluted. The test should then
be repeated on the diluted sample. The following table gives quick reference
guidelines for dilutions of various proportions.
Amount of Sample Deionized Water to Bring
Final Volume to 10 mL Multiplication Factor
10 mL 0 mL 1
5 mL 5 mL 2
2.5 mL 7.5 mL 4
1 mL 9 mL 10
0.5 mL 9.5 mL 20
INTRODUCTION•SAMPLE DILUTION TECHNIQUES 7