LaMotte TC-3000 Tri-Meter - Manual (Page 6)

Turbidity is measured by detecting and quantifying the scattering of light in water
(solution). Turbidity can be measured in many ways. There are visual methods and
instrumental methods. Visual methods are more suitable for samples with high
turbidity. Instrumental methods can be used on samples with both high and low levels
of turbidity.
Two visual methods are the Secchi Disk method and the Jackson Candle method. The
Secchi Disk method is often used in natural waters. A black and white Secchi Disk is
lowered into the water until it can no longer be seen. It is then raised until it can be
seen again. The average of these two distances is known as the “Secchi Depth”. The
Jackson Candle method uses a long glass tube over a standard candle. Water is
added or removed from the tube until the candle flame becomes indistinct. The depth
of the water measured with a calibrated scale is reported as Jackson Turbidity Units
(JTU). The lowest turbidity that can be determined with this method is about 25 NTU.
There are two common methods for instruments to measure turbidity. Instruments can
measure the attenuation of a light beam passing through a sample and they can
measure the scattered light from a light beam passing through a sample. In the
attenuation method, the intensity of a light beam passing through turbid sample is
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 TC-3000e meets the EPA specification and the TC-3000i meets the ISO
specification. The nephelometric method is most useful for low turbidity.
The TC-3000 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 TC-3000 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 TC-3000 also has a signal averaging
option to improve the stability of readings on low turbidity samples.
The TC-3000 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 TC-3000 can also be used with the formazin
calibration. Stablcal
â standards below 50 NTU should not be used to calibrate the
TC-3000.
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.
6 TURBIDITY