LaMotte LTC3000wi Turbidity & Chlorine Lab Meter - Manual (Page 13)

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 fl ame 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 a 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 scattered 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 defi nes a nephelometric measurement. The light source
for nephelometric measurements can be one of two types to meet EPA
or ISO specifi cations. The EPA specifi es 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 specifi es a light
emitting diode (LED) with a wavelength of 860 ± 30 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 LTC3000we
meets the EPA specifi cation and the LTC3000wi meets the ISO
specifi cation. The nephelometric method is most useful for low turbidity.
The LTC3000we/wi 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 high turbidity
samples. It uses a detector placed at 90° to the light source for the
nephelometric method for low turbidity samples. The LTC3000we/wi
has a signal averaging option to improve the stability of readings on low
turbidity samples.
The LTC3000we/wi 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 LTC3000we/wi can also be used with the formazin
calibration. Stablcal ® standards below 50 NTU should not be used to
calibrate the LTC3000we/wi.
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 used to meet
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