YSI 6000UPG - Manual (Page 115)

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readings. Note that YSI offers 100 NTU turbidity calibrant as a standard product (6073) and that
this standard can be quantitatively diluted with turbidity-free water to provide a 10 NTU calibrant
suspension.
The overall sensitivity (slope) of the optical sensor is generally less susceptible to drift than its
offset (probe output at 0 NTU). The 1 point calibration is designed to reset this offset without
affecting the slope and thus provide more accurate performance in water of low turbidity. The
1 point calibration must be done at 0 NTU.
While the effect of temperature on the turbidity sensor is small (approximately 0.3 % per °C ),
this factor is automatically taken into account by the sonde software providing temperature
compensated readings. However, all sensors have slightly different temperature factors and thus
to obtain the maximum accuracy in your readings, we recommend calibration at a temperature as
close as possible to that of the environmental sample.
MEASUREMENT AND CALIBRATION PRECAUTIONS
(1) For best results, use only freshly prepared turbidity standards. Under some conditions,
degradation of standards can occur on standing, particularly at lower (less than 20 NTU) values.
(2) If unusually high or jumpy readings are observed during the calibration protocol, it is likely
that there are bubbles on the optics. The bubbles can be cleaned from the surface by activating
the wiper prior to each calibration input or by manually moving the sonde (up/down and
back/forth)
(3) The output of turbidity sensors is susceptible not only to the overall cloudiness of the
environmental medium, but also to the particle size of the suspended solids which pass across the
optics on the probe face. Thus, although the turbidity of an environmental sample may appear to
the eye to be relatively stable, the displayed turbidity can vary significantly depending on the
nature of the particles in the optical path at the instant of measurement. For example, if
individual readings are taken every 4 seconds in a discrete sample study of environmental water,
variations of 0.5-1.0 NTU are common between readings. In long term, unattended studies this
effect can be even more exaggerated with spikes of up to 10 NTU sometimes observed. This
apparent jumpiness is not observed for freshly prepared turbidity standards, since the particle
size in these suspensions is very uniform.
The 6000 UPG turbidity system allows the user to either observe these real turbidity events (while
obtaining somewhat jumpy readings) or to average several readings to obtain an NTU output
which may be more reflective of the overall cloudiness of the environmental sample. The user
can select the number of samples to average on activation of the turbidity sensor in the Sensor
Setup portion of the sonde menu. If the user selects 8 at the Number of Samples to Average
prompt, the 6000
UPG will calculate an 8 point, continuous running average of turbidity readings
in Discrete sample mode. This data processing will have an adverse effect on the response time
of the sensor, but for sampling applications, this is usually a minor inconvenience since many of
the other sensors take a least 1 minute to completely stabilize. The advantage of the data
processing is a much more stable sequence of turbidity readings output to the screen.