YSI 6000UPG - Manual (Page 167)

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Question 4: Should I carry out a 1-point, 2-point, or 3-point turbidity calibration, and
what values should I use?
Even though the default calibration value in your 6000 UPG is reasonably appropriate for the
“average” 6026 sensor, you should always carry out some multipoint calibration (2- or 3-point)
prior to your first usage to make certain that your system meets the YSI accuracy specifications
provided in the 6000
UPG manual. For the accuracy required in almost all environmental
applications, a 2-point calibration is sufficient, and we recommend that the two points be a 0 and
100 NTU. The YSI turbidity sensor shows a minor degree of underlinearity below 10 NTU
which can be minimized by a 3-point calibration at 0, 10, and 100 NTU, but the effect on
accuracy of this additional calibration point may not be worth the inconvenience for the average
user. For example, if only a 0 and 100 NTU 2-point calibration is carried out, a sample which is
actually 6.0 NTU would read about 5.4 NTU on the average probe.
Once the initial multipoint calibration has been carried out, we recommend that the accuracy of
the sensor at low NTU be checked (or reset) by performing a 1-point calibration at 0 NTU before
each usage.
Question 5: How often should I perform additional multipoint calibrations?
Of course, there can be no firm answer to this question which is frequently asked not only for
turbidity but for most of the other sensors in the 6000
UPG
array. Your frequency of calibration
will depend on the conditions under which your sonde is used and on the degree of accuracy
required in your application. Periodic calibration also confirms that the sensor is performing
properly with regard to its sensitivity and general function.
We can state that our empirical testing has indicated that the optical system of the 6026 probe is
very stable and is likely to require only infrequent calibration (for example, monthly) to remain
within the accuracy specifications of the instrument (+/- 5 % of reading or 2 NTU). However,
you should initially confirm the stability of the sensor for your typical sampling or monitoring
application by frequently checking the sensor reading in a standard other than 0 NTU prior to
increasing the time between multipoint calibrations.
Question 6: What about smoothing of turbidity data with the 6000
UPG
?
As described in Section 6. Principles of Operation of the 6000 UPG manual, some processing of
raw turbidity data is usually beneficial in terms of outputting values which reflect the “average”
turbidity at the site.
The extent of the filtering is defined during activation of the turbidity sensor in the Sensor setup
menu of the sonde software. During activation, you will be asked to input a value (1 to 15) for
“Number of Samples to Average”. The larger this number, the smoother the data presentation,
but the slower the response of the turbidity sensor to abrupt changes in turbidity. For most
applications involving both spot sampling and monitoring, we recommend that the “Number of