YSI 6 series User Manual page 116

Sondes
Choosing 4-Threshold will display the following:
---------------Threshold--------------
1-Turbid=0.01
2-Chl=1
3-Other=0.001
Select option (0 for previous menu):
Setting thresholds is done in the same manner. Recommended threshold settings are 0.01 for turbidity, 1.0
for chlorophyll, 1.0 for rhodamine WT, 1.0 for BGA-PC, 1.0 for BGA-PC and 0.001 for "other".
The following descriptions provide additional information about the Data Filter feature.
1-(*) Enabled. Activating this item will result in data filtering according to the values set in (2), (3), and
(4).
2-(*) Wait for filter. If this feature is activated, readings will be available for output only after the unit has
warmed up for a time period equal to the Time Constant plus an extra 4 seconds. This feature is useful, for
example, if you are operating in SDI12 mode and want to average the data over a particular period of time.
In such a case, you would not want the filter to be engaging and disengaging, so the value of the Threshold
should be set to a large value like 1. This feature should not be activated for normal use of the sonde.
3-Time Constant. This value is the time constant in seconds for the software data filter. Increasing the
time constant will result in greater filtering of the data, but will also slow down the apparent response of the
sensors.
4-Threshold. This value determines when the software data filter will engage/disengage. When the
difference between two consecutive unfiltered readings is larger than the threshold, then the reading is
displayed unfiltered. When the difference between two consecutive readings drops below the threshold,
readings will be filtered again. For the purposes of the filter, consecutive readings are never more than 0.5
seconds apart. When sampling faster than 0.5 seconds consecutive readings will be at the faster rate and
you may want to adjust the threshold accordingly. See section 2.9.1 for details on fast sampling.
The threshold feature is intended to speed response to large changes in a reading. For example, when
changing from pH 7 buffer to a pH 4 buffer in a calibration, it is likely that the filter will disengage for a
time showing unfiltered readings until the sensor has nearly equilibrated with the new buffer. At that time,
the filter will re-engage and show filtered readings. Without disengaging the filter for awhile, much more
time would be required to come to equilibrium after large changes in reading.
During the first time constant after the filter first engages, the output reflects a simple average of all the
readings from the time the filter engaged until the present. Once the filter has been engaged for the period
of the time constant, it becomes a simple filter with a time constant equal to that set in 3-Time Constant.
Each time the filter disengages and then re-engages, this process is repeated.
Filter engagement and disengagement occurs for each sensor independently. One parameter may be filtered
while another is not because readings from one sensor are changing more than another.
Example: Moving a sonde from the air to a river water sample. Assume that the temperature of the sonde is
similar to the temperature of the water, and that the water is in equilibrium with the air. The temperature
and oxygen readings taken in the water will be very similar to those taken in the air. The conductivity
YSI Incorporated Environmental Monitoring Systems Operations Manual
Section 2
2-110