YSI 6 series User Manual page 256

Principles of Operation
sonde software to provide readings in nephelometric turbidity units (NTUs). A schematic of a YSI
turbidity sensor is shown in the following picture.
Light Source
Near Infrared LED
Two turbidity probes have been available for use with YSI 6-series sondes – the 6026 and the 6136. The
6026 probe is no longer available from YSI, but since a large number of this probe type are still in use, a
description of the sensor will be included in this section. The probes are detailed briefly below; both are
equipped with a mechanical wiper to periodically clean the sensor either by manual or automatic activation.
These wiper systems make the probe ideal for long term monitoring, but they also work well for spot
sampling applications.
 The Model 6026 probe was offered by YSI from 1995 to approximately 2002 and is characterized by
relatively small optics, a factor that results in minimal penetration of the light beam into the sample
and thus allows the use of shorter probe guards on the sonde in which it is installed. If a 6026 sensor
is selected in the Sensor menu of the sonde, the turbidity will automatically be reported in the units
"turbid NTU".
 The Model 6136 has been the only turbidity probe sold by YSI since 2002 and it is characterized by
relatively large optics compared to the 6026. Since these larger optics result in deeper penetration of
the light beam into the sample, the use of a longer probe guard is required, a slight disadvantage.
However, the larger optical cell volume of the sensor has two significant advantages: (1) There is less
background noise associated with the turbidity readings and (2) the absolute values of the readings are
significantly closer to those from typical laboratory turbidimeters (such as the Hach 2100AN) which
use a large cell volume. If a 6136 sensor is selected in the Sensor menu of the sonde, the turbidity will
automatically be reported in the units "turbid+ NTU".
In the primary standard formazin, the two turbidity sensors (6026 and 6136) will show effectively identical
behavior. In addition, AEPA-AMCO polymer beads (supplied by YSI as Model 6073) can also be used
with one important qualification. You will need to remember that the 6026 and 6136 sensors, which read
the same in formazin suspensions, will have different responses in the suspensions of the AEPA-AMCO
beads. This effect is due to the larger optical cell volume of the 6136. Thus, as noted on the label of the
YSI Incorporated Environmental Monitoring Systems Manual
Section 5
Photodetector
Optical Fiber
5-18