Xylem 6-Series Multiparameter - Manual (Page 244)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-6
4 - During pH calibration, allow the sensors time to stabilize with regard to temperature (approximately 60
seconds) before proceeding with the calibration protocol. The pH readings after calibration are only as
good as the calibration itself.
5- The true pH values of commercially-available buffers are slightly temperature dependent with the actual
pH values at various temperatures usually shown on the bottle. For example, the actual pH of YSI “pH 7
buffer” at 20 C is 7.02 rather than the value of 7.00 at 25 C. Users who wish to obtain the maximum
accuracy from their pH sensors should first determine the temperature of their buffers and then enter the
proper pH reading for that temperature (from the bottle label) when carrying out calibration of pH.
6 - Clean and store the probe according to the instructions found in Section 2.10, Care, Maintenance and
Storage of this manual.
5.6 DEPTH AND LEVEL
The sondes can be equipped with either depth or level sensors. In fact, both sensors measure depth, but by
YSI convention, level refers to vented measurements and depth refers to non-vented measurements. Both
measurements use a differential strain gauge transducer to measure pressure with one side of the transducer
exposed to the water.
For depth measurements, the other side of the transducer is exposed to a vacuum. The transducer measures
the pressure of the water column plus the atmospheric pressure above the water. Depth must be calculated
from the pressure exerted by the water column alone; therefore, when depth is calibrated in air, the software
records the atmospheric pressure and subtracts it from all subsequent measurements. This method of
compensating for atmospheric pressure introduces a small error. Because the software uses the
atmospheric pressure at the time of calibration, changes in atmospheric pressure between calibrations
appear as changes in depth. The error is equal to 0.045 feet for every 1mm Hg change in atmospheric
pressure. In sampling applications, frequent calibrations eliminate the error. Considering typical changes
in barometer during long-term monitoring, errors of ± 0.6 feet (0.2m) would be common. In applications
where this error is significant, we recommend using a level sensor in place of the depth sensor.
As with depth measurements, level uses a differential transducer with one side exposed to the water.
However, the other side of the transducer is vented to the atmosphere. In this case, the transducer measures
only the pressure exerted by the water column. Atmospheric pressure is ignored and changes in
atmospheric pressure do not affect the reading at all.
The voltage output of the transducer is directly proportional to the pressure. The sonde software converts
this voltage to a depth reading in feet or meters via calibration parameters that are factory installed.
Readings are automatically compensated for the temperature and for the density of the environmental
medium that is estimated from the measured salinity.
For more additional information on measuring level, see Appendix G, Using Vented Level.
CALIBRATION AND EFFECT OF TEMPERATURE
The depth sensor must be zeroed prior to deployment to account for atmospheric pressure. Level sensors
may also require a small adjustment prior to their first use. This procedure is carried out by following the
calibration menu instructions with the sonde in air only (do not submerge). The sensors can also be set to
any known depth via the calibration routine after they are immersed. The temperature dependence of the
sensor is automatically taken into account by the sonde software based on input from factory calibration.