In-Situ TROLL 9500 Operators Manual - Manual (Page 102)

95
TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
SECTION 13: DISSOLVED OXYGEN—OPTICAL
The values will be written to the sensor and you will be asked if
you want to see the calibration report.
TIP: For more on Calibration Reports, see Section 10 of this
manual.
The RDO sensor is now calibrated and ready to take readings. To
confirm this, take a reading by selecting the RDO sensor in the Navi-
gation tree and clicking the Read button.
TIP: If the units for the reading are not what you expect, it is
easy to change the units selection: In Win-Situ, select
Preferences on the Options menu. In Pocket-Situ, select the
Home site, then tap Setup in the command bar.
Clean-up
Rinse the sensor and front end of the instrument very thoroughly after
calibrating in sodium sulfite solution to avoid cross-contamination. For
best results, always use fresh calibration solutions.
UNITS AND CALCULATED MEASUREMENTS
Four units are available for dissolved oxygen:
TIP: To change unit preferences: In Win-Situ, select
Preferences on the Options menu. In Pocket-Situ, select the
Home site, then tap Setup in the command bar.
s
(mg/L). Since a liter of pure water weighs 1000 grams, and a mil-
ligram is 1/1000 of a gram, this is equivalent to ppm (parts per
million).
s
This is equivalent to parts per billion (ppb).
s
s
of oxygen that can be dissolved in water at a given atmospheric
pressure, water temperature, and salinity. The percent saturation
output is automatically corrected using the TROLL 9500 instru-
ment’s temperature, conductivity, and barometric pressure values
(from a baro sensor on vented cable or from a user-entered input).
If no conductivity sensor is present, salinity is assumed to be zero.
USAGE RECOMMENDATIONS AND CAUTIONS
The amount of oxygen that can be dissolved in water decreases at
higher temperatures and with increasing altitude (i.e., as baromet-
ric pressure drops) and salinity. In other words, as water becomes
warmer and saltier, it can hold less dissolved oxygen.
During tests that include the RDO channel, the TROLL 9500 instru-
ment automatically measures barometric pressure and temperature
for compensation of DO readings.
The barometric pressure value at the time of calibration is stored
in the sensor and will be used to correct DO readings for weather-
INDUCED
Exposure to direct sunlight can bleach the sensing foil over time but
this effect is minimized with the protection provided by the opaque
optical isolation layer. However, the frequent application of strong blue
light (blue LED during readings) will eventually have a bleaching effect
on the foil. In-Situ recommends sampling intervals of greater than 1
minute for long-term deployments.
Salinity
Since the absolute solubility of oxygen is lower in saline waters, it is
advantageous to compensate DO concentration readings (μg/L, mg/L,
μmol/L) to ensure that the sensor accurately reports the concentration
of dissolved oxygen in the presence of significant salinity. This can be
accomplished by storing a salinity value in the sensor before taking
measurements. The compensation algorithm is applied internally
before the concentration is reported. The degree of compensation is
minimal at very low salinities, and but may affect readings by several
percent at higher levels.
To store a salinity value in the RDO sensor, connect in Win-Situ 4 or
Pocket-Situ 4, select RDO in the Navigation tree, click Edit and in the
next screen select “Edit RDO Salinity Value.”
SENSOR CARE AND HANDLING
BIOFOULING
Organisms that produce or consume oxygen, if attached to the area
OF
To avoid this the sensor should be cleaned at regular intervals de-
pending on the required accuracy and the fouling condition at the site.