HOBO Dissolved Oxygen Logger (U26-001) Manual
5
•
Do not deploy the logger in freezing water with moving ice
where the logger could be crushed.
•
Use the optional anti-fouling guard to protect against
fouling. Unscrew the protective guard and replace it with
the anti-fouling guard.
•
If fouling is expected during deployment, use field
calibration readings from both the beginning and end of
the deployment as described in the next section. These
readings can then be entered into the HOBOware
Dissolved Oxygen Assistant to compensate for any
measurement drift due to fouling. Scrub fouling off the
logger with a plastic bristle brush.
•
When deploying the logger in saltwater with small
changes in salinity, you will need a conductivity or salinity
value from either a conductivity meter or salinometer to
enter in the Dissolved Oxygen Assistant to adjust the data
from the logger for salinity. A single meter reading will
add less than 1.1% DO error (assuming the conductivity
changes are within ±3,000 µS/cm from the calibration
point).
If the conductivity changes, then you will need a data file
with salinity or specific conductivity readings for the
entire deployment. Consider deploying a HOBO
Conductivity logger (U24-002-C) next to this DO logger to
use the resulting data file for salinity data. For U24-002-C
conductivity readings within a ±30,000 µS/cm range,
there will be less than 4% error added to the DO
measurements, and for readings over a narrower range,
the accuracy will be even better. Refer to the
HOBO
Conductivity Logger (U24-002-C) Manual
for more
details. For applications that require higher accuracy
conductivity data than the U24-002-C can provide, use a
third-party conductivity logger.
•
To generate a percent saturation series, you will need to
deploy a barometric pressure logger (such as a HOBO
Water Level Logger, U20-001-0x or U20L-0x) or have
access to a nearby weather station to gather barometric
pressure data. This data is necessary for the Dissolved
Oxygen Assistant to calculate percent saturation.
Taking Field Calibration Readings
If fouling is expected during the deployment, you can take
calibration readings at the beginning and end of the
deployment to enter in the Dissolved Oxygen Assistant. This will
adjust the data from the logger to compensate for any
measurement drift due to fouling. There are two methods for
taking field calibration readings: the first method involves
taking readings using a dissolved oxygen meter or titration
while the second method involves calibrating the logger in
100% water-saturated air. The first method is recommended
because it is quicker to get the necessary calibration readings;
the second method can take 40 minutes or more to achieve
equilibrium with temperature extremes.
To Take Calibration Readings Using a DO Meter or Titration:
1.
The logger must be logging. Take a DO measurement of the
water where the logger is being deployed using either a DO
meter or by titration. If using a meter, make sure it is
calibrated and allow time for the meter probe to stabilize
(this will occur when three meter measurements taken in a
row are within your accuracy tolerance).
If the logger is being deployed in saltwater, adjust the
meter measurements for salinity using a meter with both
conductivity and DO probes. If the saltwater has a constant
salinity, you can use a DO meter where you can enter that
salinity value to adjust the readings. If the salinity and/or
DO are changing rapidly, then you will need to get a sample
of the water in a container large enough for both the logger
and meter probe to be completely submerged. Place both
devices in the water long enough for them to stabilize and
then for the DO logger to log at least two values, and take a
concurrent meter reading.
2.
Record the reading, date, and time of the measurement in a
field notebook.
3.
At the end of the deployment, repeat steps 1 and 2.
To Take Calibration Readings Using 100% Water-Saturated Air:
1.
The logger must be logging. You will need fresh water, the
included calibration boot and sponge, and the current
barometric pressure from a HOBO U20 or U20L Water Level
logger, a barometer, or a nearby weather station.
2.
If the logger has been in salt water, clean the logger body
and sensor cap as described in the
Maintenance
section.
Make sure the sensor cap is dry before continuing.
3.
Make sure the protective guard or anti-fouling guard is
installed on the logger.
4.
Wet the small sponge with fresh water. Squeeze out any
excess water.
5.
Place the sponge in the end of the calibration boot.
6.
Insert the logger in the calibration boot so that there is
approximately a 1 cm (0.5 inch) overlap between the end of
the boot and the body of the logger. This will ensure there
is enough space between the end of the logger and the
sponge (the logger should not be pressed up tightly against
the sponge).
7.
Allow at least 40 minutes for the logger to reach
temperature equilibrium, and then write down the date and
time in a field notebook.
8.
Write down the barometric pressure at that time (note the
elevation if the barometric reading has been adjusted for
sea level).
9.
Repeat these steps at the end of the deployment.
Reading Out the Logger and Redeploying
Your readout and maintenance schedule will be determined by
the amount of fouling at the site. To read out the logger in the
field:
1.
Take a field calibration reading as described in the
Taking
Field Calibration Readings
section.
2.
If the logger was in saltwater and you did not deploy a
HOBO Conductivity Logger, then use a conductivity meter
or salinometer to take a conductivity reading. Write down
the reading and the date and time.
