22
Figure 10, Salinity box highlighted
T A K I N G M E A S U R E M E N T S
Before taking measurements, be sure the instrument has been
calibrated to ensure the most accurate readings. Turn the instrument
on and wait 5-15 minutes if using a polarographic sensor. If using a
field cable/sensor, install the sensor guard to protect the sensor and
membrane. Place the probe in the sample to be measured and give the
probe a quick shake to release any air bubbles. Allow the temperature
readings to stabilize. Next, stir the probe in the sample to overcome the
stirring dependence of the dissolved oxygen sensor. You must provide
at least 6 inches (16 cm) per second of water movement. Once the
values plateau and stabilize you may record the measurement and/or
store the data set. The dissolved oxygen reading will drop over time if
stirring is ceased. If placing the DO sensor into a stream or fast flowing
waters it is best to place it perpendicular to the flow and NOT facing
into the flow.
If using the DO sensor in an aeration tank/basin it is helpful to make
sure bubbles do not burst on the membrane. This may cause unstable
readings to occur. You should be able to prevent this by pointing the
sensor upwards so it’s facing the sky and twist tying, zip tying, or rubber
banding the bulkhead to the cable. Essentially making a simple curve
to the cable without bending or breaking the cable will allow you to
lower the sensor into the aeration tank while the sensor points skyward
and the bubbles are no longer bursting on the membrane surface.
