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TAKING MEASUREMENTS
Make sure the meter has been
calibrated and the protective
cap has been removed.
Immerse the tip of the probe
in the sample to be tested.
For accurate dissolved oxygen
measurements a water move-
ment of at least 0.3 m/sec is
required.
This movement will ensure that the oxygen-
depleted membrane surface is constantly re-
plenished. A moving stream will provide ad-
equate circulation.
To quickly check if the water speed is suffi-
cient, wait for stable reading and then move
the D.O. probe. If the reading remains stable,
measurement conditions are correct. If the
reading increases, the water movement is too
low.
During field measurements, this condition may
be met by manually agitating the probe. Ac-
curate readings are not possible while the
liquid is at rest.
During laboratory measurements, the use of
a magnetic stirrer is recommended. In this
way errors due to the diffusion of ambient
oxygen in the solution are minimized.
Anyway, the time necessary for reaching the
thermal equilibrium between probe and sample
must be allowed (a few minutes for a tem-
perature difference of several degrees).
16
PROBE & MEMBRANE MAINTENANCE
The D.O. probe body is made of reinforced
plastic for maximum durability.
A thermistor temperature sensor provides tem-
perature measurements and compensation.
When not in use, it is always recommended
to protect the probe against damage and dirt
with the supplied cap.
To replace the membrane or refill it with
electrolyte, proceed as follows:
•
Remove the protective
cap by gently twisting
and pulling it as sshown
in figure 1.
•
Unscrew the membrane
by turning it counter-
clockwise (see fig.2)
•
Wet the sensor by
soaking the bottom (2.5
cm / 1") of the probe in
HI 7041S electrolyte
solution for 5 minutes.
•
Rinse the new mem-
brane (HI 76407A, sup-
plied with the meter)
with some electrolyte
while shaking gently.
Refill with clean elec-
trolyte.
•
Gently tap the sides of
the membrane with your
finger to ensure that no
air bubbles remain
trapped inside. Do no
directly tap the bottom
as this will damage the
membrane.
•
Make sure that the rubber O-ring is seated
properly inside the membrane cap.
17
TWIST
AND
PULL
fig. 1
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