ALTITUDE & SALINITY COMPENSATION
If the sample contains salts or if you are
performing the measurements at a different
altitude than sea level, the readout values must
be corrected, taking into account the lower
degree of oxygen solubility.
ALTITUDE COMPENSATION
All the readouts are referred at sea level, thus
the displayed measurements are higher than
the actual values. In fact, altitude affects D.O.
concentration decreasing its value. The follow-
ing table reports the oxygen solubility at various
temperatures and altitudes, based on sea level
barometric pressure of 760 mm Hg.
°C
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
0 m
14.6
13.8
13.1
12.4
11.8
11.3
10.8
10.3
9.9
9.5
9.1
8.7
8.4
8.1
7.8
7.5
7.3
7.1
6.8
6.6
6.4
300 m
14.1
13.3
12.7
12.0
11.4
10.9
10.4
9.9
9.7
9.2
8.8
8.4
8.1
7.8
7.5
7.2
7.1
6.9
6.6
6.4
6.2
600 m
13.6
12.9
12.2
11.6
11.0
10.5
10.1
9.6
9.2
8.7
8.5
8.1
7.8
7.5
7.3
7.0
6.8
6.6
6.3
6.2
6.0
1200 m
12.7
12.0
11.4
10.8
10.3
9.8
9.4
9.0
8.6
8.3
7.9
7.7
7.3
7.0
6.8
6.5
6.4
6.2
5.9
5.7
5.6
1500m
12.3
11.6
11.0
10.4
9.9
9.5
9.1
8.7
8.3
8.0
7.7
7.3
7.1
6.8
6.6
6.3
6.1
6.0
5.7
5.6
5.4
1800m
11.8
11.2
10.6
10.1
9.6
9.2
8.8
8.3
8.0
7.7
7.4
7.1
6.8
6.6
6.3
6.1
5.9
5.8
5.5
5.4
5.2
900 m
13.2
12.4
11.9
11.2
10.6
10.2
9.7
9.3
8.9
8.6
8.2
7.8
7.5
7.3
7.0
6.8
6.6
6.4
6.1
5.9
5.8
Altitude, Meters above Sea Level
This gives an idea of the error that can be
introduced at different altitudes and allows you
to calculate the quantity to be subtracted to
correct your reading.
°F
32.0
35.6
39.2
42.8
46.4
50.0
53.6
57.2
60.8
64.4
68.0
71.6
75.2
78.8
82.4
86.0
89.6
93.2
96.8
100.4
104.4
9
TAKING MEASUREMENTS
Make sure the meter has been
calibrated and the protective
cap has been removed. Im-
merse the tip of the probe in the
sample to be tested.
Make sure the temperature sensor is also
immersed.
For accurate dissolved oxygen measurements
a water movement of 0.3 m/sec is required at a
minimum. This is to 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 sufficient, wait for the reading to
stabilize and then move the D.O. probe. If the
reading is still stable, the measurement condi-
tions are right, while if the reading increases the
water movement is not adequate.
During field measurements, this condition may
be met by manually agitating the probe. Accu-
rate readings are not possible while the liquid is
at rest.
During laboratory measurements, the use of a
magnetic stirrer to ensure a certain velocity in
the fluid is recommended. In this way, errors
due to the diffusion of the oxygen present in the
air in the solution are reduced to a minimum.
At all times, time necessary for thermal equilib-
rium to occur between the probe and the sample
must be allowed (a few minutes for temperature
difference of several degrees).
8
