Mettler Toledo InPro Serie Instructions D'utilisation page 35

InPro 6850 i / 6900 i / 6950 i O
Type D, 4-electrodes, InPro 6950(i)
The InPro 6950(i) is intended for measuring per-
manent low oxygen level down to 0.1 ppb. In terms
of measurement principle it is a combination of the
InPro 6850 i and InPro 6900(i). It consists of 4 elec-
trodes. The anode (platinum) and reference (silver /
silver chloride) are separated in two electrodes. A
guard ring is placed around the cathode. The cath-
ode has the highest diameter from all amperometric
oxygen sensors from METTLER TOLEDO
The electrodes show the following reactions:
Cathode: O
Anode:
Reference: No current
Guard:
Due to the larger surface of the cathode a higher
current appears during measurement. This allows for
measurement of oxygen levels below 1 ppb.
As a consequence of the reaction at the cathode and
the anode, a current flows between cathode and
anode (4 electrones per oxygen molecule), which
is proportional to the oxygen partial pressure at the
cathode.
The amperage of the current flow depends on the
surface of the cathode. Typical values are:
6800
6850(i)
6900(i)
6950 i
This current is measured and converted and indi-
cated in the transmitter into an oxygen value. In ISM
sensors this computation takes place in the sensor
and the transmitters displays this value.
© 04 / 13 Mettler-Toledo AG
Printed in Switzerland
Sensor 12 / 25 mm
2
+ 2 H O + 4 e
2 2
–
4 OH O
$
O + 2 H O + 4 e
2 2
Air current
50 – 110 nA
50 – 110 nA
250 – 500 nA
2500 – 6000 nA
4 OH
–
$
+ 2 H O + 4 e
2 2
4 OH
–
$
Zero current in %
of the air current
< 0.1
< 0.1
< 0.03
< 0.025
InPro 6850 i / 6900 i / 6950 i
35
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–
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52 206 349