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Emerson Process Management GmbH & Co. OHG 3-11
X-STREAM X2
Instruction Manual
HASX2E-IM-HS
02/2012
3
Measuring Principles
3.2.3.1 Special Hints
For trace oxygen measurements (tO
2
)another
electrochemical sensor technology is used,
see Fig. 3-9. The sensor is a self contained
disposable unit which requires no mainte-
nance. The sensor utilizes the principle of
electrochemical reaction to generate a signal
proportional to the oxygen concentration in
the sample.
This ow of electrons constitutes an electric
current which is directly proportional to the
concentration of oxygen present in the sam-
ple. In the absence of oxygen, no oxidation/
reduction reaction occurs and therefore no
current is generated. This allows the sensor
to have an absolute zero.
3.2.3 Electrochemical Trace Oxygen Measurement
Fig. 3.9 Trace Oxygen sensor design principle
The sensor consists of a cathode and anode
which are in contact via a suitable electrolyte.
The sensor has a gas permeable membrane
which covers the cathode allowing gas to
pass into the sensor while preventing liquid
electrolyte from leaking out.
As the sample diffuses into the sensor, any
oxygen present will dissolve in the electro-
lyte solution and migrate to the surface of
the cathode. The oxygen is reduced at the
cathode. Simultaneously, an oxidation reac-
tion is occurring at the anode generating four
electrons. These electrons ow to the cathode
to reduce the oxygen.
The representative half cell reactions are:
anode:
cathode:
in total:
4OH
−
+2Pb → 2P bO+2H
2
O+4e
−
4e
−
+2H
2
O + O
2
→ 4OH
−
2Pb+ O
2
→ 2P bO
I
NTRODUCTION
2
August 2000 Rosemount Analytical 748375-CROX GT/ROX GP Oxygen Analyzers
Cathode
Gas Permeable Membrane
Thin Electrolyte Layer
Anode
Oxygen
OH
-
e
-
Electrolyte
Current Signal
Contact Plates
1.3 THEORY OF TECHNOLOGY
The ROX GT / GP uses an electrochemical sensor technology to achieve the
measurement of oxygen. See Figure 1-1. The sensor is a self contained disposable
unit which requires no maintenance. The sensor utilizes the principle of
electrochemical reaction to generate a signal proportional to the oxygen concentration
in the sample.
The sensor consists of a cathode and anode which are in contact via a suitable
electrolyte. The sensor has a gas permeable membrane which covers the cathode
allowing gas to pass into the sensor while preventing liquid electrolyte from leaking
out.
As the sample diffuses into the sensor, any oxygen present will dissolve in the
electrolyte solution and migrate to the surface of the cathode. The oxygen is reduced
at the cathode. Simultaneously, an oxidation reaction is occurring at the anode
generating four electrons. These electrons flow to the cathode to reduce the oxygen.
The representative half cell reactions are:
Cathode:
4e
-
+ 2H
2
O + O
2
→ 4OH
-
Anode:
4OH
-
+ 2Pb → 2PbO + 2H
2
O + 4e
-
The resultant overall cell reaction is:
2Pb + O
2
→ 2PbO
This flow of electrons constitutes an electric current which is directly proportional to
the concentration of oxygen present in the sample. In the absence of oxygen, no
oxidation / reduction reaction occurs and therefore no current is generated. This
allows the sensor to have an absolute zero.
F
IGURE
1-1. E
LECTROCHEMICAL
S
ENSOR
T
ECHNOLOGY
This sensor is a consumable
and requires replacement pe-
riodically. (To determine if the
sensor requires replacement,
see the troubleshooting section
of this manual)
Remaining lifetime counts down
when the sensor is in contact
with oxygen.
For above reasons, the analyzer
is shipped with the sensor as
extra item in a sealed bag!
The sensor must be installed
before analyzer startup, accor-
ding the instructions shipped
with the sensor!
Always consult the separate
instructions, shipped with the
sensor, before intending to start
calibrations! Violation may re-
sult in a damaged sensor!
Prolonged exposure of the sensor to air can
cause extended start up time, reduction of
3.2 Oxygen Measurement