Instruction manual

NGA 2000 MLT Hardware
MEASURING PRINCIPLE
2 - 12
Emerson Process Management GmbH & Co. OHG
Instruction Manual
90002929
07/2006
Fig. 2-6: Reaction of galvanic cell
In consequence of it‘s design the sensor‘s lifetime is limited and depends on theoretical designed
life and oxygen concentration. The sensor output can be taken as a rough criterion for end of
lifetime: The sensor is weared when the output in atmosphere is below 70 % of the initial output.
The period till this can be calculated by
The sensor‘s designed life under constant conditions of 20 °C is approx. 900,000 hrs.
The lifetime at 21 % oxygen is then calculated to approx. 42,857 hrs, corresponding to approx.
5 years.
The given values are for reference only! The expected lifetime is greatly affected by the temperature
of the environment in which the sensor is used or stored. Increases or decreases in atmospheric
pressure have the same effect as that by increases or decreases in oxygen concentration (Opera-
tion at 40 °C halves lifetime).
Notes for analyzers with electrochemical EO
2
cell!
This sensor is not suitable for anorganic gases containing chlorene or flourene!
In addition is not suitable for sample gases containing ozone, H
2
S (> 100 ppm) or
NH
3
(> 20 ppm).
Due to the measuring principle the electrochemical oxygen cell requires a minimum internal consumption
of oxygen (residual humidity avoids drying of the cell). Supplying cells continuously with dry sample gas
of low grade oxygen concentration or with sample gas free of oxygen could result in a reversible detuning
of O
2
sensitivity. The output signal will become instable, but response time remains constant.
For correct measurement the cell needs continuously to be supplied with concentrations of at least 0.1 Vol.-
% O
2
. We recommend to use the cells if need be in alternating mode, means to purge cells with conditioned
(not dried, but dust removed) ambient air when measurement pauses.
If it is necessary to interrupt oxygen supply for several hours or days, the cell has to regenerate (supply
cell for about one day with ambient air). Temporary flushing with nitrogen (N
2
) for less than 1 h (e.g. for
analyzer zeroing purpose) has no influence on measuring characteristics.
designed life (hours)
O
2
concentration (%)
Lifetime =
OXYGEN MEASUREMENT
Resistor (6)
Thermistor (5)
(11)
(Red) (Black)
(+)
Lead-
Anode (1)
(-)
Gold-
Cathode (2)
Summary reaktion O
2
+ 2 Pb → 2 PbO
Electrolyte (3)
(ph 6)
2 Pb + 2 H
2
O → 2 PbO + 4 H
+
+ 4 e
-
O
2
+ 4 H
+
+ 4 e
-
→ 2 H
2
O