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Emerson Process Management GmbH & Co. OHG 7-35
Instruction Manual
HASAxE-IM-HS
05/2006
7 Maintenance
7-4 Replacing the Electrochemical Sensor
7-4 Replacing the ElectrochemicalSensor
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 worn-out when
the output in atmosphere is below 70 % of the
initial output. The period till then 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 therefore calcu-
lated to approx. 42,857 hrs, corresponding
to approx. 5 years.
Irrespective of all calculations above:
A Sensor is worn-out when, connected to
ambient air, the output voltage is less than
2.8 V: Replace sensor!
Note 1!
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. (Operation at 40 °C
halves lifetime).
Note 2!
Due to the measuring principle the electro-
chemical Oxygen cell requires a minimum
internal consumption of Oxygen (residual
humidity avoids drying up 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 reversib-
le detuning of O
2
sensitivity. The output signal
will become unstable, but response time
remains constant.
For proper measurement results the cell
needs to be supplied continuously with con-
centrations of at least 0.1 Vol.-% O
2
.
We recommend using the cell if need be in
alternating mode, means to purge the cell with
conditioned ambient air (not dried, but dust
removed) when measurement pauses.
If it is necessary to interrupt Oxygen supply
for several hours or days, the cell has to rege-
nerate (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 =
For replacing the electrochemical sensor the
following tools are required:
• 1 Philips screw driver # 1 for 19" instru-
ments or the square key for the field
housing's squash fasteners to remove/
open the cover/front door.
• 1 Torx screw driver # 10 for disassembling
the sensor unit.
• 1 digital volt meter (measuring range
0 ... 2 V dc minimum) with suitable cables
and probes