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New customer benefits for more reliability
with electrochemical DrägerSensors
in the Polytron 7000
Abstract
Two new functions offered by the Polytron
7000 transmitter provide the user with valuable
information about the current state of
the sensor and recommendations as to
when to replace it. The optional “Vitality”
and “Predictive Maintenance” functions are
enabled via a plug-in software dongle.
Introduction
As Danish Nobel Prize winner Niels Bohr
once said: “It is always difficult to make predictions,
especially when they relate to the
future”.
It is not possible to say with any great certainty
how long a particular electrochemical
sensor will function. Past stress on the sensor
and the particular circumstances of
each application can significantly influence
the length of time the sensor can be
expected to last.
This is why Dräger decided to take a different
approach with its Polytron 7000. An integrated
unit analyses past data to give information
about the current condition of the
sensor.
A bit like a petrol gauge in a car which
shows when the tank is half empty, the vitality
function indicates the extent to which
the sensor has already been used up. The
vitality reading is continuously recalculated and displayed. This, however, gives no indication
of how long the remaining capacity
will actually last, as this depends on the application
in hand and future events, just as
the distance which can be covered with the
petrol left in the tank of a car will depend
on how the car is driven and the type of
journey ahead.
A second diagnostic function is comparable
to a odometer. Once a high mileage has
been notched up, the driver will automatically
expect the car not to last as long and
will be prepared for the vehicle to fail suddenly
at any time. In the Polytron 7000,
stress data relating to past use, e.g. gas exposure
and unusual ambient conditions, are
stored and analysed, and displayed for the
user in the form of a recommendation.
These two indicators allow users for the
first time to initiate preventive measures in
time, or decide to replace the sensor by
way of anticipating and avoiding downtime
and emergency measures.
In order to understand what happens inside
the Polytron 7000, an explanation of some
technical details is first needed.
Design
An electrochemical DrägerSensor is a Teflon
container filled with liquid – the electrolyte
– whose open side is sealed by a gas-permeable
membrane. The gas to be measured
has to pass through this membrane to
reach the sensor. Inside, there are three
porous electrodes made of noble metals. At
the measurement electrode, the gas to be
measured is converted by means of an
(electro-)chemical reaction. This results in
a small electric current which flows to the
counter electrode and is displayed as an indication of the concentration (Figure 1).
The purpose of a third electrode (reference
electrode) is to set the operating point of a
sensor to a specific value, though it can also
be used for diagnostic purposes. This involves
briefly adjusting the sensor and then
analysing the electric sensor signal. During
this sensor self-test, the reaction must follow
a certain pattern if the sensor is to be classified
as fully functional.
Sensor properties
A sensor's most important property is its
sensitivity to gas, as this expresses the
amount of signal which will be generated
per volume of gas. A new DrägerSensor
leaves the factory with a guaranteed sensitivity:
at the end of the production process
its sensitivity is measured with target gas
and stored in the sensor's memory. In other
words, the sensor is pre-calibrated upon
delivery.
Unfortunately, the sensitivity changes, i.e.
decreases, over time (Figure 2). Once a
sensor comes to the end of its useful life, it
has only a minimum residual sensitivity. The
loss of sensitivity over time depends on the
application, ambient parameters and normal
ageing. Extreme stress and dramatically
changing ambient conditions can accelerate
the ageing process.
A sensor's sensitivity at a given time can be
determined by calibrating it using a known
concentration of test gas. The loss of sensitivity
is compensated for by adjusting the
amplification of the measurement electronics.
A DrägerSensor stores the measured values
from previous calibrations in its memory,
this way the data is available later for software
analysis.
The maximum and minimum temperatures,
as well as any temperature exposure, are
also recorded and stored.
Some sensors are exhausted when they
convert measuring gas. The measurement
current which is generated is used as a
measure of the amount of converted gas
and made available for analysis.
Sensor dongle
The Sensor Diagnostic Dongle memory
module provides a package of innovative
software to extend the diagnostic functionality
of the Polytron 7000. A dongle
can be easily retrofitted by inserting it into
one of the slots. Other dongles which are
available are the Sensor Self-Test Dongle
and the Data Dongle. Each Polytron 7000
comes with three slots. |
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One of the new functions is the Sensor Vitality
function. The current vitality value can
be displayed by clicking on -->"Sensor“--
>"Vitality“ in the menu (Figure 4). The vitality
is calculated on the basis of current and
past sensitivity values and the sensor age,
and indicates how exhausted the sensor already
is. The vitality reading is not used to
correct the measured gas concentration or
the displayed measurement value, and is
intended solely for the information of the
user.
The user can check the sensor's current
vitality at any time on the display of the
Polytron 700 transmitter (Figure 4). When
the sensor is removed, the data remain in
the sensor's memory and will be displayed
again correctly if the sensor is subsequently
used in another transmitter. In other words,
the vitality function is even available if the
sensor is calibrated in a different transmitter.
A new sensor has an initial vitality of between
95 and 100. At the end of its useful
life, a sensor's vitality reaches zero. The values in between are displayed in increments
of one, and while the sensor is in
use the Polytron 7000 continuously recalculates
the sensor's vitality.
In the case of a new sensor, the vitality is
calculated on the basis of the factory calibration
and its expected useful life (Figure
5 P1). When the sensor is next calibrated,
its change in sensitivity is determined and
compared with the previous factory calibration.
The trend which is revealed is then
used to calculate future vitality values
(Figure 5 P2).
If the sensitivity determined during calibration
deviates from the calculated vitality
value, this is corrected upwards or downwards
accordingly (Figure 5 P3 and 4).
The vitality value provides two important
pieces of information – the change over
time and the absolute value. If the value
changes noticeably, calibration should be
performed more frequently because the
sensitivity is probably decreasing quickly,
with the result that the measurement error
will grow. If the vitality falls below 25, the
user should start thinking about replacing
the sensor (Figure 5 P5).
If the adjusted vitality value is higher after
calibration, this indicates that the sensor
has either recovered or that a stable phase
has started.
A significantly lower value indicates that the
sensor has either been subjected to major
stress or that it is ageing quickly.
A dramatic loss of vitality, however, can also
be brought about by incorrect calibration. In this case, calibration should preferably be
repeated. If this produces a different sensitivity
value, the vitality will be recalculated.
The vitality function permits the user to plan
for the timely replacement of a used sensor.
A low value indicates that the sensor is approaching
the end of its useful life.
If vitality values decrease significantly over
time, calibration should take place more
often. A 10 percent loss in vitality since the
last calibration can also mean that the
measurement error has risen to 10 percent.
As an indication of the sensitivity loss, however,
vitality is not the only criterion for assessing
a sensor's performance. Over time,
other properties of the sensor, e.g. its response
time or cross-sensitivity to other
gases, may also change. Such variables
cannot always be determined by assessing
the sensitivity alone, and a more detailed
analysis is needed – this can be conducted
with the help of the Predictive Maintenance
function.
Predictive Maintenance
The new Predictive Maintenance (PM) function
uses a number of input variables to calculate
a status indicator (Figure 6) showing
the degree of sensor exhaustion. The display
shows at regular intervals a schematic diagram
of a sensor with three, two or one horizontal
bars (Figure 7). A new sensor will appear
with three bars. Two bars indicate that
the sensor has already reached the first stage
of exhaustion due to stress and wear, but
that it is still reliable. When only one bar remains
visible, the sensor has reached its limit,
and may fail at any time due to wear and
tear.
A number of different stress factors which
can be demonstrated to influence the useful
life of a sensor are used for the purposes
of analysis. These factors are measured,
calculated and stored by the transmitter
software.
The life expectancy of electrode material,
electrolyte and membrane is impeded by
temperature. For this reason, the transmitter
remembers how high the temperature
was in the past in order to work out the degree
of exhaustion. In addition, any violation of the permissible temperature limits as
specified in the data sheet will negatively
affect the value due to possible damage to
the sensor.
In some sensors, the electrode material is
worn when it reacts with the measuring gas.
Because the amount of material is known at
the time of manufacture, the maximum gas
dosage can be calculated and monitored by
the transmitter. If a sensor is exposed to a
lot of gas during its lifetime, this will accelerate
its exhaustion accordingly. Throughout
the sensor's useful life, the transmitter
measures and stores the gas dosage to
which the sensor is exposed. Once the specified limit has been exceeded, a sensor
may become insensitive and should therefore
be replaced.
The vitality, as described above, is another
input variable for the Predictive Maintenance
function. If the vitality falls below a value of
25, the final stage of PM is displayed to
prompt the user to replace the sensor.
Dräger knows from experience how long
the various sensors will last under moderate
conditions. If the sensor's age exceeds a
value at which the statistical probability of a
sensor failure is greater than 80%, this will
result in the lowest PM stage. This Methuselah
can now fail at any time.
The sensor's current condition is calculated
on the basis of these five variables (Figure 8) and displayed. Just like a battery charge
status indicator, a warning is given when it
is likely that the sensor will no longer be
able to perform its monitoring function
reliably. The sensor should then be replaced
to prevent malfunctions, false alarms and
other disruptions.
Sensor self-test
Besides the Vitality and Predictive Maintenance
functions, the sensor is continually
subjected to tests by the transmitter to allow
certain sensor errors to be detected immediately.
Such tests are performed every
10 minutes by means of electrical stimulation
lasting just a few milliseconds. This does
not affect the measurement function. The
sensor’s reaction is analysed and must
follow a given pattern. If several successive
tests are negative, the transmitter signals a fault. The result indicates whether the
electrochemical function is maintained and
the operating point is still at the optimum
setting as required.
This test is no substitute for a gas test to
check sensitivity. If the gas access is
blocked, this will not be detected in the
sensor. However, this function, which runs
automatically in the background, can be deactivated
via the "Settings" menu and then
activated manually when needed. The transmitter
then presents the result of the test
on the display. Several unsuccessful tests
are evidence of a serious sensor error, in
which case the sensor must be replaced
immediately.
Gero Sagasser,
Dräger Safety AG & Co. KGaA |
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Dräger Safety AG & Co. KGaA |
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Revalstrasse 1 |
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23560 Luebeck, Germany |
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Tel +49 451 882 0
Fax +49 451 882 2080
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