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DELTA F CORPORATION
Delta F
Solid State Coulometric Oxygen Sensor
This Technical
Bulletin provides individuals who need to monitor oxygen an appreciation
for the suitability of the Delta F Solid State Coulometric Sensor’s
capabilities. This new offering is Delta F’s first departure from the non-depleting
electrochemical sensor used in all of our traditional oxygen analyzers.
Those familiar with the working principle of the Delta F non-depleting
coulometric sensor will notice the similarities between the working
principle of the solid state coulometric oxygen sensor and our
electrochemical sensor. The flow of electrons, or current, from an
electrochemical reaction is measured directly to provide the sample gas
oxygen concentration. The nature of the solid state coulometric sensor
provides advantages over our traditional sensor in some applications.
Why Solid State Coulometric?
As with any analytical
application, It is important to understand the advantages and disadvantages
of a particular oxygen sensing technology before you buy. Experienced users
know that not every oxygen analyzer is suitable for every application. For
example, non-depleting electrochemical oxygen analyzers are highly stable,
have superior measurement sensitivity and are therefore unmatched for
ppb-level oxygen measurement. However, they would be destroyed very quickly
in applications having high concentrations of solvents in sample gases.
Paramagnetic oxygen analyzers on the other hand can be made of chemical
resistant materials and are therefore a good fit for making percent level
measurements in solvent laden sample gases. However, they lack low end
sensitivity to make low ppm measurements. The list of oxygen sensor
technologies that work well in specific applications is extensive. Delta F
is now offering a solid state coulometric oxygen sensor to satisfy a
segment of ppm and percent-level oxygen monitoring needs. This
sensor’s attributes, detailed below, fulfill some application needs
where other oxygen sensing technologies have left users dissatisfied with
their performance.
Principle
of Operation
The solid state
coulometric oxygen sensor is used to measure oxygen by counting the number
of electrons flowing through its circuit. The circuit is connected between
the sensor’s cathode electrode and its anode electrode. A Polarizing DC
voltage is used to facilitate the flow of electrons. Oxygen from the sample
gas gets to the cathode electrode by diffusing through a very small hole in
a barrier covering the cathode. Oxygen molecules in contact with the
cathode are reduced according to the following electrochemical reaction:
O2+ 4e- à 2O2-.
The oxygen ions, O2-,
are attracted though a solid electrolyte that is heated to 400°C and are
converted back to molecular oxygen at the anode electrode. The
electrochemical oxidation reaction that takes place at the anode is:
2O2- à O2+ 4e-
The quantity of
electrons (4e-) that flow through the circuit between the cathode and anode
is proportional to the concentration of oxygen in the sample gas. This flow
of electrons, or current, is measured by the analyzer electronics and the
oxygen concentration is displayed.
Attributes
& Limitations of this Sensing Technology
Users of Delta F
analyzers featuring the solid state coulometric sensor will realize the
following benefits:
Calibration/Stability:
Systems with this sensor are factory calibrated,
allowing them to be used immediately without anyone having to calibrate
them in the field prior to use. The analyzer will maintain its accuracy
over time. The sensor is not consumed when it is exposed to oxygen, it does
not require a constant purge to protect the sensor when the analyzer is not
being used and its lifetime is not dependant on how much oxygen it is
exposed to.
Sensor Size/Packaging:
The sensor is very
compact. It occupies a space less than 1” cube. Its small size allows
it to be not only packaged as a traditional “flow-through”
sensor, but also for in-situ measurement. The in-situ sensor is supplied
mounted on a bulkhead fitting. The bulkhead fitting and sensor penetrate
less than 2” from the wall it is mounted on. The in-situ design
allows you measure the oxygen content of your process environment directly.
There are no pumps or extractive sample systems required with an in-situ
sensor. In-situ measurement also eliminates consumption and disposal of
sample gas.
Ranges of Operation:
Three ranges of analyzers using the solid state
coulometric sensor are available: 0-10,000ppm, 0-25% and 0-100%. One of
these three ranges must be specified at the time of order. The 0-10,000ppm
range sensor has resolution of 0.5ppm near zero. The 0-10,000ppm range
analyzer is designed for those applications requiring accuracy at less than
1,000ppm, but greater than 0.5ppm. The 0-25% range sensor is designed for
applications requiring accuracy between 0.1% (i.e. 1,000ppm) and 25%. The
0-100% range sensor is designed for applications requiring accuracy between
1% and 100%. Speed of Response: The sensor responds very quickly to
changing oxygen concentrations. The 0-10,000ppm range sensor can be exposed
to air and in less than a minute it will measure <10ppm on pure
nitrogen. This makes the sensor attractive for users who have upset prone
applications and need to make ppm-level measurements. In addition to being
fast to respond to changing sample gas concentrations, the performance of
the sensor is not affected by reasonable changes in flow rate.
Sensor Replacement/Durability:
The sensor does not get
consumed when oxygen is being measured and has a long life expectancy.
However, in the real world there may be applications or situations that can
damage any sensor. Users can be assured that the replacement cost for a
solid state coulometric sensor is low.
Operating Pressure:
Since both the cathode
and anode electrodes are in direct contact with the sample gas, the sensor
can operate from sub-atmospheric pressure up to 100psig.
Electrolyte Maintenance:
The sensor uses a solid
ceramic electrolyte and, therefore, no periodic electrolyte maintenance
required. The sample gas will not be humidified or adulterated in any way
after it passes by the sensor. This is a key consideration for individuals
wishing to make in-situ measurements or who wish to recycle sample gas from
a “flow-through” analyzers exhaust back into the process.
No Dependence on O2 Content in Air
as Reference Gas:
The sensor does not need
a reference gas to support the measurement. Changes in barometric pressure
or elevation have traditionally affected analyzers relying on air as a
reference gas. As with any oxygen sensing technology, this sensor does have
some limitations. Background Gases: The sensor cannot be used with sample
gases containing hydrocarbons, combustibles, H2, CO, NO2,
S, or Pb. If the analyzer were sampling any of these gases, it may give
false low readings or the sensor may be damaged.
Low
Level Sensitivity/Accuracy:
This sensor is not
designed for ppb-level analysis. It is limited to those applications that
require accuracy above 1ppm. It will work well in applications requiring
measurements to be made at ppm and/or percent level, provided none of the
above mentioned gases are present. All things considered, the solid state
coulometric sensor has many characteristics that make it attractive for a
number of applications and is complementary to our standard sensor.
For more information, click here to view the Solid State Sensor.pdf
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