Measurement of dissolved oxygen
In-line measurements of the most important parameters are well
established at breweries. To evaluate the quality and the shelf life of beer,
measurement of dissolved oxygen is very common. In this article the two most
common principles, amperometric and optical, are compared when installed
in the filling process at the Krombacher Brewery in Germany.
xygen can enter at different
stages of the brewing process
and influence the taste and the
shelf life in a negative way, so the
content of dissolved oxygen is
monitored continuously at different
stages of the process.
4 – 20 mA
3 x VisiFerm DO
The principles
Oxygen content can be measured
in different ways. The most common principles are amperometric
and optical measurement. In the
classical amperometric (electrochemical) procedure as described
by Clark, the oxygen induces after
diffusion through a membrane,
and a chemical reaction with the
electrolyte of the sensor creates
a voltage differential. The induced
voltage is the measurement signal
which indicates the amount of oxygen present in the system. Essential
components of this sensor type
are an intrinsic cathode and anode
as well as the membrane body
filled with electrolyte.
The optical measurement principle
is based on fluorescent quenching
of a luminophore when oxygen
is present. In this case the sensor
contains a light source that excites
the luminophore at a specific wavelength. Once excited, the luminophore emits a characteristic light
Knut Georgy
Market Segment
Manager Process
Analytics, Hamilton
Bonaduz AG,
· BBII 5 /2012
4 – 20 mA
Figure 1: Installation situation at the filling line
with a longer wavelength. Its intensity and phase shift depends on
the amount of oxygen present.
Using specific filters, only the light
emitted by the luminophore will
be trapped and measured.
In-line measurements must fulfil
high requirements. Besides accuracy, repeatability, response time,
and detection limit, an easy installation, calibration, and maintenance
are also important. Filling lines
are the definitive test for sensors
because they must withstand
heavy pressure and temperature
changes caused by the hammering
effect of valves, as well as frequently occurring CIP cycles.
of different sensor types
In order to compare the two
different measurement principles
under harsh conditions, three
optical dissolved oxygen sensors
(VisiFerm DO) from Hamilton were
installed and tested in parallel
to the previously installed amperometric sensor for several months
in a filling line of the Krombacher
Brewery. Figure 1 shows the installation situation at the filling line.
Particular attention was paid
to treating all sensors the same
to ensure that they withstood
the same requirements.
During the testing period significant
differences in the behaviour of
the sensors was observed. The
three VisiFerm DO sensors showed
congruent results at any time
whereas the amperometric sensor,
depending on the situation, showed
a completely different dissolved
oxygen reading. The differences,
all important for the daily operations, are explained in the
When a filling line stops, closing
valves cause a pressure hammer
and in mid-term an increasing
temperature of the bound liquid.
This pressure hammer is transferred to the electrode through
the membrane to the electrolyte
of the amperometric sensor. This
leads to a massive disturbance
of the oxygen content equilibrium
on both sides of the membrane.
Hamilton optical oxygen sensors
are resistant to pressure hammers
because there is no liquid inside
the membrane cap. The optics
and electronics behind the membrane are protected by a sapphire
glass. Figure 2 shows the differences between both measurement
principles where the differences
can easily be recognised.
During the stagnation of the system,
an increase of the voltage generated by the amperometric sensor
can be observed. In the present
case, the value exceeded the determined alarm level several times.
This resulted in a further delay
of the re-start of the filling line
because the filler is controlled
amongst others by the value
of the dissolved oxygen. The
measurements of the three
VisiFerm DO showed that the
dissolved oxygen content maintained similar values during the
stagnation. The VisiFerm DO
sensors showed no stop-of-flow
effects and the filling could
be continued at any time.
Response time
When changing to another product,
after the replacement of components or sensors, or after defined
intervals, a CIP procedure is necessary. During this time very harsh
conditions regarding the temperature and pH-value dominate,
and the different sensors will be
switched off or their measurement
values are out of their calibration
range. After a CIP procedure the
piping system and the filler are
rinsed with product and the eluate
will be dismissed.
At this point it is of critical importance that the sensors become
operational very fast. Figure 3
shows that optical dissolved oxygen sensors are ready much faster
for operation than amperometric
sensors are. The difference can be
several minutes where, depending
on the capacity of the filler, 5,
10, or even 20 hectolitres of beer
must be dismissed and have to
be treated as waste water. At this
stage money is poured through
the gully and the filling performance
gets weak.
Figure 2: Stop-of-flow effects with amperometric
and optical sensors for dissolved oxygen
High plant availability is an important economic factor. Thus it’s
important that sensors are not susceptible to faults and maintenance,
and calibration take only little time.
In this respect optical sensors
have great advantages compared
to amperometric ones.
The only consumable part of
the VisiFerm DO is the sensor cap
that has to be replaced, depending
on the treatment and the number
of CIP cycles, only every four
to eight months. This procedure
takes only a few minutes including
the calibration with the new cap.
Polarisation, replacement of the
mechanical sensitive parts cathode
and anode, as well as the refilling
of the membrane with electrolyte
are obsolete. Maintenance with
the VisiFerm is easier and faster.
The optical measurement of dissolved oxygen has grown up and
is suitable for controlling sophisticated processes. The advantages
compared to amperometric measurement of dissolved oxygen
are not only the higher plant availability, the robustness, and easy
maintenance, but also the missing
hyposensitivity to CO2 that can be
found for some classical sensors.
The VisiFerm DO can thus be used
for measurement of the purity
of CO2 in the gaseous phase.
The potential abandonment
of an external transmitter,
the self-diagnosis, and the storage
of the calibration data in the sensor
head when using the VisiFerm DO
is not discussed here. When the
advantages are summarised,
the return on investment of the
change-over to optical measurement of dissolved oxygen can
quickly be recognised. In the long
term perspective, the VisiFerm DO
can contribute to easier operating
procedures and better results. 䡺
Figure 3: Comparison of response times after a CIP procedure
of an amperometric and optical sensor for dissolved oxygen
BBII 5/2012 ·