Gas bubbles are a common culprit for causing dissolved oxygen (DO) signal noise. Gas bubbles, which have a different oxygen content than the surrounding liquid media, interact with the sensor's membrane resulting in dissolved oxygen signal noise. Traditional sensor cap designs actually amplify this issue because their surface finishes have characteristics which tend to hold gas bubbles in place at the sensor.

1. In cell culture, dissolved oxygen (DO) signal noise
due to gas bubble interference is common in small
volume bioreactors which are typically used in
process development and scale-down labs. The
reason DO signal noise is most prevalent in these
settings is due to dynamics, high oxygen demands,
and vertical sensor mounting. This issue will con-
tinue to become more pronounced as processes
yield higher cell densities and higher specific
productivities, resulting in even greater oxygen de-
mands.
Dissolved oxygen, a measure of oxygen content in
the liquid phase, is a key process parameter (KPP)
because of its importance in aerobic metabolism.
DO signal noise is caused by the sensor measuring
oxygen content in gas bubbles when they collide
with the sensors membrane; the content of these
gas bubbles is not a KPP because this oxygen
content is not accessible to the cells. By having
this noise in your DO measurement it does not only
Gas bubbles are a common culprit for causing dissolved oxygen (DO) signal
noise. Gas bubbles, which have a different oxygen content than the media, inter-
act with the sensor's membrane resulting in DO signal noise. Traditional sensor-
cap designs actually amplify this issue because their surface finishes have favor-
able interactions with gas bubbles. METTLER TOLEDO has developed a
one-of-a-kind dissolved oxygen sensor-cap to combat this issue; the new Angled-
OptoCap has an increased hydrophilic surface finish which repels gas bubbles
and a new angled design which minimizes gas bubble exposure.
Eliminating Bubble Noise
In Your Dissolved Oxygen Signal
ApplicationNote

2. negatively impact process control but it also negatively
impacts experiments which support critical decisions
for investigations, process optimization, tech transfer,
trouble shooting, and quality by design. Building this
signal noise into your design space and set-points can
have unintended consequences during scale-up, and
building this signal noise into your design of experi-
ments adds unnecessary variability. METTLER TOLEDO
has designed a new DO sensor-cap for the InPro6860i
which reduces dissolved oxygen signal noise from
gas bubbles. Click on the link to view a video trial of
the Anti-Bubble OptoCap: Anti-Bubble OptoCap Video
Trial.
Dissolved Oxygen Signal Noise from Gas Bubbles
Although the purpose of a dissolved oxygen sensor is
to measure oxygen in the liquid phase, these sensors
indiscriminately measure oxygen independent of it
being in the gas or liquid phase. When the sensor
membrane is exposed to gas bubbles the oxygen con-
tent in these bubbles impact the DO reading. For oxy-
gen or air bubbles, this results in an increased signal
because the gas bubbles have a higher oxygen con-
tent than that of the process. For nitrogen or carbon
dioxide bubbles, which contain no oxygen, this results
in a reduced DO signal.
The signal noise in the oxygen measurement due to
gas bubbles is not an inaccuracy of the sensor but a
function of the sensor-cap design. There are two pri-
mary sensor-cap characteristics in traditional designs
which promote gas-bubble interference. The largest
contributor is the hydrophobicity of untreated electrop-
olished stainless steel which has favorable interactions
with gas-bubbles. A secondary contributing character-
istic to gas bubble interference is the angle of sensor-
cap. A flat tip serves as a counteracting force to rising
gas bubbles, this increases the exposure to gas-bub-
bles.
The Anti-Bubble OptoCap Designed to Reduce Signal
Noise from Gas Bubbles
The new Anti-Bubble OptoCap reduces DO signal noise
by repelling gas-bubbles and reducing gas bubbles
exposure. This is achieved by engineering two design
characteristics: a more hydrophilic surface treatment
and a 30° cap geometry.
1. More Hydrophillic Surface Treatment
By reducing the contact angle of the sur-
face finish with a proprietary mechanical
surface treatment we were able to design
a more hydrophilic cap. This reduces the
favorable interactions between the cap
and the gas phase resulting in less bub-
ble accumulation and interference. The
new surface finish maintains the N5/Ra
0.4μm surface roughness with the same
stainless steel chemical composition. The
appearance is greyish but it still fulfills
ISO requirements and is suitable for
hygienic applications.
2. 30° Cap Geometry
An angled sensor reduces the probability
that gas bubbles will collide and interfere
with the sensors membrane. The 30°
angle is optimal for reducing gas bubble
interference without impacting the ability
of the sensors to measure dissolved oxy-
gen.

3. Mettler-Toledo Process Analytics
900 Middlesex Turnpike
Building 8
Billerica, MA 01821
(781) 301-8800
Subject to technical changes
www.mt.com
For more information
OptoCap Characteristics Standard MT OptoCap
(OptoCap BT02T)
New Anti-Bubble OptoCap
(OptoCap BT02THD)
Appearance
Angle Flat 180° Angled 30°
Surface Finish Electropolished Electropolished with proprietary
mechanical surface treated
Surface Roughness N5/Ra 0.4µm N5/Ra 0.4µm
Material Stainless Steel Stainless Steel
Membrane PTFE (FDA USP Class VI) PTFE (FDA USP Class VI)
O-Ring EPDM EPDM
Length (cm) 4.2 4.7
SIP/CIP/Autoclave Compatible? Yes Yes
Meet ISO Standards? Yes Yes
Suitable for Hygienic Applications? Yes Yes
Technical Comparison: Traditional OptoCap vs. Anti-Bubble OptoCap
Performance Comparison:
To validate the improvement of the new Anti-Bubble Opto-
Cap, trials were conducted with traditional sensor-caps
from METTLER TOLEDO and Hamilton, and with the Anti-
Bubble OptoCap. These trials were performed in a 10L
bioreactor with vertical sensor mounting (refer to the
video to see the experimental set-up). As shown in the
first graph ("Traditional Sensor-Caps"), traditional sensor-
caps show a significant amount of signal noise when try-
ing to maintain a 30% DO setpoint. The Anti-Bubble
OptoCap, shown in the second graph, shows no bubble
noise when maintain a 30% DO setpoint under the same
conditions.
%DO
%DO
Video Links:
www.mt.com/inpro6860i
https://www.youtube.com/watch?v=RkU07phMOfY