In online monitoring of fermentation processes, traditional probes such as pH, dissolved oxygen (DO), and temperature probes, as well as emerging spectroscopic probes such as near-infrared (NIR) and Raman probes, serve as our “eyes” for sensing the physicochemical and biological parameters inside the bioreactor. However, if these “eyes” are installed in an inappropriate location, their “line of sight” can be easily disturbed by gas bubbles, biomass, and the flow field generated by the agitator, resulting in distorted data and potentially misleading the entire process control strategy.
Choosing the optimal installation location for a probe is therefore the first step toward ensuring accurate and reliable process data.
This article examines the principles behind common probe-installation challenges and provides practical recommendations for optimizing probe placement.
1. Identifying Sources of Interference: The “Three-Way Interaction” of Bubbles, Shear, and Optical Paths
To solve these problems, we first need to understand how the interference occurs.
1.1 Bubble Interference: The Risk of “Foamy” Data and Scattering
Direct Impact on Electrochemical Probes (DO and pH): DO electrodes measure dissolved oxygen in the liquid phase through a gas-permeable membrane. If a large number of bubbles directly attach to or continuously sweep across the electrode membrane, they can form a gas film that prevents proper contact between the liquid and the membrane. This can cause severe fluctuations or artificially high readings. The liquid junction of a pH electrode can also be temporarily blocked by bubbles, resulting in abrupt changes or unstable readings.
Critical Impact on Spectroscopic Probes: For spectroscopic probes such as NIR and Raman, which rely on interactions between light and matter, gas bubbles are one of the primary sources of interference. Bubbles can cause strong scattering and reflection of incident light, significantly reducing signal intensity while introducing substantial random noise. This can severely degrade spectral quality and cause the predictive model to fail. A continuously bubbling fermentation broth is therefore one of the greatest challenges for the successful implementation of online spectroscopy.
1.2 Agitator Interference: Shear, Impact, and Non-Uniform Flow Fields
Risk of Physical Damage: If a probe is installed too low or too close to a high-speed rotating agitator, it can be exposed to strong hydrodynamic shear forces. For pH electrodes with fragile glass membranes, DO electrodes with gas-permeable membranes, and precision optical windows, prolonged exposure to high shear can cause damage. In extreme cases, physical impact with the agitator may occur.
Hydraulic “Dead Zones” and Sample Representativeness: If a probe is installed in a stagnant zone or recirculation zone, the local parameters it measures—such as pH, temperature, or spectral characteristics—may not represent the overall conditions in the bioreactor. This can introduce time delays and spatial bias, which is particularly detrimental to process control requiring representative bulk-phase information.
1.3 Additional Challenges for Spectroscopic Probes: Optical Path and Window Cleanliness
Optical Path Length and Penetration Depth: NIR probes typically require a certain optical path length, ranging from several millimeters to several centimeters, to obtain sufficient signal. The installation location must ensure that the optical path remains unobstructed by mechanical components and that the material within the optical path is representative of the bulk process.
Window Fouling: Biomass, proteins, and metabolites in fermentation broth can readily accumulate and form deposits on the optical window. Over time, severe window fouling can obstruct the optical path and cause signal drift, eventually resulting in measurement failure. The probe should therefore be positioned where the process flow can provide a certain degree of self-cleaning action on the optical window.
2. Optimizing Probe Installation: A “Feng Shui” Approach Based on Probe Type
Based on the above analysis, we propose several principles for optimizing probe placement.
The core principle is: avoid high-bubble-density zones and high-shear regions, while positioning the probe in a well-mixed, stable-flow region that provides representative measurements.
Recommendation 1: Traditional Electrochemical Probes (DO, pH, Temperature) — Prioritize Stability and Representativeness
Optimal Location: In general, probes should be installed above the agitator impeller and behind the baffles. This position avoids the densest bubble zone rising from below the agitator while remaining within the main circulation loop, where fluid renewal is rapid and measurements are highly representative.
Critical Installation Angle: The DO electrode membrane should be installed horizontally or tilted slightly upward to prevent bubbles from accumulating on the membrane surface. Similar attention should be paid to the orientation of pH and temperature probes to minimize bubble attachment.
Absolute No-Go Zones: Never install the probe directly facing the air sparger outlet or along the path of the strong radial jet generated by the agitator.
Recommendation 2: Spectroscopic Probes (NIR/Raman) — Avoid Bubbles, Preserve the Optical Path, and Promote Self-Cleaning
Primary Objective: Maximum Bubble Avoidance. The installation location must minimize exposure to bubble streams as much as possible. This generally means:
1) Install the probe in the upper-middle section of the vessel, away from the gas-liquid dispersion zone around the agitator impeller.
2) Prefer the downward-flow side created by the baffles rather than the upward-flow side, because the downward-flow region generally contains fewer bubbles.
3) Absolutely avoid installing the probe directly behind a sight glass or in any area where bubbles can be seen vigorously circulating or accumulating.
Optical Path Design: The installation flange must ensure that the probe's optical window is fully exposed to the fermentation broth and that there are no permanent obstructions in front of it.
For transmission-mode NIR probes, the receiving and transmitting ends must be precisely aligned. The optical path should also be free from persistent large bubbles or solid agglomerates.
Window Self-Cleaning: The probe should be installed horizontally or tilted slightly downward, allowing the optical window to remain parallel to, or aligned with, the main flow direction. This uses the shear force of the flowing liquid to reduce the accumulation of contaminants on the optical window. Some probe designs incorporate automatic mechanical wipers or ultrasonic cleaning systems for the optical window. During installation, sufficient space should be reserved for these mechanisms, and their potential impact on the local flow field should be considered.
Installation-Position Verification: During an empty-vessel aeration and agitation test, a high-intensity flashlight can be used to simulate the spectroscopic optical path. Observe the planned installation location through the sight glass to determine whether the optical path is clear and whether bubble interference is minimized.
Recommendation 3: General Layout and Coordinated Planning — Use Baffles for Unified Probe Deployment
Flow Guidance and Stabilization by Baffles: Baffles can break up vortices and establish a stable axial circulation pattern. All probes should benefit from this stabilized flow field. Installation behind or near a baffle is therefore recommended.
Coordinated Placement of Multiple Probes: It is recommended to plan the installation of different types of probes—including electrochemical probes, spectroscopic probes, and potentially other sensors—in the same area of the vessel, typically at approximately 90° to the agitator motor. A vertically layered arrangement can be adopted: The spectroscopic probe may be installed at a higher position because of its strict bubble-avoidance requirements. DO and pH probes can be installed in the optimal region of the main circulation zone. Lower-level ports can be reserved for sampling or future expansion. Unified planning simplifies cabling, maintenance, and data correlation analysis.
Safe Distance from the Gas Inlet: All probe installation areas should maintain a sufficient distance from the air sparger and its gas discharge zone.
3. Verification and Commissioning: “Health Check” After Installation
After the probes have been installed, systematic verification is essential.
3.1 Empty-Vessel Test (Critical!)
Electrochemical Probes: Under aeration and agitation, monitor the baseline stability of the DO and pH readings. The readings should remain stable without excessive fluctuations.
Spectroscopic Probes: Under aeration and agitation, acquire a background spectrum or water spectrum.
Check whether:
the signal intensity is sufficient;
the signal remains stable; and
the spectral noise remains within an acceptable range.
This is one of the most direct methods for evaluating the effectiveness of bubble avoidance.
3.2 In-Vessel Comparative Validation
During the early stage of fermentation, cross-check the online readings—especially pH and estimated substrate/product concentrations—against offline analytical results. Any systematic deviation should trigger a reassessment of the probe position or model calibration.
3.3 Dynamic Response Testing
Change the agitator speed or aeration rate and observe whether the probe responses follow the expected trends based on fluid dynamics and mass-transfer principles. For example:
After increasing the agitator speed, the DO reading should increase smoothly.
The spectral signal should not undergo severe attenuation simply because the number of bubbles has increased.
Summary: Good Data Starts with the Right Location
The probe installation is essentially a precise combination of fluid dynamics, optical/electrochemical principles, and process requirements. For traditional probes, the key priorities are: Avoid bubbles + Maintain stability; For spectroscopic probes, there are three major challenges: Maximum bubble avoidance + Clean opticsal path + Representative sampling
A poorly selected installation point can make even the most advanced sensor unreliable. Conversely, a carefully selected “optimal location” can maximize probe performance and provide a solid and reliable data foundation for process optimization and control.
Remember this core principle: Choose a measurement location where your probe can “see the big picture”—with good mixing and high representativeness—without being blinded by “wind and sand”—bubbles and fouling—or being battered by “rushing currents”—high shear. In the dynamic microcosm of a bioreactor, this is a critical step toward precise process sensing and intelligent process control.
Post time: Sep-04-2026