Mettler Toledo InPro 6850i: Performance Review from 5 Peer-Reviewed Studies
The InPro 6850i is Mettler-Toledo's 3-electrode polarographic dissolved oxygen sensor and the ISM digital successor to the analogue InPro 6800. Unlike the ppb-detection 6950i, this is a workhorse standard-range probe designed for the 10-100% air-saturation regime that runs in mainstream fermentation. Four of the five citations in this review name the Mettler polarographic InPro directly across an unusually diverse set of organisms: Neves 2023 uses the InPro 6800 in a 2 L Pseudomonas taiwanensis fed-batch [1], Peyraud 2012 in a 500 mL Methylobacterium extorquens bioreactor [2], Mauri 2026 in a 1 L Ogataea polymorpha methanol-to-malate fermenter [3], and Posadas-Navarro 2025 in a stirred-tank Azotobacter vinelandii alginate campaign [4]. Hanson 2007 supplies the class-level accuracy anchor: 98.7% Pearson correlation between polarographic and optical DO in NS0/CHO mammalian cell culture [5]. Choose the InPro 6850i for a 316L stainless-steel CIP/SIP-cycled bench, pilot or production fermenter running standard DO control, where the ISM predictive-maintenance metadata and the separate platinum anode's low-signal stability are worth the capital premium. Prefer the analogue InPro 6800 if you are staying off ISM lock-in or running non-Mettler transmitters. Reserve the 4-electrode InPro 6950i for sub-ppb trace-oxygen work — brewing packaging, anoxic fermentation. For single-use bioreactors, choose an optical or single-use DO sensor instead; the reusable 316L body is a mismatch to the disposable format.
Mettler Toledo InPro 6850i at a glance
The InPro 6850i, from Mettler-Toledo Process Analytics (Urdorf, Switzerland), is a polarographic (amperometric) dissolved oxygen sensor built around a Clark-type electrochemical cell with a distinguishing feature: a new 3-electrode measurement system with a separate platinum anode, which increases signal stability at low oxygen levels compared with the earlier 2-electrode analogue InPro 6800. The 6850i sits within the wider InPro 6000 amperometric family alongside the trace-DO 4-electrode InPro 6900/6950(i), and is reachable through the same M400 or M800 transmitter platform via Mettler's ISM digital bus. Spec values below come from Mettler's InPro 6000 series technical datasheet 52206266; field performance data comes from the peer-reviewed studies cited throughout this review.
| Specification | Value |
|---|---|
| Measurement principle | 3-electrode amperometric (polarographic), Clark cell, in accordance with ISO 5814 and EN 25 814; separate platinum anode for improved low-signal stability |
| Operating range | 3 ppb to oxygen saturation (aqueous) |
| Accuracy (Mettler-claimed) | ±[1% + 3 ppb] in aqueous media; ±[1% + 6 ppb] in liquids containing CO2 |
| Response time (t98) | < 90 s at 25 °C (air → N2) |
| Sensor signal in ambient air | 50-110 nA at 25 °C |
| Body diameter | 12 mm (InPro 6850i /12/70, /12/120, /12/160, /12/220 variants) or 25 mm (/25/80) |
| Housing / wetted materials | 316L stainless steel (AISI 316L / 1.4404); silicone and PTFE; EHEDG and FDA flat-surface compliant hygienic design; N5 surface polish |
| Membrane | Reinforced PTFE/silicone gas-permeable membrane with internal steel mesh (replaceable) |
| Permissible temperature (measuring) | 0-80 °C |
| Mechanical temperature resistance | −5 to 140 °C; steam-sterilisable and autoclavable |
| Permissible pressure (measuring) | 0.2-6 bar (2.9-87 psi) with standard membrane; up to 9 bar with reinforced membrane; mechanical resistance max. 12 bar |
| Process connection | PG 13.5 threaded process fitting (standard bioreactor port) |
| Output signal | ISM digital (Intelligent Sensor Management) via K8S IP68 connector to Mettler M400 / M800 transmitter |
| Compliance | USP Class VI wetted O-rings (silicone); EHEDG and 3-A hygienic; FDA flat-surface recommendation |
| Ex certification | ATEX SEV 14 ATEX 0169 X (Ex ia IIC/IIIC); IECEx SEV 14.0026X; FM IS/I,II,III/1/ABCDEFG/T6 |
| Indicative capital cost | Mid four-figure GBP per probe plus M400/M800 transmitter; Mettler does not publish list prices — quotes are dealer-mediated |
Spec values are taken directly from the InPro 6850i /12/70 product page, the InPro 6000 series technical datasheet 52206266, and the ISM DO sensor documentation. These are vendor claims; the literature synthesis below is independent.
What the peer-reviewed literature says
Five peer-reviewed sources ground this review, and four of them name the Mettler polarographic InPro DO sensor directly. That is an unusually strong evidence base for a workhorse standard-range DO probe — better than the family sibling InPro 6950i and better than most of the review pages in the sensor cluster on this site. The four named-InPro-6800 deployments span three orders of magnitude of scale (500 mL to stirred-tank pilot), four organism classes (a Pseudomonas, a facultative methylotrophic Alphaproteobacterium, a methylotrophic yeast, and a nitrogen-fixing Azotobacter), and four different bioreactor platforms (Eppendorf BioFlo120, Infors-HT, and a custom coaxial hybrid-impeller stirred tank). Hanson 2007 supplies the class-level accuracy anchor for the polarographic pathway. The 6850i inherits the same physics, upgraded with a separate platinum anode for signal stability and with ISM digital output for predictive maintenance.
Neves et al. (2023) in Microbial Biotechnology is the most detailed named-InPro-6800 deployment in the reviewed literature [1]. The RWTH Aachen team ran Pseudomonas taiwanensis VLB120 for propionate production in 2 L stirred-tank bioreactors (Eppendorf BioFlo120 units, DASware Control 5.3.1), 1.5 L working volume of MSMB12 mineral salt medium, fed-batch strategy. Their published Materials and Methods identify the DO instrument by full model and vendor location: “The DO tension was measured with an InPro 6800 Polarographic oxygen sensor (Mettler Toledo, Giessen, Germany).” DO was maintained above 30% air-saturation by a cascaded agitation loop between 400 and 1000 rpm to prevent oxygen limitation, aeration held at 0.7 L/min, temperature at 30 °C. The DO signal drove glucose feeding during fed-batch phase — the sensor was on the critical path of the process control. Three fed-batch strategies were benchmarked; a maximal propionate titre of 2.8 g L−1 was achieved without a reported sensor failure or drift event across the campaign. For an operator specifying the 6850i as a standard-range fed-batch DO probe, this is the closest published analogue.
Peyraud et al. (2012) in PLoS ONE named the same probe in a smaller bench-scale bioreactor a decade earlier [2]. The Vorholt group at ETH Zürich cultivated Methylobacterium extorquens AM1 in a 500 mL Infors-HT batch bioreactor at 28 °C, 400 mL working volume, 1000 rpm, aerated at 0.1 L/min, on a mixed methanol-succinate substrate to trace the co-consumption dynamics with 13C labelling. The methods state: “The partial pressure of dissolved oxygen was monitored using polarographic oxygen sensors (InPro 6800, Mettler-Toledo).” The DO signal shows expected excursions during mid-consumption, end of methanol utilisation, and end of succinate utilisation, coherent with the 13C-CO2 exhaust gas readings. The probe was not the subject of the study but performed as a stable methanol-depletion indicator across the co-consumption window — a use pattern that hangs on membrane cleanliness and stable baseline signal, both features the 6850i preserves and improves on with the 3-electrode design.
Mauri et al. (2026) in FEMS Yeast Research is the most recent named-InPro-6800 deployment and moves the probe into a methylotrophic yeast fermentation on green methanol [3]. The Blank group at RWTH Aachen scaled a two-phase methanol-to-L-malate fermentation with Ogataea polymorpha from shake flask to a 1 L stirred-tank Eppendorf BioFlo120 controlled by DASware Control 6.0. Their methods identify the DO probe as “a DO probe (InPro 6800 Polarographic Oxygen Sensor, Mettler Toledo, Greifensee, Switzerland),” paired with a Hamilton EasyFerm Plus pH probe and a Pt100 temperature sensor. pH was held at 5.0 by acid/base dosing, DO above 30% by cascaded agitation (300-1200 rpm), temperature at 37 °C, airflow 36 L h−1. DO-based automated feeding was the most effective of three strategies tested, achieving a molar yield of 0.1 molMA molMeOH−1 and 0.5 g L−1 h−1 productivity. Once again the DO signal drove the fed-batch feed logic — the sensor is on the process's critical path, and once again the probe performed across the full campaign without a reported failure.
Posadas-Navarro et al. (2025) in Chemical Engineering & Technology puts the InPro 6800 into a stirred-tank alginate fermentation with coaxial hybrid impellers [4]. The UNAM Mexico group ran Azotobacter vinelandii for alginate production, using the InPro 6800 polarographic DO probe as the primary dissolved oxygen instrument through bacterial growth, alginate accumulation and the viscosity climb that accompanies alginate synthesis. The probe stayed in service through the mixing-hydrodynamics campaign without a reported failure event, and the DO signal was reliable enough to support the paper's conclusion that hybrid impellers yield higher biomass, more alginate and more viscous cultures than a traditional Rushton turbine. Viscosity climb is a stress test for a polarographic membrane probe because it changes the boundary layer at the membrane surface; the 6800 platform held.
Hanson et al. (2007) in Biotechnology and Bioengineering is the canonical class-level accuracy anchor [5]. The University of Maryland Baltimore County / FDA team benchmarked optical fluorescent DO and pH sensors against traditional electrochemical (polarographic) probes in NS0 and CHO mammalian cell cultures in 24-well plates. They reported a Pearson correlation of 98.7% between the optical and electrochemical DO signals and 99.7% for pH. This is the number every subsequent optical-DO paper cites when arguing that fluorescent chemistry can substitute for the older polarographic reference. The result is symmetric — it means an InPro 6000-class polarographic probe agrees with modern optical alternatives to within a couple of per cent across the mammalian air-saturation regime. For an operator specifying the InPro 6850i for standard cell culture DO control, this is the class-level accuracy anchor.
Performance data from cited studies
| Study | Conditions | DO probe named & how used | Response / drift | Conclusion (for the InPro 6000 amperometric class the 6850i sits within) |
|---|---|---|---|---|
| Neves 2023 [1] | 2 L Eppendorf BioFlo120 stirred-tank, 1.5 L working; MSMB12 mineral salt medium; Pseudomonas taiwanensis VLB120 fed-batch; 30 °C; DO cascade 400-1000 rpm; aeration 0.7 L/min; propionate production | “InPro 6800 Polarographic oxygen sensor (Mettler Toledo, Giessen, Germany)”; DO held >30% air-sat.; DO signal triggered glucose feeding across fed-batch phase | No probe failure or drift event reported across three fed-batch strategy benchmarks; probe on critical path of feed control | Named-6800 deployment as the standard-range fed-batch DO instrument in a mainstream 2 L bench bioreactor — the closest published analogue to the 6850i's intended use case |
| Peyraud 2012 [2] | 500 mL Infors-HT batch bioreactor, 400 mL working; Methylobacterium extorquens AM1; 28 °C, 1000 rpm, 0.1 L/min air; methanol-succinate co-consumption tracked with 13C labelling | “polarographic oxygen sensors (InPro 6800, Mettler-Toledo)”; pO2 tracked continuously through mid-consumption, methanol-depletion, succinate-depletion phases | Stable baseline preserved through the co-consumption window; DO excursions coherent with exhaust 13C-CO2 signal | Named-6800 deployment as a methanol/substrate-depletion signal in a bench-scale methylotroph batch bioreactor; performance depends on stable membrane baseline — a feature the 6850i improves via 3-electrode design |
| Mauri 2026 [3] | 1 L Eppendorf BioFlo120 stirred-tank; Ogataea polymorpha two-phase methanol-to-L-malate fermentation; 37 °C; DO cascade 300-1200 rpm; airflow 36 L/h; pH 5.0; DO-based fed-batch feeding | “InPro 6800 Polarographic Oxygen Sensor, Mettler Toledo, Greifensee, Switzerland” paired with Hamilton EasyFerm Plus pH probe; DO signal drove the feeding strategy | No probe failure reported across DO-based, pH-stat and pO2-stat feeding regimes; probe on critical path of automated feed control | Named-6800 deployment as the primary DO instrument in a methylotrophic yeast bioprocess-development campaign in 2026; validates the InPro 6000 platform for green-methanol yeast fermentation on modern Eppendorf BioFlo120 hardware |
| Posadas-Navarro 2025 [4] | Stirred-tank bioreactor with coaxial hybrid impellers (multiple configurations vs Rushton reference); Azotobacter vinelandii alginate fermentation; viscosity climb through polymer accumulation | “Mettler-Toledo InPro 6800 polarographic DO probe” as the primary DO instrument | No probe failure or drift event reported through the viscosity-climb regime, which changes the boundary layer at the membrane surface | Named-6800 deployment as a robustness test — a fermentation whose rheology stresses the boundary layer at the membrane; probe held across the campaign, and the 6850i inherits the same reinforced steel-mesh membrane |
| Hanson 2007 [5] | NS0 and CHO mammalian cell culture in 24-well plates; head-to-head comparison of optical fluorescent DO and pH sensors against traditional electrochemical (polarographic) probes; University of Maryland Baltimore County + FDA | Class-level: “traditional electrochemical (polarographic) probes”; not named to InPro model but the polarographic class the InPro 6000 family belongs to | Not the paper's focus; correlation held across multi-day mammalian cultivations without reported failure | An InPro 6000-class polarographic probe agrees with modern optical alternatives at Pearson 98.7% across the mammalian cell-culture air-saturation regime — the class-level accuracy anchor for the 6850i's intended standard-range use |
Every row is a separate peer-reviewed publication; see References section for full citations. Conditions and metrics are paraphrased from the authors' text and tables, not from vendor literature. Four of the five named studies identify the Mettler polarographic InPro DO sensor directly by model; the fifth (Hanson 2007) is the class-level accuracy benchmark for the polarographic pathway.
Limitations and failure modes reported
Across the reviewed studies the following limitations and failure modes are class-level physics or maintenance costs, not defects specific to the 6850i. Each bullet is tagged with the citations that describe or imply it.
- The four named-InPro-6800 deployments used the analogue 6800, not the 6850i. No peer-reviewed paper we located names the 6850i specifically. The 6850i is the ISM digital 3-electrode successor of the same platform — same polarographic physics, same 316L stainless housing, same PTFE/silicone reinforced-mesh membrane, same PG 13.5 fitting — and the transition from 6800 to 6850i is an upgrade path within a single Mettler platform, not a change of measurement principle. Treat the four named-6800 studies as directly applicable to the 6850i for platform-level performance and reserve the ISM-specific benefits (calibration history, cycle counts, predictive maintenance metadata) as vendor claims that no independent peer-reviewed benchmark quantifies [1] [2] [3] [4].
- Membrane and electrolyte service is a recurring cost. Every polarographic DO probe carries a periodic membrane/electrolyte refresh cadence to hold calibration over multi-day runs. The reinforced PTFE/silicone-with-steel-mesh membrane on the InPro 6000 platform is more rugged than the standard PTFE membrane it replaced, but the physical service cadence remains. Budget for a spare membrane/electrolyte kit per probe in the annual consumables spend [1] [3].
- The 6850i needs a polarisation warm-up period before first use. Every Clark-cell polarographic probe does; the operator has to build a polarisation window into the process start-up. Not a defect — a class-level physics point — but it is worth stating for a new operator (see Peyraud 2012 Materials and Methods for a typical batch start-up sequence) [2].
- Amperometric DO sensors consume the oxygen they measure. A class-level physics point relative to non-consumptive optical alternatives. In a stirred bioreactor with realistic kLa the effect is negligible; in a small stagnant vessel or a low-mass-transfer measurement chamber, it can bias the reading. Check your specific hydrodynamics before commissioning if you are running at low DO or in a partially aerated cabinet [5].
- The reusable 316L stainless body is a mismatch to single-use bioreactors. None of the four named-InPro-6800 deployments used a single-use vessel; all four used reusable stainless-steel or glass bench and pilot bioreactors, and the platform is designed for that form factor. For a single-use campaign, choose the Mettler optical InPro 6860i or single-use DO sensor instead [1] [3].
When the literature and vendor documentation recommend the InPro 6850i
Recommended for
- Standard 10-100% air-saturation DO control in 316L stainless-steel bench and pilot bioreactors — the workhorse regime the peer-reviewed literature validates: 500 mL Infors-HT, 1 L and 2 L Eppendorf BioFlo120, stirred-tank pilot with a range of impeller geometries [1] [2] [3] [4]
- Fed-batch fermentations where the DO signal drives the feed strategy — both Neves 2023 (glucose fed-batch on Pseudomonas) and Mauri 2026 (methanol/glycerol two-phase fed-batch on Ogataea) place the InPro 6800 on the process's critical control path without reported failure [1] [3]
- CIP/SIP-cycled facilities standardised on the Mettler M400/M800 transmitter stack — the ISM digital bus and on-board predictive-maintenance metadata reduce the paper-log burden a fleet of analogue probes carries, and the sensor is FDA/USP Class VI + EHEDG + 3-A hygienic compliant per the InPro 6000 datasheet
- Cross-check reference for optical DO sensors in a mixed sensor stack — polarographic InPro is the class the fluorescent optical DO literature is still validated against (Hanson 2007 Pearson 98.7% agreement) [5]
Caveats / not recommended for
- Sub-ppb trace-oxygen work (brewing packaging, anoxic fermentation) — the 3-electrode 6850i's aqueous accuracy is ±[1% + 3 ppb] and its operating range starts at 3 ppb; below that, the 4-electrode InPro 6950i is the correct family variant (Mettler InPro 6000 technical datasheet)
- Single-use bioreactor deployments — the reusable 316L stainless body is a mismatch to the disposable format; none of the reviewed peer-reviewed studies used the InPro platform in a single-use vessel [1] [3]
- Facilities running non-Mettler transmitters or unwilling to standardise on ISM — the analogue InPro 6800 stays the pragmatic pick and is what the peer-reviewed literature actually deployed [1] [2] [3] [4]
- Small stagnant vessels or low-mass-transfer measurement chambers where the sensor's own oxygen consumption becomes a non-negligible bias — a class-level physics limit of amperometric DO [5]
Use cases documented in the peer-reviewed literature
Specific deployments reported in the cited studies. Each card corresponds to a real published bioprocess use case; the first four name the Mettler polarographic InPro DO probe directly.
Pseudomonas taiwanensis propionate production, 2 L BioFlo120
InPro 6800 in a 2 L Eppendorf BioFlo120 fed-batch, 1.5 L working volume MSMB12. DO cascaded 400-1000 rpm above 30% saturation. DO signal triggered glucose feeding; three fed-batch strategies benchmarked to 2.8 g/L propionate.
Neves 2023 [1]Methylobacterium extorquens methanol-succinate co-consumption
InPro 6800 in a 500 mL Infors-HT bioreactor, 400 mL working, 28 °C, 1000 rpm, 0.1 L/min. pO2 tracked with 13C-CO2 exhaust to trace substrate co-consumption dynamics; stable baseline preserved across the run.
Peyraud 2012 [2]Ogataea polymorpha methanol-to-L-malate, 1 L BioFlo120
InPro 6800 paired with Hamilton EasyFerm Plus pH probe in a 1 L Eppendorf BioFlo120. Two-phase glycerol-then-methanol strategy, DO cascade 300-1200 rpm, DO-based automated feeding — 0.5 g/L/h malate productivity.
Mauri 2026 [3]Azotobacter vinelandii alginate, coaxial hybrid impellers
InPro 6800 as primary DO instrument in a coaxial hybrid-impeller stirred bioreactor through bacterial growth, alginate accumulation and viscosity climb. Probe held through the boundary-layer stress test; hybrid impellers beat Rushton on biomass and alginate.
Posadas-Navarro 2025 [4]Comparing the InPro 6850i against alternatives?
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Frequently asked questions
What is the Mettler Toledo InPro 6850i, and how does it differ from the InPro 6800?
Which peer-reviewed studies actually name the InPro 6800 or 6850i?
What is the measurement range and accuracy of the InPro 6850i?
Can the InPro 6850i be CIP-cleaned, SIP-sterilised and autoclaved?
How does the 3-electrode InPro 6850i compare to an optical DO sensor for cell culture?
What does ISM add to the InPro 6850i, and is it worth specifying over the analogue InPro 6800?
What failure modes are reported for the InPro 6800 / 6850i in bioprocess?
Which InPro model should I specify for a new stainless-steel bioreactor at 1-2 L working volume?
References
- Neves D, Meinen D, Alter TB, Blank LM, Ebert BE. (2023). Expanding Pseudomonas taiwanensis VLB120's acyl-CoA portfolio: Propionate production in mineral salt medium. Microbial Biotechnology 17(1):e14309. DOI: 10.1111/1751-7915.14309.
- Peyraud R, Kiefer P, Christen P, Portais J-C, Vorholt JA. (2012). Co-Consumption of Methanol and Succinate by Methylobacterium extorquens AM1. PLoS ONE 7(11):e48271. DOI: 10.1371/journal.pone.0048271.
- Mauri A, Yovcheva M, Demling P, Buhl EM, Ballerstedt H, Blank LM. (2026). Optimization and scale up of L-malic acid production from methanol by the methylotrophic yeast Ogataea polymorpha. FEMS Yeast Research 26:foag017. DOI: 10.1093/femsyr/foag017.
- Posadas-Navarro D, Carrillo-Tavera S, Córdova MS, Trujillo-Roldán M, Ramírez-Argáez M, Herrera-Valencia E, Ascanio G. (2025). Improving the Efficiency of a Bioreactor Equipped With Mixed-Flow Impellers. Chemical Engineering & Technology. DOI: 10.1002/ceat.70039.
- Hanson MA, Ge X, Kostov Y, Brorson KA, Moreira AR, Rao G. (2007). Comparisons of optical pH and dissolved oxygen sensors with traditional electrochemical probes during mammalian cell culture. Biotechnology and Bioengineering. DOI: 10.1002/bit.21320.
Further reading
- DO Sensor InPro6850i/12/70. Vendor product specification page.
- Instruction Manual: O2 Sensors InPro 6850i, 6900i and 6950i. Official Mettler calibration and maintenance instructions.
- InPro 6000 Series Oxygen Measurement Sensors Technical Data (52206266). Full datasheet covering the InPro 6800, 6850i, 6900(i) and 6950(i).