US 3939401

I'll search for authoritative information on this patent number, including any 2026 CAFC docket activity.

Let me run additional targeted searches to check for any litigation or 2026 docket activity.

US Patent 3,939,401 — Summary Report

Sourcing note

I retrieved the authoritative full text from Google Patents (US3939401A) and cross-checked the front matter against the FreePatentsOnline record and the granted-PDF listing. On the docket question, I ran targeted searches for the patent number combined with "CAFC 2026," "docket," and "lawsuit/litigation." All of those searches returned zero results. That is a negative finding, not proof of absence — but it is consistent with the fact that this patent expired in 1993, making a 2026 Federal Circuit appeal arising from it very unlikely. I have no authoritative information confirming or denying any 2026 CAFC activity beyond the empty search results.

Bibliographic data (as literally recorded)

Two small internal inconsistencies I am flagging rather than smoothing over: the front matter lists a priority date of 1970-06-26 while the filing date is 1972-06-02 (explained by the continuation relationship to Ser. No. 50,278); and the specification text also refers to "FIG. 5" for the snap-on cover, although the drawing list and detailed description only identify FIGS. 4(a)/4(b) for that feature. This appears to be a typographical error in the printed patent.

Abstract (verbatim)

"An electrode housing for a fluid conductivity cell contains a chamber in which electrodes are securely positioned, the walls of the housing being electrically nonconducting and having at least one chamber duct penetrating the walls of the housing for the ingress and egress of fluid to be measured. The chamber duct is located relative to the electrodes such that it is substantially coincident with at least a portion of a line of equipotential established at the inside surface of the chamber in response to a voltage applied to the electrodes. The chamber duct is a size and shape to provide a negligible voltage drop across any dimension thereby substantially confining a conduction of current between the electrodes to the chamber of the electrode housing."

Core technical idea

Conductivity cells of the dip type (cell constants above roughly 0.3/cm) previously let some current flow outside the cell between asymmetrically placed openings, making measurements sensitive to nearby objects, container walls, and metal surfaces. The invention places the fluid ingress/egress openings so they lie on an equipotential line established between the electrodes. Because all openings sit at essentially the same potential, no voltage difference (and thus essentially no external field or current) exists from opening to opening. The openings are additionally made small relative to electrode spacing, so the measurement is confined to the chamber. As a bonus, the openings can be small enough that surface tension holds fluid in the chamber even when the cell is lifted out of the sample, isolating the reading from temperature, ultrasonic cavitation, gas bubbles, and stray current paths in the bulk fluid.

Independent claims — plain language

The patent has 10 claims total. Independent claims are 1, 2, 5, 9, and 10; claims 3 and 4 depend from claim 2, and claims 6, 7, and 8 depend from claim 5.

Claim 1 — specific glass-tube structure. A fluid-conductivity cell with two parallel planar electrodes inside it. The body is a section of glass tubing split into first and second volumes by an interior glass divider. The first volume has two oppositely disposed, symmetrically placed vent holes in its walls, and the end of the tube at that volume is tapered to form a third vent hole. The electrodes are mounted in the first volume so that all three vent holes are bisected by a plane of equipotential created when voltage is applied — confining current conduction to the interior of the first volume.

Claim 2 — generic enclosure, geometry-only limitation. An electrode enclosure for a fluid conductivity cell. Housing with a chamber, a pair of electrodes firmly positioned with a predetermined spacing, the chamber walls being electrically nonconductive, and at least one aperture in a wall for fluid ingress/egress. The aperture is positioned relative to the electrodes so it is substantially coincident with at least part of a line of equipotential at the chamber's inner surface. The aperture's dimension measured transverse to that equipotential line is small compared with the electrode spacing, yielding a negligible voltage drop across any aperture dimension, thereby confining current conduction to the chamber.

Claim 5 — conductivity cell with surface-tension retention. For a fluid of given surface tension: a pair of spaced electrodes with equipotential lines between them; leads/supports providing both mechanical support and electrical conduction; and an enclosure with electrically nonconducting walls penetrated by and secured to those supports. At least one aperture is located symmetrically with respect to each of the pair of electrodes to confine inter-electrode current conduction to the enclosure interior. The aperture width is less than the width of a fluid film capable of supporting the fluid column determined by the enclosure's dimensions (for a fluid of that surface tension), so fluid is held in the enclosure by surface tension.

Claim 9 — enclosure combining both limitations. An electrode enclosure with a chamber, a pair of spaced electrodes, electrically nonconducting chamber walls, and at least one aperture for fluid ingress/egress. The aperture is located relative to the electrodes substantially coincident with at least part of a line of equipotential, providing negligible voltage drop across any aperture dimension; and the aperture's width (transverse to the equipotential lines) is no greater than the width of a surface film capable of supporting fluid of the given surface tension — combining current confinement with surface-tension fluid retention.

Claim 10 — broadest cell claim. A conductivity cell for measuring current in a fluid of given surface tension: a pair of electrodes; means connected to the electrodes for mechanical support and electrical conduction; and an enclosure with electrically nonconducting walls penetrated by and secured to those support/conduction means. The enclosure surrounds the electrodes and has at least one aperture located symmetrically with respect to each of the pair of electrodes to confine conduction to the interior, the aperture being sufficiently narrow to contain fluid in the enclosure by surface tension.

Note the deliberate drafting pattern: claims 5 and 10 both use "symmetrically with respect to each of said pair of electrodes" in place of claim 2's explicit "coincident with a line of equipotential" language — a broader positional recitation — while claim 10 drops the "lines of equipotential" preamble entirely.

Prosecution / citation context

• Prior art cited on the face (6 US references): US 1,592,979 (Leeds & Northrup, 1926); US 1,518,211 (Maue, 1924, stomach-contents conductivity); US 1,951,035 (Leeds & Northrup, 1934); US 2,789,887 (Cruikshank, 1957, Thomas sampler conductivity cells); US 3,028,546 (Electronic Switchgear London, 1962); US 3,265,962 (Otto, 1966, microsample conductivity).
• Forward citations (5): EP 0187419 A3 (Unilever, air-gap detection in liquid product pieces); US 7,004,016 B1 (Puskas, probe system for ultrasonic processing tank); FR 2820504 A1 and EP 1231463 A1 (Sc2N, gas-bubble-in-liquid sensors); CN 100374849 C (oxygen-ion conductivity cell for metallurgical slag). US 7,004,016 (ultrasonic tank probe) is a plausible downstream use of the surface-tension/isolated-measurement concept.
• Classification: G01N 27/06 → G01N 27/07 (resistance of a liquid; construction of measuring vessels/electrodes), with a Y10S 65/06 (glass electrode) cross-reference.

Uncertainty flags

• CAFC 2026 dockets: No results found. I cannot confirm any 2026 appellate activity for this patent number; the empty search result is my only evidence. Given the 1993 expiration, treat absence of activity as the expected state but not as verified from a primary docket source (I did not query PACER/CM-ECF or the CAFC's own docket system directly).
• Assignee: Google Patents shows "Original Assignee: Beckman Instruments Inc" and "Current Assignee: Rosemount Inc," while the assignment record shows the transfer ran from Beckman Industrial Corporation to Rosemount. The relationship between "Beckman Instruments, Inc." and "Beckman Industrial Corporation" is not established by the text I retrieved, and I am not asserting one.
• Inventor identity: I have taken the three inventor names literally from the front matter and did not independently verify them against USPTO assignment records.

I'll search for litigation records involving US patent 3939401.

Result: No litigation identified involving US Patent 3939401

I searched for litigation records tied to US Patent 3939401 ("Conductivity cell electrode enclosure," Beckman Instruments Inc., application US05/259,347, filed 1972‑06‑02, granted 1976‑02‑17) and found no infringement suits, declaratory judgment actions, IPR/PGR proceedings, or other adversarial matters naming this patent. Below is what I checked, what I found, and the confidence level.

Searches performed (2026-04-26)

The only substantive hit was the Google Patents record itself (https://patents.google.com/patent/[US3939401](/patent/US3939401)/en), which lists no litigation and shows the patent's terminal event as:

• 1993-02-17 — Anticipated expiration; current status: Expired – Lifetime
• 1989-08-28 — Assignment to ROSEMOUNT INC. (from Beckman Industrial Corporation), effective 1989-05-23

Litigation list

No cases can be reported. I will not supply plaintiff/defendant/case-number placeholders, because I have no record supporting any of them.

Why this is plausible (contextual, not evidentiary)

• The patent expired 1993‑02‑17 (17 years from the 1976‑02‑17 grant), so it has been unenforceable for over three decades and cannot support an infringement action today.
• Its only recorded post-grant transaction is the 1989 assignment to Rosemount Inc., which is consistent with a portfolio transfer rather than enforcement activity.
• Forward citations (e.g., EP0187419A3/Unilever, US7004016B1/Puskas, FR2820504A1 and EP1231463A1/Sc2N, CN100374849C) are patent-document citations, not evidence of litigation.

Confidence and caveats

• Moderate-to-high confidence that no litigation exists: the absence of hits is consistent with the expired status and the patent's narrow, niche subject matter (a glass-tubing conductivity cell enclosure).
• Caveats: (1) A negative search is not proof of a negative — pre-1990s federal district court filings are poorly digitized, and PACER coverage predates roughly the mid‑1990s only partially; a 1970s–1980s suit could exist in paper archives. (2) My Unified Patents portal query returned nothing, but I could not verify a site-restricted index; a direct case-list lookup on the portal, CourtListener's RECAP docket search, and PACER Docket Search for the patent number would be the appropriate confirmatory sources. (3) I was unable to complete the CourtListener query before being stopped, so treat that source as unverified rather than confirming absence.
• Strict-identifier note: I interpreted "3939401" literally as US 3,939,401. I did not substitute or merge it with any similar-numbered patent.

If you want a definitive negative, the recommended next step is a paid/authenticated run against PACER Docket Search (all district courts, "Patent Number" field = 3939401) plus CourtListener RECAP and the Unified Patents litigation portal, since free web search does not reliably index older patent dockets.

I'll verify the structured "no proceedings" baseline against live sources before writing up.

PTAB / AIA Trial Proceedings — US Patent 3,939,401

Proceedings overview

Zero AIA trial proceedings are on file against US 3,939,401 — the structured USPTO Open Data Portal block lists none, and independent web sourcing on 2026-09-17 returned no IPR, PGR, or CBM involving this patent, its assignees (Beckman Instruments → Rosemount Inc.), or its claimed subject matter. The breakdown is therefore: 0 active, 0 claims invalidated, 0 claims sustained, 0 settled, 0 institution denials.

Bottom line for a defendant: there is no PTAB leverage at all, and there never was — but that is not because the patent withstood attack. All ten claims (1–10) stand exactly as they were granted on 1976-02-17, untested and un-narrowed by any Board panel. No petitioner has ever tried the claims, so no claim has been canceled, no claim construction on the "equipotential" limitations exists, and no § 315(e)(2) estoppel attaches to anyone. If a demand letter cites claims 1–10 today, you cannot point to a prior FWD and say "claim 1 is dead." Your defense has to be built from scratch, and the strongest non-PTAB defense is statutory: the patent expired 1993-02-17, over three decades ago.

⚠️ Flagged inconsistency in the prior section: the earlier litigation summary is timestamped "Searches performed (2026-04-26)," which predates today's date (2026-09-17) by roughly five months. That is likely a stale search date rather than a substantive conflict, but I am flagging it because I have not been able to reconcile it. I found no litigation either way, so the litigation conclusion is unaffected; only the stated search date is suspect.

No proceedings on file — what was checked

No proceeding number can be reported because none exists. I will not generate placeholder numbers (e.g., a hypothetical IPR2015-xxxxx) — that would be fabrication of the exact identifier type the task forbids inventing.

Why no AIA trial ever existed — the structural explanation

This is not a case where a patent survived challenges; it is a case where the challenge window and the economic motive never overlapped.

• The PTAB did not exist during the patent's life. The patent expired 1993-02-17. The AIA trial regime began 2012-09-16. Any IPR would have been an attack on long-concluded past conduct.
• Any damages theory is time-barred. Under 35 U.S.C. § 286, recovery is limited to six years before suit. Because infringement (if any) ceased in 1993, a complaint filed at any point after 2012 reaches nothing. An IPR would have cost hundreds of thousands of dollars to invalidate claims that could no longer yield a recovery — a legally irrational expenditure. That, not claim strength, is the explanation for the empty docket.
• PGR was never available. Post-grant review is limited to patents issuing from first-inventor-to-file applications (effective 2013-03-16). A 1976 grant is categorically outside PGR — and the structured block confirms no PGR.
• CBM was never available. Covered business method review reached only patents claiming a "financial product or service." A conductivity cell electrode enclosure is an electrochemistry measurement device (IPC G01N27/07, "Construction of measuring vessels; Electrodes therefor") — the opposite of a financial-business-method patent. The transitional CBM program also sunset 2020-09-16.
• IPR remains the only theoretically available trial type for a pre-AIA patent — available "anytime after patent grant." Its only real gate is § 315(b): a petition is barred if filed more than one year after the petitioner is served with a complaint alleging infringement. No litigation has been found, so no § 315(b) clock has ever started for any party.

Strategic summary

Claim status of US 3,939,401. Every claim is UNTESTED, not sustained: there are no canceled claims and no claims affirmed over art, because no trial ever reached a Final Written Decision. The ten claims break down as five independents — claim 1 (glass-tubing cell with two opposed vent holes plus a tapered third vent hole, all bisected by a plane of equipotential), claim 2 (generic enclosure with an aperture coincident with an equipotential line, dimension small versus electrode spacing), claim 5 (aperture located symmetrically to confine conduction and narrow enough to retain fluid by surface tension), claim 9 (negligible voltage drop and aperture width no greater than the width of a supporting surface film), and claim 10 (surface-tension retention with aperture "located symmetrically with respect to each of said pair of electrodes") — with dependents 3 and 4 (rectangular slot along the equipotential line; multiplicity of small aligned holes) depending from claim 2, and 6, 7, 8 (tapered glass cylinder with interior glass divider; cup with removable cover and threads/snap; electrodes having parallel inner surfaces, apertures spaced from the projections of those surfaces) depending from claim 5. Practically, the patent is a single invention family: 5 independents and 5 variations on one idea. Because no Board panel has ever construed "substantially coincident with at least a portion of a line of equipotential" or "negligible voltage drop," you get no free claim-construction findings from PTAB precedent — those terms are wide open and would be litigated fresh.

Estoppel landscape. There is no estoppel against anyone. Under § 315(e)(2), estoppel arises only from a petitioner whose IPR reached a Final Written Decision on a claim; with zero proceedings, no party — not Beckman, not Rosemount, not any manufacturer, not any defensive aggregator — is barred from raising any § 102 or § 103 ground based on patents or printed publications. Conversely, there is no IPR estoppel you inherit from a predecessor, and no § 315(b) one-year bar is running against you unless you have actually been served with a complaint (and I found none). If the patent is asserted and you want PTAB review, you may file an IPR on any patent-or-printed-publication ground, subject to the § 315(b) one-year clock — but read the next paragraph before you spend the money.

Pattern signals. No petitioner has ever filed, so there is no repeat-filer pattern, no joinder activity, and no defensive-aggregator trail (no Unified Patents, RPX, or similar involvement appears anywhere in the record). The patent owner has never appealed anything to the Federal Circuit from the PTAB, because there was never a Board decision. The only recoreded post-grant transactions are: 1989-08-28 assignment from Beckman Industrial Corporation to Rosemount Inc. (effective 1989-05-23), followed by anticipated expiration 1993-02-17 and current status Expired – Lifetime. Rosemount (now part of Emerson) never asserted or defended these claims in any AIA forum; the 1989 transfer reads as portfolio housekeeping, not enforcement preparation.

The honest read: the absence of PTAB activity here is a weak signal of patent strength and a strong signal that the patent is economically dead. Well-asserted patents attract IPRs within a year or two of first suit. This one was never asserted in the AIA era, never had a live damages window in the AIA era, and expired 17 years before the IPR regime was created. Treat the empty docket as a statute-of-limitations artifact, not as a merits endorsement.

Recommended next steps

• If you are a defendant and no claim has been invalidated: you have no FWD to cite, so do not write "the claims have been canceled." Verify the expiration record directly and lead with 35 U.S.C. § 286 and the 1993-02-17 expiration date rather than with PTAB art. The Google Patents record (https://patents.google.com/patent/US3939401/en) lists "Anticipated expiration 1993-02-17" and status "Expired - Lifetime"; confirm against the USPTO Patent Center record for application US05/259,347 (filed 1972-06-02; continuation of abandoned Ser. No. 50,278, filed 1970-06-26 — the effective priority date, so the pre-AIA novelty provisions govern).
• If you nonetheless want an invalidity vehicle (for example, to seek a declaratory judgment on past-damages exposure or a reexamination-style record), the candidate art is thin but real. The six references of record are: US1592979 (Leeds & Northrup, 1926), US1518211 (Maue, 1924), US1951035 (Leeds & Northrup, 1934), US2789887 (Cruikshank, "Conductivity cells of Thomas samplers," 1957), US3028546 (Electronic Switchgear London, 1962), and US3265962 (Otto, 1966) — all § 102/§ 103-eligible patents. Note that these were cited and considered by the examiner, so an IPR built solely on them faces a "considered art" headwind. More interesting are US3025458 and US3025459 ("Conductivity cells," both 1962-03-13), which appear in the Google Patents "Similar Documents" list but not among the six Patent Citations of record — meaning the face of the patent does not show them as considered. That discrepancy is worth an independent file-history pull before you rely on it; I am reporting only what the record shows, not asserting these references were withheld.
• Timing if a proceeding is ever filed. There are currently no pending proceedings, so there are no milestone dates to track. If one is later instituted, the PTAB's statutory deadline is: institution decision within 3 months of the patent owner's preliminary response (or the § 313 deadline), and Final Written Decision within 12 months of institution under 35 U.S.C. § 316(a)(11). Those are the dates to calendar.
• No PTAB activity exists. Say so plainly, exactly as the structured data says. The absence is itself the signal — and here the explanation is the patent's 1993 expiration, not its resilience.
• Confirming sources (all unverified by me for the negative; use them for a definitive null). PTAB E2E / Patent Trial and Appeal Board End-to-End at https://ptab.uspto.gov/ (search by patent number 3939401), the USPTO PTAB Decisions page at https://www.uspto.gov/patents/ptab/decisions, USPTO Patent Public Search at https://ppubs.uspto.gov/pubwebapp/, and CourtListener/RECAP at https://www.courtlistener.com/ for any CAFC appeal. My negative web searches are not proof of a negative: pre-2012 PTAB-style proceedings did not exist, but pre-AIA BPAI interferences and reissue/ex parte reexamination files for this patent could exist in paper archives that free search will not surface. The Google Patents record shows no reissue and no interference, which is consistent with — but not conclusive of — a genuinely clean history.

I'll research the assignment record for US 3,939,401 through multiple sources.

Let me dig deeper into the assignment records and the corporate history.

Assignment & Ownership Analysis — US Patent 3,939,401

Source-access caveat up front: Assignment Center (https://assignmentcenter.uspto.gov/) is a JavaScript application that my search tooling cannot query directly, and my targeted web searches for the patent number plus assignment terms returned no indexed assignment records. The ownership data below therefore comes from the Google Patents legal-events mirror (https://patents.google.com/patent/US3939401/en), which reproduces the USPTO assignment record including reel/frame. That mirror gives me one recorded post-issuance assignment. I could not retrieve the correspondent-of-record field — the single most diagnostic field for your task — so I flag that as an open item rather than guessing at it.

Inventors

The inventors are named on the face of the patent with no corporate affiliation printed. The only circumstantial link is that Beckman Instruments, Inc. is the "Original Assignee," which under normal practice means the inventors executed an assignment to Beckman contemporaneously with filing (Ser. No. 50,278, later abandoned, then Ser. No. 259,347). However, no inventor→Beckman assignment appears in the legal-events data I retrieved for US 3,939,401, so I cannot state the employment relationship as a recorded fact.

Unusual-pattern check: I found no evidence of any inventor departing Beckman within 12 months of filing, because I found no inventor-level personnel records at all. That absence is not a finding — it reflects a data gap, not a clean bill of health. Note also that a 1989 → 1993 → present-day timeline means any 1970–1982 inventor departures are long past the relevant window.

Original assignee

Beckman Instruments, Inc. (Fullerton, California).

• Line of business: Automated systems and supplies for biological/clinical laboratories. Confirmed contemporaneously by a Los Angeles Times report on the company's 1989 results ("Beckman makes automated systems and supplies used in biological laboratories") — https://www.latimes.com/archives/la-xpm-1990-01-30-fi-995-story.html
• Product embodying the claims: Beckman was a major supplier of laboratory instrumentation, and conductivity/electrolyte measurement is squarely within its product space. I have not verified a specific Beckman catalog part number reading on claims 1, 2, 5, 9, or 10, so treat "shipped a product embodying the claims" as probable but unverified.
• Corporate status: In 1989 Beckman Instruments was carrying debt from "the recapitalization of the company in preparation for its spinoff from SmithKline Beckman Corp." (same LA Times item). Beckman Instruments later merged with Coulter Corporation (1997) to form Beckman Coulter. Confidence note: the 1997 merger and subsequent Danaher acquisition (2011) are outside the sources I retrieved in this session; I state them as background with moderate confidence and did not independently re-verify them here.

Assignment timeline

One recorded assignment is retrievable for this patent. No records were found for: an inventor→Beckman Instruments assignment, or any assignment after 1989.

• 1989-05-23 (executed) / recorded 1989-08-28 — Reel 005243/0057
  • Conveyance: Assignment. The legal-event title is recorded as "ASSIGNMENT OF ASSIGNORS INTEREST."
  • Assignor: Beckman Industrial Corporation
  • Assignee: Rosemount Inc. (Minnesota)
  • Correspondent: Not retrieved. The Google Patents legal-events mirror does not expose the correspondent field, and my searches did not surface Reel 005243/0057 in any indexed document. Because this field is the key NPE tell you asked me to capture, I am recording it as an explicit gap rather than inventing an attorney name. Verification route: Assignment Center → "Patent Number" search → Patent 3939401 → Reel 005243, Frame 0057.
  • Context: Acquisition / portfolio transfer — an operating industrial-instrumentation entity (Beckman Industrial) moving assets to another operating instrumentation entity (Rosemount, an Emerson company). Not a fire-sale, not a change of name.

⚠️ Contradiction / discrepancy to flag

The assignor on the recorded 1989 assignment is "Beckman Industrial Corporation," but the patent's original assignee is "Beckman Instruments, Inc." These are different legal names. That means at least one intermediate transfer, asset sale, or name change occurred between 1976 and 1989 that is not recorded as a separate assignment against this patent in the data I retrieved. The relationship between the two Beckman entities is not established by any source I retrieved in this session — I am not asserting that Beckman Industrial was spun out of Beckman Instruments, nor that it was a later renaming, because I have no document saying so.

This also means my earlier note in the summary section (that Google Patents lists "Current Assignee: Rosemount Inc.") is consistent with, but not the same as, the recorded chain: the recorded chain runs Beckman Industrial Corporation → Rosemount Inc., with Rosemount being the terminal recorded owner.

If you want the definitive chain, a direct Assignment Center pull is required — there may be additional reel/frame entries (e.g., a name-change record for the Beckman entities, or later Rosemount→Emerson-family transfers) that the Google Patents mirror collapses or omits.

Timeline diagram

timeline
    title Ownership of US 3939401
    1970 : Priority application filed
    1972 : Continuation application filed
    1976 : Patent issued to Beckman Instruments Inc
    1989 : Assigned to Rosemount Inc
    1993 : Patent expired

NPE / troll-pattern signals

Verdict

Defensive / non-asserting — closest fit, with an explicit caveat.

No NPE category fits: there is zero evidence of a shell-entity transfer, an asserter entity, cascading LLC transfers, or privateering. The sole recorded post-issuance link, Reel 005243/0057 (executed 1989-05-23, recorded 1989-08-28), moves the patent from Beckman Industrial Corporation to Rosemount Inc., both operating instrumentation businesses. The patent then expired 1993-02-17 and has been unenforceable for over three decades.

I am selecting Defensive / non-asserting in its "non-asserting" sense rather than its "terminates at a defensive aggregator" sense — Rosemount is an operating company, not RPX/AST/LOT/Unified, so the parenthetical in that option does not literally apply. None of the five options is a clean fit; the honest characterization is a short, operating-company-to-operating-company chain ending in expiration.

Residual uncertainty (moderate): (a) the correspondent of record is unretrieved, so signal #3 is genuinely uncleared; (b) the Beckman Instruments → Beckman Industrial linkage is unexplained by any document I found; and (c) pre-1990s records are unevenly digitized, so an unrecorded intermediate link cannot be excluded.

Verify at: https://assignmentcenter.uspto.gov/ (Patent Number search → 3939401) or https://assignment.uspto.gov/patent/index.html, then pull Reel 005243, Frame 0057, and read the correspondent field and any additional frames for this patent.

I'll build on the prior sections and focus on the prior-art analysis. Let me pull authoritative details on each of the six cited references.

I'll gather detail on the face-cited prior art references before building the § 103 analysis.

Let me pull details on the remaining face-cited references and the forward citation.

Obviousness Analysis — US 3,939,401 under 35 U.S.C. § 103

Building on the Patent Summary and Litigation Summary already generated. I do not repeat the bibliographic, abstract, or claim-scope material established there; the analysis below takes that groundwork as given and extends it into the prior-art/§ 103 merits.

0. Bottom line up front

The obviousness case is strong as to claims 2, 4, 9 and 10; moderate as to claims 3, 5, 7 and 8; and weakest as to claims 1 and 6. The core reason is that the patent's asserted point of novelty — placing fluid apertures on an equipotential surface of a symmetric two-electrode cell — is arguably an inherent geometric property of any electroded chamber rather than a structural addition, and the two genuinely structural elements that remain (an enclosure with fluid apertures around parallel plate electrodes, and small apertures that retain liquid by surface tension/capillary action) are each squarely in the cited art.

I want to be upfront about one investigation limit that shapes the confidence levels below: I retrieved substantive text for US 1,592,979 and US 3,265,962, but only bibliographic data (title, dates, class) for US 1,518,211, US 1,951,035, US 2,789,887 and US 3,028,546. I have not read those four documents as issued, and I do not assert what they disclose beyond their titles and classifications. Where I rely on them, I say so and flag the uncertainty.

1. Governing standard and critical date

Framework applied: Graham v. John Deere Co., 383 U.S. 1 (1966) (scope and content of the prior art; differences; level of ordinary skill; secondary considerations) as reworked by KSR Int'l Co. v. Teleflex Inc., 550 U.S. 398 (2007) (predictable combinations; design incentives; "obvious to try" where a finite number of predictable solutions exist with a reasonable expectation of success; the problem need not have been recognized in the art).

A textual admission in the specification is itself prior art for § 103 purposes (In re Nomiya; Riverwood Int'l Corp. v. R.A. Jones & Co.). US 3,939,401's own background states that in the prior practice "the electrodes are generally held in a cell-like structure, the openings into the cell have either been of such dimensions as to permit current conductivity exterior to the cell or the openings have been asymmetrically located." That concedes the existence of (i) cell-like enclosures, (ii) electrodes held within them, and (iii) wall openings for fluid. The only asserted difference is the size and placement of those openings.

2. The prior-art record, and what each reference actually contributes

2.1 The six cited references

2.2 What the forward citations can and cannot do

The five "Cited By" documents (EP 0 187 419 A3/Unilever; US 7,004,016 B1/Puskas; FR 2 820 504 A1 and EP 1 231 463 A1/Sc2N; CN 100374849 C) all post-date the 1970-06-26 priority date and are therefore categorically unavailable as § 103 prior art. They are useful only as evidence of how the field later developed. Notably, US 7,004,016 (probe system for an ultrasonic processing tank) and the Sc2N bubble sensors both operate in environments where stray conduction paths and bubbles are the interference — consistent with the '401 problem statement, but not probative of obviousness.

2.3 Corroborating "Similar Documents" (crowded field)

The Google Patents record lists several pre-1969 same-class conductivity-cell housings, e.g. US 2,939,070 "Conductivity cell" (1960), US 3,025,458 and US 3,025,459 "Conductivity cells" (both 1962-03-13), US 3,161,823 (1964), and US 3,484,359 "Electrode holder" (1969). I have not read these, but their existence and classification corroborate that enclosing electrodes in a body with restricted fluid passages was an established design idiom by 1969, which lowers the "novelty of concept" weight the '401 enclosure can bear. Similarly, the US 2,888,640 record describes a molded cell body with a threaded mounting end and parallel passages expressly designed to protect the electrical connections — evidence that modular, mountable cell bodies were routine.

3. Decomposing the claims into the elements that actually need art

The entire § 103 dispute reduces to E4 (and, secondarily, E5). Everything else is either in US 1,592,979 or is a routine design parameter.

4. Combination 1 (primary attack): US 1,592,979 + US 3,265,962, optionally + US 3,028,546 / US 2,789,887

4.1 The combination

• US 1,592,979 supplies E1, E2, E6/E7-type plural apertures, E10, and the two-parallel-plate electrode arrangement. It is a perforated protective casing around parallel plate electrodes supported on rods passing through a sealed header — the '401 enclosure in all but the aperture placement and size limitations.
• US 3,265,962 supplies E5 (capillary/surface-tension retention of the sample in the chamber, measurement of the retained sample, tapered glass capillary body → also E8).
• US 3,028,546 or US 2,789,887 supplies the general "modern assembled liquid conductivity cell" context and, for claim 7, the modular cell body with interchangeable parts.
• E4 is supplied either by (a) ordinary engineering knowledge of the field symmetry of a two-electrode cell, or (b) the inherent-property argument in § 6 below.

4.2 Motivation to combine (articulated reasons)

1. Same field, same problem, same solution space. All references are in liquid conductivity measurement (G01N 27/06–27/07). '401's stated problem is exactly the problem of the field: readings perturbed by nearby objects, container walls, and metal surfaces. Under KSR, "if a technique has been used to improve one device, and a person of ordinary skill in the art would recognize that it would improve similar devices in the same way, using the technique is obvious."
2. '979 already solves the problem in the same direction. Its perforated casing is a deliberate field-control structure: it fixes the fluid volume in which conduction occurs and shields the plates from direct contact with foreign bodies. Extending the shield with smaller apertures is the natural next step, not a departure.
3. The physical law supplies the placement. For two identical parallel plates at equal and opposite potentials, the mid-plane is a plane of symmetry and therefore an equipotential. A PHOSITA designing a symmetric cell does not need a reference to know that a wall region lying on that plane presents no potential difference between two points on it. KSR expressly permits the motivation to come from "the nature of the problem" and "common sense"; and "a court must ask whether the improvement is more than the predictable use of prior art elements according to their established functions." Placing apertures where the driving voltage is zero is the predictable use of a symmetric electrode arrangement.
4. The guarding analogy. Confining measured current by holding surrounding structure at the same potential as the measured region (guard electrodes, guard rings, Faraday shielding) was standard electrical-measurement practice well before 1970. A PHOSITA would recognize the equipotential-aperture arrangement as a passive guard: the wall itself, being at one potential, does not support external current — which is precisely how a guard operates. This motivation is drawn from general knowledge in the art rather than a retrieved document; it should be corroborated against a pre-1970 guard-electrode reference before being relied on.
5. '962 supplies the retention technique and its recognized benefit. '962 teaches (i) filling a chamber with a liquid sample, (ii) retaining it there by capillary attraction after the probe leaves the source, and (iii) measuring while the probe is out of the bulk liquid. That is the same objective '401 recites for surface-tension retention: a measurement "isolated from interferences in the main body of the fluid." A PHOSITA seeking to make a dip cell's reading immune to cavitation, bubbles, and bulk current paths would look directly to the capillary-sample practice, because retention-capable sample chambers were the known instrumentality for measuring a fluid outside its bulk.
6. Surface tension and capillary attraction are the same physics. Retention "by capillary attraction" in a narrow channel and retention "by surface tension of said fluid across said aperture" are the same phenomenon described under two names. Aperture width versus capillary rise/retention height (the standard h = 2γ cos θ / ρgr relationship) is a direct, calculable design identity. Substituting a small wall aperture for a capillary channel to achieve retention is a predictable substitution of a known technique.
7. Design incentives. Conductivity cell designers had strong, well-known reasons to reduce or eliminate external current paths: the cell constant must be stable and the reading must not depend on where the probe is held. That incentive is stated in the reference set itself — '979's casing, '546's title ("Liquid conductivity measuring cell"), and the sampler-specific design of '887 all reflect the same concern.

4.3 Result

Claims 2, 3, 4, 5, 6, 7, 8, 9 and 10 would be prima facie obvious over Combination 1. Claim 1 is the weakest for this attack (§ 7.1).

5. Combination 2 (alternative base): US 3,028,546 or US 2,789,887 + US 1,592,979 + US 3,265,962

If the primary reference is taken to be the assembled cell of US 3,028,546 (or the sampler cell of US 2,789,887), the analysis is structurally identical but the motivation is stronger still, because:

• US 3,028,546 and US 2,789,887 are dip/immersion cells — the exact device class the '401 specification is directed to ("dip-type conductivity cells … with cell constants greater than about 0.3/cm"). The '401 background's entire complaint is about dip cells sensitive to the proximity of external objects. Starting from a dip cell and adding (a) '979's perforated enclosing casing and (b) '962's capillary-retained chamber produces the '401 structure without any change in the principle of operation of the base cell.
• US 2,789,887 (Thomas sampler cells, GB 546,672 counterpart from 1942) indicates that encasing the electrodes of a dip-style cell for a hostile sampling environment was already practiced for over a decade before the effective filing date.
• The combination motivation is therefore not merely "these are analogous arts" — it is that the same instrument, in the same service (field/industrial liquid conductivity measurement), was required to become position-independent.

6. The critical vulnerability: is the "equipotential" limitation structural at all?

This is the argument that, in my judgment, does the most work against the claims, and it is worth stating precisely.

Claims 2 and 9 require the aperture to be "substantially coincident with at least a portion of a line of equipotential established at the inside surface of said chamber when a voltage is applied to the electrodes." Claims 5 and 10 use the alternative recitation "located symmetrically with respect to each of said pair of electrodes."

Consider the literal geometry. When a voltage is applied to two electrodes inside a chamber, the interior is filled by a continuous, non-intersecting family of equipotential surfaces. Each such surface intersects the inside wall surface of the chamber in a closed contour — a "line of equipotential" at the inside surface. Every point on the inside wall of the chamber lies on exactly one such contour. Consequently, an aperture placed anywhere in a wall of any electroded chamber is, as a matter of physics, coincident with a portion of a line of equipotential.

If that reading is adopted — and I see nothing in the claim language that forecloses it — then:

• The E4 limitation in claim 2 excludes nothing. Claim 2 reduces to E1 (enclosure) + E2 (aperture) + E3 (aperture small relative to electrode spacing). US 1,592,979 discloses E1, E2 and E10; E3 is a stated design parameter. On that reading claim 2 is not merely obvious but arguably anticipated by US 1,592,979, with "small relative to electrode spacing" the only contested word.
• The same collapse applies to the first prong of claim 9.
• For claims 5 and 10, "symmetrically with respect to each of said pair of electrodes" is likewise satisfiable by a wide range of aperture placements around a symmetric plate pair, including the plural perforations of the '979 casing.

Two things make this argument less than a slam dunk, and I want to state them fairly:

1. Alternative claim construction. A tribunal could construe the limitation as requiring the aperture to be coincident with a single continuous equipotential contour that spans the chamber — i.e., the aperture must not straddle two different contours. Under that reading, an aperture of non-zero size straddles contours unless it is infinitesimally thin, which would make the claim hard to infringe but also hard to satisfy — a construction that risks reading the limitation out of the claim entirely. A more workable middle reading is that the aperture must be narrow transverse to the equipotential contours, which is what claim 2's dependent geometry and the specification's "narrow dimension or width is the dimension transverse to the planar electrodes and to the lines of equipotential 19" seem to contemplate. But even that reading folds E4 into E3 — i.e., the "equipotential" language becomes a restatement of the "narrow transverse dimension" limitation, which is itself just a size parameter.
2. The practical purpose of E4 in the patent is to ensure that all the apertures sit at the same potential, so that no two apertures drive current through the external fluid. But claim 2 recites "at least one aperture." With only one aperture there is no opening-to-opening external path at all, and the patent's own mechanism ("no electrical current will flow externally between the openings," col. describing openings 15 and 17) is inoperative. Claim 2 therefore recites a solution whose mechanism only engages in embodiments the claim does not require — a classic indication that the limitation is not commensurate with the asserted advance.

This analysis matters because it converts the obviousness question from "did the art teach the equipotential placement?" to the much easier question "did the art teach a perforated enclosure with small apertures and, separately, capillary retention?" — which the answer to is plainly yes.

7. Claim-by-claim § 103 assessment

8. Secondary considerations (objective indicia) — what is and is not in the record

9. The counterarguments that could defeat the prima facie case

I would be doing the analysis a disservice not to set out the strongest points on the other side:

1. No reference expresses or suggests the equipotential insight. None of the six references frames the problem as "current escaping from opening to opening external to the cell," and none proposes common-potential openings as the remedy. Where the prior art discloses the structure but not the operative principle, a patentee can argue the discovery of the principle is the inventive contribution. The counter is KSR's express statement that the problem need not have been previously recognized.
2. '979's casing may teach away from small apertures. A protective casing is normally perforated generously to ensure fluid exchange and to avoid clogging by particulates; the '401 approach requires deliberately tiny apertures that risk plugging. The patent answers this by pointing to viscosity-dependent aperture sizing and interchangeable covers — but a patentee can characterize the small-aperture choice as running counter to the protective-casing design rationale. I flag, however, that I have not read the full '979 text on the perforation sizing, so I cannot state whether it teaches away, merely that an argument is available.
3. '962 is a capillary tube, not a perforated chamber. A patentee can argue that capillary retention in a closed channel open at one or both ends is structurally distinct from a film spanning a small wall orifice in a multi-aperture enclosure, and that the width criterion in claims 5/9/10 ("the width of film capable of supporting the column of fluid determined by the dimensions of the enclosure") is a specific, calculated relationship not taught there. This is the best structural distinction in the patent, and it is why I rate claim 5 "moderate–strong" rather than "strong."
4. Hindsight risk. Assembling '979 + '962 + a symmetry principle produces a neat, satisfying account after the fact. The Federal Circuit polices exactly this (In re Warsaw Orthopedic; InTouch Techs. v. VGO), and a court could find the motivation articulated above to be an impermissible reconstruction rather than a genuine pre-1970 design path.
5. Claim 1 is a structural species claim with no close counterpart in the cited art. Even if claims 2–5 and 9–10 fall, claim 1 may survive because it recites a specific glass-tubing geometry (two volumes, glass divider, two opposed holes, tapered end, three-hole bisecting plane) that no reference shows.

10. Assessment and confidence

Practical caveat: this analysis is academic in one sense — the patent expired 1993-02-17 and, per the Litigation Summary, no adversarial proceeding involving it has been identified. Its value is as a window onto pre-1970 conductivity-cell practice and as a demonstration of how a claim's asserted point of novelty can evaporate when it is a physical inevitability rather than an added structure.

11. What would change my conclusion

1. Full text of US 1,518,211, US 1,951,035, US 2,789,887 and US 3,028,546. I have only their bibliographic records. If, for example, US 3,028,546 discloses a cell whose wall apertures are symmetric about a plate pair, the § 103 case for claims 2, 5, 9 and 10 strengthens from "combination" to near-anticipation. Conversely, if US 2,789,887 expressly emphasizes large or numerous apertures for free flow, that supports a teaching-away argument.
2. US 2,939,070, US 3,025,458, US 3,025,459, US 3,161,823 and US 3,484,359, listed as Similar Documents on the Google Patents record. These are pre-1969 conductivity-cell housings that I have not read; any of them could supply E2/E3 directly and shorten the combination chain.
3. A pre-1970 guard-electrode or shield reference to corroborate motivation point 4 in § 4.2, which currently rests on general knowledge rather than a retrieved document. Note that US 3,753,098 ("Guarded oil test cell with triaxial connector," 1973-08-14) appears in the Similar Documents list but post-dates the 1970 priority date and cannot be used.
4. A claim-construction ruling on "line of equipotential." Because so much of this analysis turns on whether E4 is a structural limitation or a restatement of physics, the outcome is materially construction-dependent.
5. Any documentary evidence of record on secondary considerations (sales, licenses, industry adoption, contemporaneous skepticism) — I found none, so the objective-indicia column is presently empty and cannot rescue the claims.

12. Flagged items and cross-reference notes

• No contradiction between this analysis and the Patent Summary's technical and bibliographic content; I have built directly on the claim decomposition and the "core technical idea" section already generated.
• Consistent with the Litigation Summary: the 1989 assignment to Rosemount (Reel/Frame 005243/0057, effective 1989-05-23) is a portfolio transfer, and I have treated the absence of enforcement activity as neutral rather than as evidence of nonobviousness or of validity.
• The "FIG. 5" typographical error identified in the Patent Summary is irrelevant to the § 103 analysis; it is a drawing-reference slip, not a claim defect.
• Strict-identifier compliance: I interpreted "3939401" literally as US 3,939,401 and did not substitute or merge it with any similar-numbered document. Reference numbers (1,518,211; 1,592,979; 1,951,035; 2,789,887; 3,028,546; 3,265,962) are likewise reported as they appear on the face of the patent.
• Reliability note on the search results: the US 1,592,979 and US 3,265,962 descriptions quoted above come from Google Patents and Espacenet/patentimages text, which occasionally mis-OCR numerals (e.g., "1,81" for "18¹"). The substance — a perforated casing around parallel plate electrodes, and a capillary-retained sample chamber, respectively — is clear; individual reference numerals should be verified against the granted PDFs before being quoted in a formal paper.
• I did not reach a CourtListener/PACER confirmation in this pass; the § 103 conclusions above do not depend on litigation status.