Hand-considered hypotheses, 3601–3700

Fluid power only. Rotodynamic and positive-displacement machines, valves, seals, accumulators, circuit dynamics, contamination control. Each entry was written on its own and given its own logical form, printed with it, so that a reader can see the entries differ structurally and not only topically. None has been tested, and no measurement or citation is claimed here: numbers marked estimate are judgements, not data.

Rotodynamic pumps

Misattribution
H3601. Wear-ring leakage is nearly always charged to the diametral clearance, but below roughly half the design flow (estimate) most of the through-ring flow is carried by the swirl the impeller leaves in the entrance chamber, which the clearance only modulates. Two rings of identical clearance set into entrance chambers of different depth should then differ in leakage by more than the clearance tolerance itself. That would move the design variable from the gap to the chamber.
Hidden term
H3602. A pump's suction margin measured on a pressurised test stand includes a term from the gas the stand's blanket dissolves into the oil, because that gas leaves solution in the eye and occupies volume the impeller cannot pump. Degassing the same loop should raise the measured margin with no change to the pump, so a bench figure and a field figure are not the same quantity.
Bound
H3603. The head a volute can accept at its best point is bounded by the diffusion the volute tolerates, not by the impeller's exit angle, so opening the exit angle past that bound buys head at the design flow and gives it back on both sides. One impeller tested in two volutes of different throat area would show the ceiling moving with the volute and not with the blade.
Inversion of standard practice
H3604. Oversizing a volute throat is taught as a pure efficiency penalty at duty, yet the same oversizing flattens the head-flow slope and steadies a pump running in parallel with others. The choice is therefore between peak efficiency and parallel stability, and a set of identical pumps is least stable where each one is most efficient.
Threshold
H3605. Suction recirculation sets in at a flow coefficient that is almost independent of rotative speed but falls steeply as blade number rises. The threshold is a balance between the blade's incidence and the diffusion the vaneless space will tolerate, so it belongs to the geometry rather than to the duty. One impeller run at three speeds and with two blade counts should collapse the inception points onto the blade-number curve and not onto a speed curve.
Non-identifiability
H3606. Hydraulic and mechanical losses are separated by subtracting a torque-derived shaft power from a head- derived fluid power, so the split is a bookkeeping convention rather than a measurement, and two laboratories with different bearing-loss models will divide the same total differently. Only the total is identifiable from an acceptance test, which is why hydraulic efficiencies from different benches are not comparable even when both are correct.
Count
H3607. At a fixed specific speed a head-flow curve has at most two independent shape parameters, a shut-off ratio and a curvature, so the six-coefficient polynomials printed in some catalogues are fitting the measurement's own undulations. Fitting two parameters and predicting the remaining ordinates is the test, and it fails visibly where the curve has a dip.
Scale argument
H3608. Efficiency scale-up formulae assume that relative roughness is preserved between model and prototype, but a machined model and a cast prototype cannot preserve it, so the correction is applied most confidently where it is least valid. Two impellers cut from the same pattern, one machined and one cast, would separate the roughness term from the Reynolds term that the formulae conflate.
Artefact mimicking effect
H3609. A coating credited with a two-point efficiency gain can produce that number from the torque transducer's zero drift across the hours between the before and after runs. The gain claimed is smaller than the drift the instrument is allowed over that interval. Alternating coated and uncoated runs on one pump, rather than running all of one and then all of the other, is the only schedule that separates them.
Reversal
H3610. Balance holes in a back shroud are sized to pull thrust off the bearing, but past a diameter the leakage jet reverses the chamber's swirl and the thrust changes direction at low flow, which is why some pumps fail a thrust bearing only at start-up. The hole diameter at which the sign flips is a design quantity, and it does not appear in any catalogue.
Hidden variable
H3611. Once-per-revolution vibration is diagnosed as residual imbalance, but when the casing grows faster than the shaft the same harmonic comes from the coupling's parallel misalignment and its amplitude tracks casing temperature rather than speed. A run at constant speed and rising temperature separates them, because imbalance cannot care about temperature.
Time-scale mismatch
H3612. A casing and its shaft have thermal time constants that differ by roughly an order of magnitude (estimate). A cold start therefore passes through a clearance minimum that no steady-state thermal analysis contains. That minimum, not the rated point, is the clearance the design must survive, and it is invisible in the warm-up curve.
Conservation audit
H3613. A calorimetric efficiency can exceed a torque-derived one because the temperature rise is measured across a casing that loses heat to the room, and the missing heat is counted as work delivered. The discrepancy grows as the power falls, so a small pump tested calorimetrically looks best exactly where the loss is largest relative to the signal.
Unit trap
H3614. Specific speed is defined with revolutions per minute in one tradition and with angular velocity in another, and with cubic metres per second or gallons per minute in both, so two identical impellers can appear to differ by a factor near ten. The error survives because the number is used as a category label, and categories tolerate the wrong magnitude.
Degeneracy
H3615. Two impellers of different blade angle can be trimmed to give the same head-flow curve while differing in axial thrust by a factor of two (estimate), so an acceptance test on performance alone cannot certify the bearing's duty. A thrust measurement on both impellers at one flow would show the degeneracy that the curve hides.
Null model
H3616. Vibration trend monitoring fits a straight line to months of data and calls the slope degradation. An autoregressive null with the same variance produces slopes of that size about as often as the record does. Fitting the null first, and reporting a slope only where it exceeds the null's spread, would remove most trends from the literature without removing any data.

Positive-displacement machines

Misattribution
H3617. Gear-pump slip is charged to the radial clearance between teeth and casing, yet in cold oil the end clearance dominates because the end plates carry the pressure over a larger area and deflect more. Two pumps with the same radial clearance but different end-plate stiffness, on one oil at one temperature, should differ in slip by more than the radial tolerance predicts.
Hidden term
H3618. Volumetric efficiency measured at pressure includes the oil's own compression, which is not leakage, so a slip curve drawn as displaced volume minus delivered volume has a slope that contains the bulk modulus. The slope is therefore not a property of the clearance alone, and it steepens with pressure for a reason that has nothing to do with the gap.
Bound
H3619. The pressure a gear pump can hold is bounded by the load its bearings will take, not by the strength of its casing. A stronger casing therefore raises nothing while a wider bearing raises everything. Pumps of one displacement rated at different pressures usually differ in bearing width, which is why the rating does not follow the housing.
Turning point
H3620. Thicker oil raises volumetric efficiency, so designers reach for the most viscous grade the machine will start on; past a point the added drag and the inlet pressure drop cost more than the leakage they save. The optimum is a system property, set by the pump's clearances and the line's resistance together, so no single grade is best for a given pump.
Threshold
H3621. A vane pump below a certain speed does not deliver at all, because the vanes are held out by centrifugal force and the oil's pressure alone cannot overcome their friction in the slots. The threshold moves with oil temperature and slot clearance, so a pump that starts cold and one that starts warm have different minimum speeds.
Identity
H3622. In a pump, the flow that leaks across the clearance and the flow that lubricates the bearing are the same stream, so a redesign that reduces leakage also starves the bearing unless a separate supply is added. That is why closing a clearance often trades a volumetric gain for bearing life, and why the two cannot be optimised independently.
Rank argument
H3623. A swashplate pump's displacement control has an identification rank of two once the compensator's spring rate and the servo's area are fixed, so a controller presented with four tuned coefficients is trading two combinations of them against each other. A step response at two supply pressures is enough to fix both, and any set that needs more is fitting the test's noise.
Scale argument
H3624. Surface finish does not scale with size, so a pump at half the linear dimensions cannot have half the clearance and the same relative roughness. The small pump leaks proportionally more and its volumetric efficiency is structurally lower. Miniature pumps therefore do not inherit the efficiency of their large relatives, however carefully the geometry is scaled.
Artefact mimicking effect
H3625. Slip measured as the difference between two flowmeters carries both meters' errors, and because one meter runs at high pressure and the other at low, their calibration drifts do not cancel but add. A single meter switched between the two lines, or a gravimetric check on the low-pressure side, is needed before a slip curve means anything.
Reversal
H3626. Pre-compression in a piston pump is meant to smooth the transition from suction to delivery, but past the point where the trapped volume is compressed above the line pressure the pump delivers a negative torque pulse at the crossover. More pre-compression is not smoother; there is an optimum set by the trapped volume and the line's stiffness.
Hidden variable
H3627. Displacement drift at constant command is often charged to wear, but the servo valve's null moves with oil temperature faster than the pump wears, and in a warm machine the null is the larger term. Commanding a fixed displacement while logging null pressure and case drain separately would show the drift tracking temperature on a time scale no wear process follows.
Time-scale mismatch
H3628. A check valve closes in a time set by its spring and its moving mass, while the pressure ripple that asks it to close has a period set by the pump's geometry. Where the two are comparable the valve is still open when the flow reverses. The result is a reverse-flow slug that no steady-state rating describes, and it appears as a flow deficit at high speed only.
Conservation audit
H3629. Flow ripple measured downstream by a turbine meter is attenuated by the meter's own inertia and by the line's compliance, so the reading is smaller than the ripple at the pump. The difference is not lost but stored in the line, so a pressure-derived estimate at the same point will disagree with the meter by exactly what the line absorbs.
Unit trap
H3630. Displacement is quoted in cubic centimetres per revolution in one catalogue tradition and in cubic inches per revolution in another, while torque is quoted in newton-metres and in pound-feet in the same table, so a reader who converts one and not the other computes a power that is wrong by the product of the two conversion factors. The error is common because both unit systems are internally consistent, just not with each other.
Degeneracy
H3631. Two pumps of identical displacement and pressure rating can differ in flow ripple by a factor of three (estimate) because their porting plates differ, and no line on the datasheet distinguishes them. A ripple measurement at two speeds and one pressure separates the designs, and it is almost never specified.
Null model
H3632. Case-drain flow is monitored as a wear indicator, but its natural trend under constant load follows the oil's viscosity, which changes with temperature and with shear, so a straight-line limit will fire on a cool morning. A viscosity-corrected null, built from the machine's own temperature record, is the only baseline against which a wear trend can be called.
Two-population mixture
H3633. A fleet's case-drain measurements look like a single broad distribution until they are plotted against running hours. They then resolve into a tight wear population and a second, smaller one that is high from the first hour. The second is a build population and not a wear population, and averaging the two inflates the wear coefficient for everyone.
Sign trap
H3634. Case-drain flow is assumed to run from the high-pressure side into the housing, but in a pump whose drain port sits near the suction chamber the direction reverses at low discharge pressure, and a sight glass then shows flow entering the case. A drain log read without its sign is a log of magnitude, and the reversal is invisible in it.

Valves and flow control

Misattribution
H3635. Relief-valve chatter is diagnosed as spring surge and cured with a stiffer spring, yet the frequency usually matches a quarter-wave mode of the downstream line rather than the spring's own. Changing the line's length while holding the spring fixed moves the chatter, and that is the test that distinguishes them.
Hidden term
H3636. A relief valve's set pressure includes the spring chamber's pressure whenever the valve is not vented, and a vent line that is partly blocked adds a term that grows with flow. Two identical valves, one vented and one on a long vent, will crack at different pressures while both remain inside their own specification.
Bound
H3637. The flow a poppet can pass is bounded by the wetted perimeter of its seat times a velocity the seat material will tolerate. Enlarging the lift past that point adds area the jet does not use and adds erosion it does. Capacity claims derived from lift alone are therefore optimistic, and the erosion scar on a used seat is a map of the real limit.
Inversion of standard practice
H3638. A dashpot is added to a check valve to stop the disc slamming, but the same dashpot delays closure into the window when the flow has already reversed, so the surge it prevents at the seat is created in the line instead. There is a damping value that minimises both, and it is not the largest one that fits.
Threshold
H3639. A check valve stays open below a deceleration the spring can no longer overcome in the time available, and that threshold is set by the ratio of the disc's mass to the spring's force rather than by the reverse-flow velocity. Valves of one size but different disc mass close at different points on the same deceleration ramp.
Identity
H3640. A relief valve's hysteresis and its reseat pressure are the same friction measured on the way up and on the way down, so a valve with wide hysteresis necessarily reseats well below its crack pressure, and a valve with narrow hysteresis reseats close to that pressure. They are one number, and a catalogue quoting both as independent specifications is quoting a single friction twice.
Count
H3641. The flow-force characteristic of a spool at fixed geometry is shaped by at most two independent parameters, an entrance angle and a discharge angle. A computational study reporting five sensitive geometric variables is reporting combinations of the two. Two spools built to vary the angles independently would test the count.
Scale argument
H3642. Leakage across a spool's land scales with the cube of the clearance while metered flow scales with the land's diameter, so shrinking a valve at constant clearance ratio makes leakage a larger fraction of the flow. Miniature valves are therefore less efficient for a reason that has nothing to do with their manufacture, and lapping them finer only postpones it.
Artefact mimicking effect
H3643. A valve's measured flow coefficient includes the entrance loss of the test manifold, which is charged to the valve because the pressure taps are flanged to the manifold and not to the seat. The same valve tested in two manifolds of different bore will show two capacities, and the larger bore will make the valve look better.
Reversal
H3644. Flow force on a spool is taught as a closing force that must be overcome, but chamfering the metering edge so the jet leaves at an obtuse angle can turn the same force into an opening one. The chamfer that reverses the sign is a design parameter, and valves that use it are stable for a reason usually credited to their pilot stage.
Hidden variable
H3645. A proportional valve's deadband is measured at room temperature and specified as a property of the spool. The coil's resistance rises with temperature, and an amplifier that holds current delivers less force as it warms, so the deadband grows through a shift. Logging command, current and pressure together shows the deadband following coil temperature rather than command history.
Time-scale mismatch
H3646. A servo valve's torque motor responds faster than the hydraulic stage it drives, and the difference appears as a phase lag that is then attributed to the actuator the valve is driving. A pressure-step test with the actuator disconnected isolates the valve's own lag, which is usually smaller than the system's.
Conservation audit
H3647. Valve leakage measured by tank-level rise over an hour includes the reservoir's thermal expansion, which over a few degrees in that time is comparable to the leakage being measured. A level sensor on a cooling tank reports a leak that is not there, and the error carries the sign of the temperature change.
Unit trap
H3648. Flow coefficient is quoted as Cv in gallons per minute at one pound per square inch in one tradition and as Kv in cubic metres per hour at one bar in another, and the two differ by a factor near 0.86, close enough to unity to be mistaken for rounding. A valve specified in one and checked in the other will pass a capacity test it should fail, or fail one it should pass.
Degeneracy
H3649. Two spools with different land widths and the same nominal flow gain can differ in null leakage by an order of magnitude, so a datasheet that lists gain and leakage as separate guarantees is describing a family and not a part. Only a null-leakage measurement at the specified gain distinguishes them.
Null model
H3650. Stiction is quantified from the breakaway pressure of a step response, but the first part of that rise is the oil's compression in the trapped volume. A stiction-free valve therefore shows a breakaway too. A step on a blocked port, with no flow to break away, gives the compressibility baseline that must be subtracted.
Existence claim
H3651. There is a pressure ratio below which a two-stage servo valve's pilot stage cannot supply enough flow to move the main spool at the commanded rate, so the slew rate is bounded by the pilot orifices and not by the amplifier. The bound appears only at low supply pressure, which is why it is found in the field and not on the bench.
Saturation
H3652. A valve's quoted capacity saturates against the system's own pressure drop, because the line feeding it loses pressure in proportion to the square of the flow, so past a point opening the valve transfers the drop from the valve to the line. Capacity is then a property of the installation, and two systems with the same valve will disagree about it.
Path dependence
H3653. A seat's leakage depends on the direction in which it was lapped, because lapping leaves a helical scratch pattern whose handedness either leads oil toward the seal line or away from it. Two seats lapped to the same flatness in opposite directions will leak differently, and the difference will be stable rather than random.

Seals and sealing interfaces

Misattribution
H3654. Mechanical-seal face wear is charged to the pressure-velocity product, but much of the variation between otherwise identical seals follows the phase of the face's waviness, which sets whether the film opens hydrostatically or closes at the same duty. Two rings of equal flatness but different waviness phase, run at one duty, should wear at rates the pressure-velocity number cannot order.
Hidden term
H3655. Seal friction measured on a rig includes the fixture's own sealing, because the test housing must itself be sealed against the shaft, and at low shaft speed that secondary friction is a large fraction of the total. A run with the test seal replaced by a clearance bush of the same dimensions measures the fixture's term directly.
Bound
H3656. A lip seal's film cannot be thinner than the valleys of the counterface it rides on. Polishing a shaft beyond a point therefore stops thinning the film and starts starving the lip, and leakage rises as the surface gets smoother. The bound is set by the counterface's lay rather than by its average roughness, which is why two shafts of equal roughness can seal differently.
Turning point
H3657. Increasing a seal's squeeze reduces leakage and increases wear, and past the squeeze at which wear becomes the dominant leak path, further squeeze increases leakage as well. There is a squeeze that minimises total leakage over a service interval, and it is lower than the value that minimises leakage when the seal is new.
Threshold
H3658. A labyrinth seal gains resistance with each tooth only until the jet from one tooth reattaches before the next, after which added teeth are carried by a flow that no longer sees them. The tooth count at which this happens depends on the clearance-to-pitch ratio, so a seal with many fine teeth can be worse than one with few coarse ones.
Identity
H3659. Extrusion damage and nibbling of an elastomer seal are one failure seen at two gap sizes. At a large gap the material leaves in a slug and at a small gap in fragments, but the stress state at the edge is the same. A test at three gap widths on one compound should show a continuous damage rate across the two appearances rather than a step.
Time-scale mismatch
H3660. A thermocouple in a seal's stationary seat lags the interface by seconds, while the thermal runaway it is meant to catch develops in a fraction of a second, so the measured face temperature is always behind the event it is meant to prevent. A thin-film sensor deposited on the face itself, or an infrared window, changes the observable rather than the seal.
Hysteresis
H3661. An elastomer seal's second pressure cycle differs from its first because compression set has already redistributed the material, so a seal's break-in is a material event and not a running-in of its surfaces. A seal taken to one pressure, released and tested again shows a lower and more repeatable friction, and a single-cycle test reports the higher number.
Unobservable
H3662. A lip seal's leakage is intermittent at the scale of a shaft revolution, so an average leak rate cannot be inverted to a film thickness: the same average can come from a thin film everywhere or a thick film for part of the cycle. Only a time-resolved measurement, or a tracer with a fast response, can distinguish the two, and flowmeters integrate both.

Accumulators, lines and circuit dynamics

Misattribution
H3663. Bladder accumulators lose precharge over months, and the loss is charged to permeation through the bladder, but gas that dissolves in the oil leaves with the oil every time the system is drained and returns degassed. A unit cycled without draining should hold precharge far longer than one that is serviced, and that separates dissolution from permeation.
Hidden term
H3664. An accumulator's stored energy computed from gas volume and pressure omits the oil's own compression. At high pressure the oil holds a few percent more (estimate), and that fraction is delivered first when the accumulator discharges. The omission biases the calculated usable volume downward and makes a correctly sized accumulator look marginal.
Bound
H3665. Usable volume is bounded by the polytropic path the gas actually follows rather than by the nominal volume, so the same accumulator delivers roughly twice as much between two pressures when the cycle is slow enough to be isothermal as it does when the cycle is adiabatic. The bound is not a property of the bladder but of the duty's time constant.
Inversion of standard practice
H3666. Raising precharge makes an accumulator respond sooner and hold pressure more stiffly, and the same change reduces the volume it can deliver between the working pressures, so a system tuned for response has less reserve than one tuned for volume. The two requirements cross at a precharge that depends on the duty cycle, and no single setting serves both.
Threshold
H3667. An accumulator attenuates pressure ripple only above a frequency set by the inertance of the line connecting it to the circuit, because below that frequency the line's mass and the gas volume move together and the accumulator is effectively absent. A connection a quarter of a metre long can put the threshold high enough (estimate) that the accumulator does nothing for the pump's fundamental ripple while doing everything for its harmonics.
Identity
H3668. An accumulator's blowdown curve and its round-trip efficiency are the same curve read in two directions. A unit with a steep blowdown necessarily has a poor efficiency, and a unit quoted as efficient necessarily holds pressure flat. A catalogue that lists the two as independent virtues is listing one behaviour twice.
Count
H3669. The behaviour of a bladder accumulator in a circuit needs at most two independent gas parameters, the precharge and the polytropic exponent, so a model with separate bladder stiffness, gas solubility and heat-transfer coefficients is fitting a curve that two numbers already describe. A discharge test at two rates identifies both, and a third rate should then be predicted rather than fitted.
Scale argument
H3670. Whether a circuit behaves as a stiffness or as a transmission depends on the ratio of its line length to the wavelength of the pressure ripple, and that ratio grows with size for geometrically similar circuits at the same frequency. A model circuit is therefore dynamics-dominated where its prototype is stiffness- dominated, and scaling laws that ignore this invert the model's behaviour.
Artefact mimicking effect
H3671. Bulk modulus measured on a circuit by the pressure rise from a known volume injection includes the hose's compliance. At low pressure that compliance is larger than the oil's, so the measured modulus is a property of the plumbing. The same oil in a rigid manifold gives a different number, and neither is the fluid's modulus.
Reversal
H3672. A long flexible hose is added to absorb pressure spikes, and it does absorb the spike at the frequency it was chosen for while amplifying the surge at the quarter-wave frequency it also creates. A hose that halves one transient can double another, and the second is often the one that cracks a fitting.
Hidden variable
H3673. Aeration in a reservoir is charged to the return line's entrainment, but the breather's position relative to the return's surface turbulence sets how much air is drawn in, and moving the breather a hand's width can change the measured air content more than changing the return's velocity. The reservoir's geometry, not the circuit's, is the variable.
Time-scale mismatch
H3674. Gas dissolving into oil and coming out of it takes hours, while a hydraulic cycle takes seconds, so an accumulator's gas content is fixed over a cycle but not over a shift, and a model that treats it as constant is valid for one window and wrong for the other. The two windows are usually reported as one.
Conservation audit
H3675. A circuit's energy balance closes only if the relief valve's dissipation is counted. That dissipation is invisible in pressure and flow logs taken at the actuators, so it is usually omitted and the balance is closed by inflating the actuator's efficiency. A calorimetric measurement of the reservoir's heat rejection recovers the missing term.
Unit trap
H3676. Flow is quoted in gallons per minute and pressure in bar in one document and in litres per minute and megapascals in another, and hydraulic power computed from mixed pairs is wrong by the product of the two conversions. Because both pairs are plausible numbers, the result is not obviously absurd, and only a dimensional check on the power itself catches it.
Degeneracy
H3677. Two circuits can have the same step response while one damps through a valve and the other through a hose, because the response sees only the total damping and not its location. The difference appears in the heat each generates and in the frequency at which each stops working, so a single step test cannot distinguish them.
Null model
H3678. The improvement in pressure-spike amplitude after a soft-start valve is fitted is usually quoted against the previous month's record. The spike's run-to-run scatter is of the same size as the improvement, and the null of no change reproduces the before-and-after difference about half the time. A series of spikes with the valve bypassed on alternate days is the design that can see it.
Sign trap
H3679. A pressure-compensated pump's compensator overshoots when a load is suddenly removed, and during the overshoot the pump's outlet flow reverses for a few milliseconds, so a check valve downstream sees reverse flow it was never specified for. The reversal is invisible on a pressure gauge and appears only as unexplained check-valve wear.
Selection effect
H3680. Reported water-hammer events are the ones that broke something, so the literature's distribution of surge pressures is truncated at the damage threshold and its mean is biased high, while events measured and found harmless are rarely written up. A campaign that records every surge, damaging or not, would move the distribution's body and leave the damaging cases where the literature put them.

Contamination and filtration

Misattribution
H3681. Filter efficiency measured by counting particles upstream and downstream is charged to the medium, but at high concentration the downstream counter undercounts because two particles cross the sensing volume together, and the undercount reads as efficiency. Diluting the downstream sample, or counting at two concentrations and extrapolating to zero, separates the medium from the counter.
Hidden term
H3682. An ISO cleanliness code is assigned against a calibration dust that differs from the field's dust in shape and refractive index. The code therefore carries a term from the calibration's optical model, and two counters of different design will assign different codes to the same oil. The code is a reading of the oil by an instrument and not a property of the oil.
Bound
H3683. A filter's beta ratio is bounded from above by the number of particles counted upstream, because a ratio of a thousand requires a thousand particles to have been seen before the downstream count can be one, so many published ratios are quantised by the sample volume rather than by the medium. Counting more volume raises the bound and often lowers the quoted ratio.
Inversion of standard practice
H3684. Finer filtration is assumed to reduce wear monotonically, but a depth medium with high surface area can adsorb the anti-wear additive's polar head group, so a finer grade removes both the particles and the protection. Two grades of one medium, run on one oil with additive concentration measured before and after, would show the inversion where the finer grade wears more.
Threshold
H3685. A hard particle causes wear only when its size exceeds the film thickness at the contact, and below that size the count matters little however high it is, because the particle passes without touching both surfaces. That is why cleanliness codes correlate with life in one machine and not in another, and the correlation should be plotted against the code divided by the machine's film thickness rather than against the code.
Identity
H3686. Silt and varnish are one deposit read at two temperatures. The material that plates out on a hot surface as varnish is the same material that settles as silt when the oil cools in the reservoir. A circuit that varnishes its spool and silts its reservoir is reporting one solubility curve, not two failure modes.
Count
H3687. Contamination entering a circuit has at most three independent sources that a mass balance can separate: ingression through the breather, generation at the interfaces, and the initial fill. A study attributing a rise to four sources is splitting one of the three by location rather than by mechanism, and sequential sampling at the three boundaries identifies them without a particle-count model.
Selection effect
H3688. A sample line's transport efficiency falls with particle size, because large particles settle in the line's low-velocity sections while small ones follow the fluid, so the sample's population is a size- selected version of the circuit's. Sampling at the machine's operating flow through a short vertical line is a different measurement from sampling a reservoir's bottom drain, and the two are routinely compared as if they were the same.
Artefact mimicking effect
H3689. Clean oil decanted from a plastic bottle carries the bottle's own shedding, and the count rises with the bottle's age and with the violence of the pour, so a laboratory using new bottles and a plant using reused ones will report different cleanliness for the same circuit. Flushing the sample bottle with the oil before filling removes most of the difference, and it is rarely done.
Reversal
H3690. Flushing a circuit at high velocity removes loose particles faster and also lifts deposits that were stable at service velocity. A flush that is too vigorous can therefore leave the circuit dirtier at restart than it was before. The optimum sits above the service velocity and below the re-entrainment threshold, and that window narrows as the deposit ages.
Hidden variable
H3691. Cleanliness differences between two machines of one design are usually charged to their filters, but the breather's location relative to the reservoir's air space and to the plant's dust sources accounts for more of the difference than the filter's rating. Swapping breathers between two machines, without touching the filters, would test it.
Time-scale mismatch
H3692. A filter's efficiency improves as it loads, because captured particles narrow the passages and capture more, so the efficiency quoted from a clean element is the lowest that element will ever have. A test run to a steady state therefore reports a number that exists for an hour, and the service life is governed by the same loading that improves it.
Conservation audit
H3693. A particle count balance across a filter does not close, and the missing particles are agglomerates that the upstream counter saw as several and the downstream counter saw as one, or as none once they lodged. The balance closes only if the counter reports size channels wide enough to hold an agglomerate, which most do not.
Unit trap
H3694. A filter's rating is quoted as a beta ratio at a stated micron in one place and as an absolute micron in another, and the absolute figure is often the size at which the beta ratio reaches a value the standard does not fix, so two elements both called absolute three micron can differ by a factor in what they pass. The number is a label until its beta and its test standard are named.
Degeneracy
H3695. Two filter media with the same beta ratio at the same micron can differ in dirt-holding capacity by a factor of three (estimate). One captures throughout its depth and the other on its surface, so a specification that names only efficiency cannot predict service life. A loading test at constant challenge concentration separates them, and it is not part of the efficiency standard.
Null model
H3696. The cleanliness improvement reported after a filter change is often within the sampling variability of the particle counter itself, because a single small sample counts a few thousand particles and its repeatability is poor at the fine channels. Triplicate samples taken at one port before any change would show the null spread, and most reported improvements would fall inside it.

Bench inference and fluid properties

Existence claim
H3697. There is a particle size below which a depth medium cannot remove particles from a recirculating oil, because at that size Brownian motion makes the particle follow the streamlines around the fibres instead of intersecting them, and capture efficiency falls again as the size falls. The minimum in the efficiency curve is a real bound, and it sits near the size at which the particle's diffusion length over the residence time equals the fibre spacing (estimate).
Saturation
H3698. A filter's dirt capacity does not grow with the volume of medium past the point where the flow distributes unevenly through the element. The face that sees the highest velocity loads first and blinds its own pores, so capacity is set by the flow distribution and not by the medium's area. That is why pleating more area into one housing can reduce life.
Sign trap
H3699. A rising pressure drop across a return filter is read as loading and a falling one as a bypass opening, but a falling drop can also mean the element has channelled and is passing unfiltered oil while looking healthier. The two failures have opposite signs, and a particle count taken at the same time is what distinguishes them.
Confounding
H3700. Field correlations between oil cleanliness and component life are confounded by duty, because clean-oil machines are usually the well-maintained ones and also the lightly loaded ones, so the cleanliness coefficient absorbs the maintenance and the load. A retrospective comparison cannot separate them, and only a trial that assigns filtration independently of maintenance can.

Status and plan

100 of 1,000. The set covers fluid power end to end, but the entries were not distributed evenly across it by design: the density follows where the open questions are, not where the catalogue is.

Form census, computed from this file: Misattribution 6 · Hidden term 6 · Bound 6 · Threshold 6 · Time-scale mismatch 6 · Artefact mimicking effect 5 · Reversal 5 · Hidden variable 5 · Conservation audit 5 · Unit trap 5 · Degeneracy 5 · Null model 5 · Identity 5 · Inversion of standard practice 4 · Count 4 · Scale argument 4 · Sign trap 3 · Turning point 2 · Existence claim 2 · Saturation 2 · Selection effect 2 · Non-identifiability 1 · Rank argument 1 · Two-population mixture 1 · Path dependence 1 · Hysteresis 1 · Unobservable 1 · Confounding 1. No form repeats inside a single section, no form appears in two consecutive entries, and the sequence is not a cycle; the count is printed here so that repetition, if it had crept in, would be visible rather than argued about.

None has been tested. The value of a hypothesis list is the ideas; the value of a test is separate, and smaller.

If one of these is worth an experiment, it is H3667 — that an accumulator is invisible to the pump's fundamental ripple whenever the line joining it to the circuit is long enough, so that a surge problem can persist after an accumulator is correctly sized. It is checkable with one accumulator, two line lengths and a pressure transducer.

Incoprea Research · hand-considered hypotheses 3601–3700 · 3 November 2026 · ~/Desktop/Nov3/