The interface detector is one of the smallest pieces of equipment that MARPOL Annex I names by regulation, and one of the most consequential for a tanker’s discharge record. It does one job: it finds the boundary between the settled oil on top of a slop tank and the clean water beneath it. That boundary is the line a watchkeeper cannot cross when decanting water overboard. Reg 32 makes the instrument mandatory on oil tankers of 150 gross tonnage and above, requires that it be approved by the flag Administration, and ties it to the load-on-top discharge regime that the rest of MARPOL Annex I Chapter 4 builds around. Without a working interface detector, the lawful slop-water decant under Regulation 34 cannot be carried out safely, because the operator has no reliable way to know when the suction is about to reach oil.
Where Regulation 32 sits in MARPOL Annex I
Reg 32 is in Chapter 4 (Requirements for the cargo area of oil tankers), Part B (Equipment), of the consolidated MARPOL Annex I. The text carries forward, almost word for word, the requirement that earlier consolidations of Annex I carried as Regulation 15(3)(b) before the 2004 renumbering reorganized the Annex into chapters. Anyone reading an older copy of the rules will find the same provision under the old Reg 15 number; the substance did not change at renumbering, only the citation did. The current heading is “Oil/water interface detectors,” and the requirement reads, subject to the provisions of paragraphs 4 and 5 of Regulation 3, that oil tankers of 150 gross tonnage and above must be provided with effective oil/water interface detectors approved by the Administration.
The cross-reference to Regulation 3 paragraphs 4 and 5 is the exemption hook. It lets the Administration waive or modify equipment requirements for tankers engaged solely on voyages that do not pass through areas where a discharge would be a problem, or for tankers that hold an exemption on a defined route, but the waiver is the exception rather than the rule. For a tanker trading internationally, Reg 32 applies in full from 150 GT upward, the same gross-tonnage threshold that triggers the Regulation 31 ODMCS and the cargo-side discharge controls. The two thresholds are aligned on purpose: a tanker large enough to need a monitor on its overboard line is a tanker large enough to need an instrument that finds the interface that monitor is protecting.
The detector also belongs to a family of equipment that Annex I lists for the cargo area: the slop-tank arrangement under Regulations 29 and 30, the oil discharge monitoring and control system under Reg 31, and the crude oil washing system under Regulation 33 where COW is fitted. Reg 32 names the one instrument in that group that a crew member holds in their hand or reads at a gauge, the one that turns the abstract idea of a settled interface into a number on a tape.
The load-on-top problem the detector solves
To see why the interface detector matters, start with what a slop tank holds after a voyage. Under the load-on-top method, a tanker that has cleaned its cargo tanks and washed down does not pump the resulting oily water overboard. Instead it collects all the tank washings, the dirty ballast, and the oil residues into one or two dedicated slop tanks, arranged and piped under Regs 29 and 30. The tank then sits, and gravity does the separation. Oil is less dense than seawater, so over hours to days the mixture stratifies: a layer of recovered oil floats on top, a layer of relatively clean water settles to the bottom, and a diffuse emulsion band, the rag layer, sits between them.
The economic point of load-on-top is that the recovered oil on top is kept and the next cargo is loaded on top of it, hence the name. The environmental point is that the clean water at the bottom can be decanted overboard within the Regulation 34 limits, leaving the oil on board. Both points depend on knowing precisely where the boundary lies. Decant too little and the tanker carries water it did not need to keep, eating cargo space and slop capacity for the next voyage. Decant too much and the suction reaches the rag layer and then the oil, and the overboard line starts discharging the very thing the whole exercise was meant to retain. That is a pollution event, an Oil Record Book falsification risk, and a detention-grade Port State Control finding all at once.
The interface detector is the instrument that prevents the second outcome. It reads the depth of the oil/water boundary directly, so the operator decanting the tank can watch the interface fall toward the suction and stop the pump with a known margin of clean water still below the bell-mouth. The detector does not decide the discharge; the watchkeeper and the Regulation 31 ODMCS do that. What the detector supplies is the one measurement nothing else on the ship can give: the live position of a boundary that is invisible from the deck and that moves as the tank empties.
How the decant actually runs
A real decant operation reads like a sequence of gauge checks. The chief officer first confirms the tank has had enough settling time, often 24 to 36 hours for a heavy crude slop, less for a light product. The interface detector is then lowered, or its fixed sensor read, to establish the ullage of the oil surface and the depth of the oil/water interface. The difference between the two gives the thickness of the recovered oil layer, a number the officer records before discharge begins. A thin oil layer over a deep water layer is an easy, low-risk decant; a thick oil layer over a shallow water layer means the margin is small and the discharge must be slow and closely watched.
With the interface located, the overboard discharge is lined up from the bottom of the slop tank through the cargo pump and the ODMCS sampling point to the overboard valve. The pump starts on the clean water well below the interface. As water leaves, the interface falls and the oil layer descends with it. The officer re-checks the interface at intervals, and the ODMCS oil content meter watches the effluent continuously. As the falling interface approaches the suction, the oil content of the sampled effluent begins to climb. The decant stops on whichever trigger fires first: the interface detector showing the boundary near the suction, the oil content meter reading rising past the operator’s set point, or the ODMCS automatically halting the discharge before the Regulation 34 instantaneous rate of 30 liters of oil per nautical mile is breached.
That redundancy is the safety case. The interface detector is the predictive instrument, telling the operator where the boundary is before any oil reaches the line. The ODMCS is the reactive instrument, catching oil that has already reached the line and shutting the valve. Reg 32 and Reg 31 are written to be used together; neither is sufficient alone. A tanker relying only on the oil content meter would be running blind on layer thickness and would routinely stop the decant later than it should, discharging the leading edge of the rag layer. A tanker relying only on the interface detector would have no automatic stop if a slug of oil broke through unexpectedly. The pair gives both a forecast and a backstop.
What the regulation requires of the instrument
Reg 32 sets three substantive requirements and leaves the engineering detail to the type-approval specification. First, the detector must be effective, meaning it must actually find the interface under the conditions a slop tank presents: stratified hydrocarbons, a rag layer, an inert atmosphere, ship motion. Second, it must give a rapid and accurate determination, so a single reading taken in minutes, not a slow titration or a laboratory analysis. Third, it must be approved by the Administration, which is the flag State or a recognized organization acting for it, against a published specification.
That specification is IMO Resolution MEPC.5(XIII), the Specifications for Oil/Water Interface Detectors, adopted on 13 June 1980. MEPC.5(XIII) is the document a type-approval certificate cites. It sets the performance the instrument must meet across a range of representative oils, the accuracy of the interface reading, the response behavior, and the safety requirements for use in a tank that is part of a cargo system. Among its provisions, the equipment must read the position of the interface to an accuracy within 25 mm, so that an operator decanting a slop tank can stop the overboard flow with a real margin of water still below the probe rather than chasing the oil layer down to the last centimeter. The specification frames the test against fluids spanning the density and viscosity range a tanker handles, from a light distillate through a medium crude to a residual fuel, so that approval is not granted on one easy oil.
The resolution also distinguishes portable from permanently installed equipment and sets installation requirements for the fixed type. For a permanently installed detector, the sensor must be positioned with regard to the internal structure of the tank and the ship’s motion, the control and display unit should be in the cargo control room or a similar space, and the in-tank parts must withstand the impact of jets from tank-cleaning machines. Those installation rules exist because a fixed sensor lives in a hostile place: an inerted, hydrocarbon-laden tank that gets blasted by high-pressure washing water on every clean.
Portable versus fixed detectors
Reg 32 does not prescribe a single physical form, and both portable and permanently installed detectors are in common service. The choice follows ship size, age, and operator preference, not a regulatory mandate, provided whichever is fitted holds an Administration approval to MEPC.5(XIII).
A portable interface detector is the more familiar type on many tankers. It is a hand-held or reel-mounted instrument: a sensor at the end of a marked tape or cable is lowered through a vapor-lock or sounding pipe at the ullage opening until it reaches and passes through the interface. The instrument signals the transition between water and oil, usually with a tone or a meter deflection, and the operator reads the depth off the calibrated tape. The portable type doubles as the ullaging and sounding instrument on tankers where a combined portable electronic gauge measures ullage, interface, and temperature from the same probe. Its advantage is that one approved unit can serve every slop and cargo tank in turn, which suits Reg 32’s requirement that the detector be available for use in tanks other than the dedicated slop tanks.
A permanently installed detector places one or more sensors at fixed heights inside the tank, wired back to a readout. The fixed type gives a continuous indication without sending anyone to the tank top, which matters during a long decant where the interface is checked repeatedly, and it integrates more readily with the cargo control room’s mimic and with the Regulation 31 ODMCS readout. Its cost is mechanical exposure: the in-tank sensor must survive the tank-washing environment MEPC.5(XIII) describes, and a fixed sensor fouled by a rag-layer film or wax can read wrong while looking healthy. Many operators run a portable detector as a check on a fixed installation for exactly that reason, and as the back-up that keeps the tanker compliant when the fixed unit is defective.
The measurement principle
Different approved detectors find the interface by different physics, and the practitioner should know which one is fitted because the failure modes differ. The common principles are capacitance, optical, and conductivity, sometimes combined in one probe.
A capacitance detector exploits the dielectric difference between oil and water. Water has a relative permittivity near 80; most crude oils and products sit between about 2 and 4. A capacitive sensor lowered through the layers sees a large, abrupt change in measured capacitance as it crosses from oil into water, and the instrument reports that step change as the interface. Capacitance probes are durable and common, but a thick emulsified rag layer or a heavy water-in-oil emulsion blurs the dielectric step and can soften the reading.
A conductivity, or resistance, detector uses the same density difference from the electrical side: seawater conducts well, oil is effectively an insulator. The sensor reads a sharp conductivity rise on entering the water layer. Conductivity sensing gives a clean, decisive transition in clean systems but can be confused by a conductive emulsion or by salt-water bridging across an oily probe tip.
An optical detector reads the change in refractive index or light transmission between the two phases. A light source and detector in the probe see the medium go from the relatively opaque or differently refracting oil to the clearer water, and the instrument flags the change. Optical sensing resolves a sharp interface well and is less troubled by salinity, but a fouled optical window or a dark, opaque water phase can defeat it.
Whatever the principle, the reading the operator wants is a depth: the ullage or sounding at which the interface lies, referred to the same datum as the cargo-tank gauging system so the two numbers can be compared. A capable approved detector resolves the interface to within a small fraction of the tank height, accurate enough that the operator can set a decant margin of a known number of centimeters of clean water above the suction and trust it.
Relationship to ullage and the cargo gauging system
The interface detector does not work in isolation from the tank’s ordinary gauging. Every cargo and slop tank already has a level-measurement system that reads ullage, the empty distance from a fixed reference point at the top of the tank down to the liquid surface, or its complement, the sounding from the bottom up. The cargo total in a slop tank is the liquid level read by that gauging system. The interface detector adds the one measurement the level gauge cannot give: where, inside that single body of liquid, the oil ends and the water begins.
In practice the operator works with three numbers against a common datum. The oil-surface ullage comes from the tank gauging system or the top of the detector’s own pass through the surface. The interface depth comes from the detector. The tank-bottom datum is fixed. From those, the oil-layer thickness is the interface depth minus the oil-surface depth, and the clean-water depth available for decant is the interface depth measured up from the bottom. Keeping the detector’s datum consistent with the cargo gauging datum is what lets the chief officer write a single coherent set of figures into the cargo log and the Oil Record Book, rather than two unreconcilable depth scales. A portable combined ullage-temperature-interface probe gives all three from one instrument and one datum, which is why that form is popular.
The detector’s reading also feeds the slop-volume accounting that runs across a voyage. The thickness of the recovered oil layer measured before decant, against the layer thickness measured after, tells the operator how much water left the tank, a figure that should reconcile with the quantity the ODMCS counted overboard and with the Oil Record Book entry. A decant where the detector-derived water volume and the ODMCS-counted volume disagree badly is a sign that something, a fouled sensor, a wrong datum, or an unexpected oil carry-over, needs investigating before the next discharge.
Intrinsic safety in the inert and flammable atmosphere
A slop tank is one of the more dangerous places on a tanker to introduce an electrical instrument. During and after cargo operations the tank is held inert, with the oxygen content kept below the level that supports combustion, typically under 8 percent by volume by the inert-gas plant. But inerting is not permanent or perfect, and the atmosphere above a slop layer carries hydrocarbon vapor that, if air enters, sits in or near the flammable range. Any instrument lowered into that space, or permanently installed in it, must not be a source of ignition.
That is why MEPC.5(XIII) and the flag approval treat intrinsic safety as a pass/fail equipment requirement, not an operator’s choice. An approved interface detector is certified intrinsically safe for the relevant gas group and temperature class, meaning its electrical circuits cannot release enough energy, by spark or by surface heating, to ignite the vapor even under fault conditions. The certification covers the probe, the cable, and the readout as a system, because intrinsic safety is a property of the whole circuit, not one component. Operators verify that the unit carries a current intrinsic-safety certificate, that its cable and connectors are the certified ones, and that no repair has substituted a non-approved part, because an uncertified battery change or a swapped cable can void the safety case while leaving the instrument apparently working.
The same constraint shapes the deck procedure. A portable detector is introduced through a vapor-lock arrangement that lets the probe pass while keeping the tank atmosphere sealed from the deck and the deck atmosphere sealed from any ignition source. Bonding the instrument to the ship’s structure before it enters the tank guards against an electrostatic discharge between a charged probe and the tank, a separate ignition hazard from the instrument’s own circuit. These are the practices that turn an approved instrument into a safe operation; the approval certifies the hardware, the procedure protects the use.
The discharge interlock: Reg 32 with Regs 29, 31 and 34
Reg 32 is written to interlock with three neighboring regulations, and a lawful cargo-side discharge needs all four working together. The slop-tank arrangement under Regulations 29 and 30 provides the settling tank and the piping that lets oil and water separate and lets the clean water be drawn off from below. The interface detector under Reg 32 finds the boundary inside that tank. The Regulation 31 ODMCS measures the oil content of the effluent leaving the tank and stops the discharge automatically if oil breaks through. The Regulation 34 discharge criteria set the legal envelope the whole operation must stay inside: not in a special area, more than 50 nautical miles from the nearest land, the tanker proceeding en route, the instantaneous rate of discharge of oil not exceeding 30 liters per nautical mile, and the total quantity within 1/30,000 of the previous cargo for existing tankers.
Read together, the chain runs: Regs 29 and 30 build the tank, Reg 32 finds the interface, the operator decants the clean water, Reg 31 watches and limits the effluent, and Reg 34 defines when and how much may go overboard. Remove any link and the chain breaks. A tanker with a perfect ODMCS but a dead interface detector cannot tell where the boundary is and so cannot decant safely, only blindly. A tanker with a good interface detector but an inoperative ODMCS has lost the automatic backstop and the legal monitoring record, so it may not discharge at all under Reg 34. The regulations are deliberately not redundant copies of one safeguard; they are a series of different safeguards, each covering a gap the others leave.
The same logic explains why Reg 32 extends the detector beyond the dedicated slop tanks. The requirement that it be available for use in other tanks where oil and water are separated and from which effluent is to be discharged direct to the sea covers the case of a cargo tank used as an additional settling tank, or a ballast tank that has held oily water and is being decanted before the tanker proceeds to clean ballast. Wherever a tanker intends to draw separated water off the bottom of a tank and put it over the side, Reg 32 wants the interface located first.
Recording the decant in the Oil Record Book Part II
The interface detector’s reading is part of the evidentiary record of every slop decant. The operation it controls is logged in the Oil Record Book Part II, the cargo and ballast record book that Regulation 36 requires every oil tanker of 150 GT and above to keep. Part II uses lettered operation codes, and a slop-water decant falls under the discharge-of-water-from-slop-tanks entries. The entry records the identity of the tank, the ullage of the interface and of the oil retained, the quantity of effluent discharged, the ship’s position at the start and end of the discharge, the times the overboard valve was opened and closed, the ship’s speed, and the fact that the ODMCS was operating throughout.
Those figures have to reconcile across instruments. The interface ullages in the Oil Record Book should match the cargo-log gauging figures and the detector reading; the discharged quantity should agree with the ODMCS-counted volume; the position and time should match the ODMCS printout, which records position and time independently and is retained for at least three years. A Port State Control officer reading a decant entry checks that internal consistency. An Oil Record Book that records a clean slop decant with no interface ullage, or with an interface figure that cannot be reconciled with the oil retained, is the kind of entry that turns a routine inspection into a detailed examination of the tanker’s discharge practice.
The reverse case matters too. If the interface detector is defective and the tanker cannot determine the boundary, the proper course is to retain the slop on board, not to decant on a guess and write a plausible figure. A fabricated interface reading in the Oil Record Book is a far more serious matter than a tanker that arrives with slop tanks it could not safely decant. The discipline the regulation expects is that the record reflects what the instruments actually showed, including their failure.
Port State Control checks and common deficiencies
Reg 32 is a standing item on a Port State Control oil-tanker inspection, and the checks are concrete. The officer confirms that the tanker carries an interface detector at all, that it is the type and model named on its type-approval documentation, that the approval is to MEPC.5(XIII) and issued by or on behalf of the flag Administration, and that the instrument is in working order. Working order is tested, not assumed: the officer may ask the crew to demonstrate the detector, switch it on, and produce a reading, and may check the calibration record and the intrinsic-safety certificate.
Common deficiencies follow a pattern. The detector is on board but cannot be demonstrated to work, often a flat battery, a corroded probe, or a damaged cable on a portable unit. The instrument is present but its calibration is out of date or undocumented, so the reading cannot be trusted. The approval certificate is missing, expired, or refers to a different model than the one fitted. A fixed sensor has been left defective with the defect unrectified and no portable back-up provided, leaving the tanker with no means to find the interface. The intrinsic-safety certification has lapsed or has been compromised by a non-approved repair. On some inspections the gap is the procedure rather than the hardware: the crew cannot explain how they use the detector during a decant, which tells the officer the instrument is carried for the certificate, not used for the operation.
A defective interface detector is read in the context of the whole cargo-discharge system. An officer who finds the detector inoperative will look hard at the ODMCS, at the Oil Record Book Part II decant entries, and at whether recent decants were carried out safely without the instrument the regulation requires. Where the deficiency removes the tanker’s ability to discharge slop legally, or where it points to a pattern of discharging without proper monitoring, it can support detention under the regional Port State Control regime. The detector is a small instrument, but its absence undermines the whole cargo-side discharge case the tanker has to make.
A worked decant in numbers
Take a product tanker of 30,000 GT that has finished discharging a gasoil cargo and washed its tanks, collecting the washings in one slop tank. After 30 hours of settling, the chief officer gauges the slop tank: the liquid surface stands at an ullage of 2.0 meters in a tank 18 meters deep, so the tank holds about 16 meters of liquid. The interface detector, lowered on its tape, signals the oil/water boundary at a sounding of 11.0 meters from the bottom. That puts the oil-layer thickness at meters and the clean-water depth available for decant at 11.0 meters, the water sitting between the bottom and the interface.
The officer decides to leave a 1.0 meter margin of clean water above the suction, so the planned decant draws the interface down from 11.0 meters to 1.0 meter of water remaining, removing roughly 10.0 meters of water depth. The decant runs from the bottom suction through the ODMCS sampling point. The interface detector is re-read every few meters of falling level; the ODMCS oil content meter holds near zero on the clean gasoil-washed water, well under any alarm point. The officer stops the pump when the detector shows the interface approaching 1.0 meter above the suction, with the ODMCS confirming the effluent is still clean. The Oil Record Book Part II entry records the slop tank identity, the before-and-after interface ullages, the roughly 10 meters of water depth discharged converted to cubic meters by the tank tables, the start and stop positions and times, and the ODMCS-operating confirmation. Every figure in that entry traces to a real instrument reading: the interface depths to the Reg 32 detector, the discharged volume to the tank tables and the ODMCS count, the limit compliance to Reg 34.
That margin discipline is the practical heart of Reg 32. The detector lets the officer choose a clean-water margin as a deliberate number, decant to it, and stop with the boundary still safely above the suction, rather than chasing the interface down until oil appears in the meter. The instrument converts a risky operation into a measured one.
Limitations
The interface detector finds a boundary; it does not judge whether the water below that boundary is clean enough to discharge. A slop layer can stratify into oil and water with the water still carrying dissolved or finely emulsified hydrocarbon above the Regulation 34 limit even though the gross interface is well above the suction. That is precisely why Reg 32 does not stand alone: the Reg 31 ODMCS measures the actual oil content of the effluent, the thing the interface detector cannot read. An operator who treats a high interface as a license to decant freely, without watching the oil content meter, has misunderstood what the detector tells them.
The rag layer is the detector’s hardest target. A thick, stable emulsion band between the oil and water gives no sharp boundary for a capacitance, conductivity, or optical sensor to lock onto, and the instrument may report a soft or ambiguous reading across that band. Heavy crudes, weathered residues, and slops that have been agitated by rough weather emulsify worse and rag worse. In those tanks the prudent operator reads the detector as the top of the safe-decant zone, not as a precise line, and leaves a larger clean-water margin.
The instrument also depends on adequate settling time, which Reg 32 does not and cannot mandate. A tank decanted before it has separated has no clean interface to find; the detector will read whatever partial stratification exists, and the decant will run into emulsion. Settling time is an operational judgment that varies with oil type, temperature, and ship motion, and no gauge substitutes for it. A detector reading taken too soon is accurate about a tank that is not yet ready.
Fouling and calibration are continuing limitations of any sensor that lives in an oily, waxy, washing-blasted tank. A capacitance probe coated in an oily film, a conductivity tip bridged by emulsion, or an optical window fouled by wax can read confidently and wrongly. The regulation’s answer is type-approval, calibration discipline, and, on many tankers, a portable detector cross-checking a fixed one, but none of those removes the underlying reality that the instrument can deceive a crew that does not maintain and verify it. The detector is a measurement aid, not an autopilot; it informs the operator’s decision and does not replace the operator’s judgment about a tank that does not behave as expected.
Finally, Reg 32 is an equipment-carriage and equipment-use requirement, not a discharge authorization in itself. Carrying an approved, working interface detector does not by itself make any discharge legal. The discharge is lawful only when the Regulation 34 conditions on area, distance, speed, rate, and total quantity are all met, with the Reg 31 ODMCS operating and the operation recorded in the Oil Record Book Part II under Regulation 36. The detector is one necessary instrument in that chain, not the authorization at the end of it.
Calibration, maintenance, and verification
An approved detector is only as good as its last verified calibration, and Reg 32’s requirement that the instrument be effective is read by inspectors as a requirement that it be maintained, not merely owned. Calibration ties the instrument’s signal to a true depth and a true oil-versus-water decision. For a portable detector read off a marked tape, calibration covers the tape datum and the sensor’s transition behavior; for a fixed installation it covers the sensor heights, the readout scaling, and the alarm or indication thresholds. Operators keep a calibration record that an officer or a surveyor can produce on request, because an instrument with no calibration history is an instrument whose readings cannot be defended.
Routine maintenance follows the tank environment. The probe and any in-tank sensor get cleaned of oily film and wax that would shift a capacitance or optical reading; the cable and connectors are checked for the cracking and corrosion that a salt and hydrocarbon atmosphere causes; the battery on a portable unit is checked and changed only with the type the intrinsic-safety certificate names, because the wrong cell voids the safety case. The vapor-lock or sounding-pipe fitting the portable probe passes through is kept clear and gas-tight, so that a reading can be taken without breaking the tank’s inert seal. None of this is exotic, but it is the difference between a detector that gives a trustworthy interface on the day a decant is needed and one that reads plausibly while being wrong.
Verification before a critical decant is its own habit. Many chief officers function-check the detector on deck before lowering it, confirm the tape datum against a known reference, and, where a fixed unit is the primary instrument, cross-check it with a portable detector at the first reading of the operation. The cross-check costs minutes and catches a fouled fixed sensor before it sends the decant past the interface. A detector that disagrees with the cargo-tank gauging on the oil-surface position, or with a second instrument on the interface depth, is a detector to distrust until the disagreement is explained, not a number to write into the Oil Record Book.
How the requirement evolved
Reg 32 did not appear new at the 2004 renumbering. The provision it carries was already in MARPOL 73/78 Annex I from the start, under the old Regulation 15, which grouped the retention-of-oil-on-board equipment for the cargo area: the oil discharge monitoring and control system, the slop-tank arrangement, the oil/water interface detector, and the associated piping. When the consolidated Annex I was reorganized into chapters and adopted by Resolution MEPC.117(52) in 2004, entering force in 2007, that single dense Regulation 15 was split across several stand-alone regulations in Chapter 4: the slop-tank arrangement became Regs 29 and 30, the ODMCS became Reg 31, the interface detector became Reg 32, and the discharge criteria became Reg 34. The renumbering made the Annex easier to read and cite, one subject per regulation, without changing the technical requirements.
That history matters for two practical reasons. First, older approval certificates, surveys, and ship documents cite the interface detector under Regulation 15(3)(b), and a surveyor reading a vintage file should recognize that as the same requirement Reg 32 now carries. Second, the type-approval specification predates both numberings: MEPC.5(XIII) was adopted on 13 June 1980, before the consolidated reorganization and indeed in the early life of MARPOL 73/78 itself. A detector approved decades ago to MEPC.5(XIII) remains validly approved under Reg 32, because Reg 32 points to that specification, not to a newer one. The instrument’s physics has not changed: oil still floats on water, and the boundary still has to be found.
The continuity also explains why the interface detector sits alongside instruments of very different sophistication. The Reg 31 ODMCS has been revised repeatedly, most recently to the MEPC.108(49) specification of 2003 for tankers built from 2005, as oil-content metering improved. The interface detector specification has been stable since 1980 because the measurement it makes is simpler and the physics is fixed. Reg 32 is one of the parts of Annex I where a 1980 specification still governs current equipment, a reminder that not every safeguard in the Annex needs constant revision to remain fit for its job.
See also
- MARPOL Annex I: the full Annex on prevention of pollution by oil, of which Reg 32 is the cargo-area interface-detector requirement.
- The MARPOL Convention: the parent instrument and its structure of annexes.
- MARPOL Annex I Regulations 29 and 30: slop tanks and piping: the settling tank and pumping arrangement whose interface the detector reads.
- MARPOL Annex I Regulation 31: the ODMCS: the oil discharge monitoring and control system that works with the detector to control a decant.
- MARPOL Annex I Regulation 33: crude oil washing: the alternative cargo-residue method that reduces the slop the detector then manages.
- MARPOL Annex I Regulation 34: cargo-area discharge limits: the legal envelope every slop decant must stay inside.
- MARPOL Annex I Regulation 36: the Oil Record Book Part II: where the decant the detector controls is recorded.
- Port State Control: the inspection regime that checks the detector’s presence, approval, and working order.