Three fixed oxygen transmitters dominate the marine-cold-storage and engine-room shortlist as of 2026-09-17: the CAATM IP66 O2 unit with wireless remote commissioning, the CAATM industrial fixed O2 detector (same 0-30% VOL, IP66 stainless housing) and the Safegas SG100 (IP66/IP68, IECEx/ATEX Ex db IIC T6 Gb) [S1][S2][S3].
Selection on a vessel is not a generic "buy a gas detector" decision: the unit must survive salt spray, 5-15 Hz vibration from the main engine, and the confined-space rules that govern entry into cargo holds and reefer compartments [S3]. For a fuller primer on instrument family, see the oxygen detector reference.
Core spec envelope that wins a P/O in 2026
All three current marine-oriented O2 transmitters publish an identical measuring range of 0-30% VOL O2, the band that covers normal air (20.9% VOL), OSHA's lower alarm threshold at 19.5% VOL, and the upper fire-risk threshold at 23.5% VOL named in the SG100 datasheet [S3]. Resolution sits at 0.1% VOL on the SG100, with stated accuracy of ≤ ±2-5% FS depending on the electrochemical cell selected [S3].
Enclosure protection is the harder differentiator. CAATM specifies IP66 in stainless steel anti-corrosion housing for its marine cold-storage O2 unit [S1][S2][S4], while the Safegas SG100 steps up to IP66/IP68 (submersible conditions) with a die-cast aluminum body and an optional SS316 variant for corrosive sites [S3]. For a primer on sensor categories across gases, see gas detector.
Hazardous-area certification: why IECEx/ATEX marks the line
The SG100 carries explicit Ex db IIC T6 Gb / Ex db ia IIC T6 Gb markings under both IECEx and ATEX schemes, qualifying the unit for Zone 1 and Zone 2 [S3]. That dual-cert is the practical differentiator when the O2 detector is sited near diesel bunker lines, paint lockers, or battery rooms, where an un-rated IP66-only unit is a procurement rejection waiting to happen. CAATM also ships a "CCS Certified Marine Combustible Gas, Oxygen (O2), R22 Refrigerant Gas Detection System" as part of its marine catalogue, addressing the ship-classification gate directly [S1].
Operating-temperature limits of -20 °C to +50 °C on the SG100 cover most non-Arctic vessel routes; cold-storage reefer holds on reefer ships sit at -25 °C to -30 °C and are addressed instead by detectors mounted in adjacent service spaces with sample draw, not by the sensor head inside the hold [S3].
Wiring, outputs, and how the O2 detector talks to the bridge
Output topology separates the marine-grade units. The SG100 ships 4-20 mA (3-wire), RS485 Modbus, and 1-5 V as standard, with HART and either LoRa or 4G wireless as factory options, plus three programmable relays (low, high, fault) for direct horn/fan/valve actuation [S3]. CAATM's marine O2 detector offers wireless remote commissioning via handheld remote and an HD digital tube display, with the same alarm-relay logic implied in the controller line-up (CA-91A / CA-91D / CA-91K) [S1][S2][S4].
Supply voltage on the SG100 is 12-24 V DC, well-matched to a vessel's 24 V emergency bus, with power consumption under 2.5 W for the electrochemical O2 cell and under 4 W for flammable-gas operation [S3]. Cable entries are 1 x 3/4 NPT and 1 x M20, the standard marine-gland complement [S3]. For a deeper look at the analog-vs-digital layer, the dissolved-oxygen-meter reference covers related 4-20 mA loops.
Decision matrix: CAATM IP66 O2 vs CAATM industrial fixed O2 vs SG100
Lining the three units up on four shipboard decision criteria: (1) Hazardous-area rating: only the SG100 publishes explicit IECEx/ATEX Ex db IIC T6 Gb for Zone 1/2 [S3]. (2) Enclosure: CAATM both = IP66 stainless; SG100 = IP66/IP68 aluminum with SS316 option [S1][S2][S3]. (3) Output richness: SG100 leads with 4-20 mA + RS485 + 1-5 V + HART + LoRa/4G + 3 relays; CAATM emphasizes wireless remote commissioning and HD digital display [S3][S1]. (4) Class-society angle: CAATM carries a CCS-certified marine multi-gas system line that the SG100 datasheet does not mention [S1][S3].
For a reefer ship with class-cert pressure, the CAATM CCS line is the lower-friction P/O. For an offshore platform, FPSO, or petrochemical barge requiring Zone 1 certification and Modbus integration to the DCS, the SG100 is the safer spec. For cost-sensitive coastal vessels where only IP66 and an O2 alarm are needed, the CAATM IP66 O2 unit is the practical pick [S1][S2][S3]. For related marine safety and HVAC choices, the marine HVAC and marine valve references cover adjacent spec territory.
Why the safe band matters: alarm philosophy
SG100 documentation spells out the three-band alarm logic the rest of the industry also follows: oxygen deficiency below 19.5% VOL (loss of coordination, unconsciousness, death below 10% VOL), normal at 20.9% VOL, and enrichment above 23.5% VOL with sharply elevated ignition risk [S3]. Confined-space entry, inert-gas purging with N2/Ar/CO2, and welding in enclosed areas are the three common onboard drivers of an O2 alarm, so detector placement at tank manholes, reefer access hatches, and CO2 scrubber rooms covers most of the exposure [S3].
The wireless remote commissioning feature on both CAATM units, via a magnetic wand or handheld remote on the SG100, lets the crew calibrate or change setpoints without opening the housing, which is the key advantage in a salt-spray environment where every cover-open is a corrosion event [S1][S2][S3]. For dust-loaded adjacent spaces such as bulk-cargo corridors, the dust detector primer covers the related particulate-monitoring spec.
What to track next on the marine O2 detector
Two signals worth watching through 2026 Q4: (1) CCS, DNV, and Lloyd's class-society published lists of accepted marine gas-detection systems, which gate which IP66/68 + ATEX O2 units can be specified for newbuild contracts [S1]; (2) HART and Modbus integration on marine alarm controllers (CA-91D / CA-91K with 10-detector RS485 bus), which is moving the bus topology from 4-20 mA point-to-point to RS485 multi-drop on retrofits [S1]. Procurement should re-verify the SG100 SS316 option lead time and the CAATM 5-day delivery window against the next docking schedule. For a broader gas-detection sourcing view, the toxic gas detector suppliers map and the combustible gas detector sourcing map cover adjacent 2026 supply-chain ground.