Introduction: Marine oxygen sensors come in range tiers, and the model number tells you which tier you are holding before you connect a single cable.
On a nitrogen generator skid, the oxygen sensor is a small probe with a G1/8 inch thread and a 100 cm cable, and the fastest way to tell two spares apart is the label: one reads 0-1000 ppm, another 0-25%, a third 0-96%. A shipboard electrician swapping a sensor during a port call has to know what those numbers mean before the probe goes in, because each model is built for a different job. The range tiers behind OC-38, OC-39, and OC-40 answer three different questions, their millivolt outputs follow the same ladder, and trace and bulk oxygen readings are never a straight swap.
Oxygen monitoring aboard ship splits into tiers based on how much oxygen the sampled gas actually contains. A marine nitrogen generator drives oxygen down to a trace level, so the useful reading is in parts per million. Tank and cargo-space inerting lives in the percent band, because the safety question there is whether oxygen has been pushed below a percentage threshold. High-percent monitoring covers the opposite case, where oxygen is the main gas rather than a contaminant. The three OxysenZir models map onto those three situations, and each one carries its own range on the label.
That shared hardware is exactly why a mixed-up label is easy to miss. Two sensors can thread into the same boss and reach the same terminal with the same cable length while watching completely different oxygen bands. Checking the range first is the habit that keeps a replacement in the right tier.
Each model produces its own low-level millivolt band, and the numbers move upward with the tier. OC-38 outputs 0.49 to 0.55 mV at 1000 ppm, the top of its range. OC-39 outputs 3.98 to 4.05 mV across its percent span. OC-40 outputs 4.98 to 5.05 mV. The trace sensor works in the smallest band of the three, while the two percent models sit close together but not at the same level. Those figures are worth reading as a ladder rather than as three random numbers. A trace-level measurement produces the smallest signal, the 0-25% model sits around 4 mV, and the 0-96% model sits around 5 mV. When a spare is pulled from a locker and the printed label is worn or smudged, the millivolt band is a quick way to confirm which tier the sensor belongs to, because a value below 1 mV does not belong to a high-percent probe. The published figures cover range and output only. Accuracy, response time, and compatibility with a particular monitoring setup are separate questions, so a replacement is best checked against the system requirement rather than guessed from the mV number alone. What the tiers do clearly is separate a trace job from a bulk job before any electrical question comes up.
A ppm sensor answers a purity question; a percent sensor answers an atmosphere question. On a nitrogen generator, the gas leaving the unit is supposed to be almost entirely nitrogen, so the number that matters is the small oxygen residue still inside it. A reading of a few hundred ppm is useful information, while 21% would mean the generator is not separating anything at all. In inerting duty the decision thresholds sit in whole percentages, whether a fuel tank, a cargo hold, or a ballast water treatment arrangement has been brought low enough to be safe. A reading of a few ppm tells nobody whether a space is properly inerted. The two scales also sit in very different sizes, which is why the range labels cannot be treated as interchangeable. OC-38's entire span of 1000 ppm equals just 0.1% oxygen, so a purity monitor's whole working range fits inside a tenth of one percent. OC-39's span of 25% is 250,000 ppm, which means the small shifts a purity monitor watches for fall far below its reporting scale. OC-40's 0-96% span goes the other direction and covers the oxygen-rich side of the scale, where the sample is mostly oxygen rather than mostly nitrogen. Nitrogen purity work is essentially a question of how much oxygen remains after separation. Industrial gas references describe nitrogen as an inert bulk gas with well-documented behavior, while oxygen behaves as the reactive counterpart in the same mixture. That is the background behind the tier split: the smaller the expected oxygen concentration, the finer the band the instrument has to work in. In practice, the tier decision comes before everything else. A requisition that says only "oxygen sensor" leaves the range wide open, and an electrician who fits the wrong tier ends up with a monitor that either sits at its ceiling or barely moves. Naming the model, OC-38, OC-39, or OC-40, names the range, the task, and the millivolt band in one step.
OC-38, OC-39, and OC-40 are three range tiers rather than three quality levels. OC-38 watches trace oxygen up to 1000 ppm, which is a tenth of one percent. OC-39 covers 0-25%, the band that takes in ordinary air and inerted tank atmospheres. OC-40 covers 0-96% for streams that are mostly oxygen. Their millivolt outputs climb the same ladder, from 0.49-0.55 mV through 3.98-4.05 mV to 4.98-5.05 mV. For anyone holding a replacement probe, reading the range on the label first is the simplest way to keep a ppm job and a percent job from being mixed up, and the full model listing is a useful next stop for matching a tier to a specific monitoring point.
A：They differ mainly by measurement range and output band. OC-38 covers 0-1000 ppm and outputs 0.49-0.55 mV at 1000 ppm, which places it in trace oxygen duty. OC-39 covers 0-25% and outputs 3.98-4.05 mV, and OC-40 covers 0-96% with an output of 4.98-5.05 mV. All three share a G1/8 inch thread and a 100 cm cable, so the range on the label is the main thing that separates them.
A：A ppm-level sensor is used when oxygen is a small impurity that has to be watched closely, which is the situation in nitrogen purity monitoring, where the whole span of 1000 ppm equals just 0.1% oxygen. A percent-level sensor takes over when the question is about the atmosphere itself, such as ordinary air near 21% or an inerted space held at a low percentage. Matching the sensor to the size of the number being watched keeps the reading meaningful.
A：The millivolt band identifies which tier a sensor belongs to. OC-38 works in the smallest band at 0.49-0.55 mV at 1000 ppm, OC-39 sits around 3.98-4.05 mV, and OC-40 sits slightly higher at 4.98-5.05 mV. The numbers rise with the range, so a label showing a fraction of a millivolt points to the trace model while a label near 5 mV points to the high-percent model.
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