Commercial Diving Glossary: 80 Terms in Your Scope of Work, Decoded
By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.
Commercial Diving Contractors Alabama | Commercial Diving Contractors Alaska On the piling job in Corpus Christi, the current was running two knots and my tender kept shouting, 'What's your pneumo?' I was 90 feet down, hanging onto a downline, and I had no idea what he meant. That was 28 years ago. I've never made that mistake again. Since then I've logged more than 4,000 dives across dams, bridges, water tanks, and power plant intakes. I've been a supervisor for the last 19 of those years, and I've come to realize the hardest part of the job isn't the pressure — it's the language. The terms in a scope of work can be as foreign as a codebook if you haven't been on the deck. So this is a glossary I wish someone had handed me on that first day, decoded the way I'd say it over the comms.
The Rig Above the Water
When you turn up for a job, the first thing you talk about is the surface spread. That's the boat or barge, the compressor, the HP or LP air supply, and the umbilical that feeds the diver below. The diver wears a helmet — call it a Kirby Morgan, even if it's another brand — and a full dress of gear that includes a dry suit, weights, and a harness. The umbilical bundles air, comms, and a pneumo line, which reads depth back to the surface. The tender minds the umbilical and the pneumo, and the supervisor runs the show. A stage is the metal platform you ride down, and the basket is the one you get hauled up in if you can't make it on your own. Beyond the basics, you've got the winch and the crane, the taglines, the shackles, and the mooring lines. I've seen a diver get dragged by a taut tagline because it was snagged on a valve, and the tender didn't spot it. You learn to check every line like it's the last one you'll ever trust. The whole spread is sometimes just called the rig. Depends on who you're working for.
Gas and the Deep
Most inshore work is air diving — 21% oxygen, 79% nitrogen. Go deeper than 150 feet or so, and you start looking at nitrox, heliox, or trimix. The number that matters is PPO2 — the partial pressure of oxygen. You want it between 0.16 and 1.4 bar. Too low, you're hypoxic; too high, you risk oxygen toxicity. Dive computers and gas analyzers keep us honest, and so do the old dive tables. MOD is the maximum operating depth for your mix, and EAD is the equivalent air depth that gives you the same narcotic effect. If you're working a long stretch, you might go to saturation, living in a pressurized habitat and riding a bell to the job site. The bends — decompression sickness — is what happens when gas bubbles out of your tissues too quickly, and that's why we make deco stops on the way up. The dive schedule tells you how long to sit at 20 feet, or 10 feet. The surface interval is how long you wait between dives, and the standby diver sits fully dressed while you're down, ready to go help. We log everything: bottom time, max depth, decompression time, mix used. If you think that's overkill, I've seen a diver with DCS screaming in the chamber at 3 a.m. It's not overkill.
The Work Itself
Underwater, the work is inspections, repairs, installations, and salvage. For inspections, we use CCTV, ultrasonic testing, and magnetic particle testing to check welds and metal thickness. Sometimes an ROV does that, and we watch from a screen topside. For repairs, we carry grinders, hydraulic torque wrenches, and saws. Exothermic cutting — burning — is for cutting steel and concrete. The burning bar is a rod that burns under water, and it works because it doesn't need an external oxygen supply. Lifting bags move heavy loads. They're filled from air bottles, and you rig them with slings and shackles. A J-tube is a curved conduit for cables, and penetration is the term for breaking into a sealed structure. It's not always dramatic. Half the time I'm sitting in the mud, using a grinder to round off a bolt so a pile can be lifted. But the vocabulary is the same, and the more precise you are, the less likely you'll be misunderstood. 'Pass me the dogging line' is a lot better than 'give me that rope.' Misunderstanding is what gets people hurt.
Safety, Codes, and the Paperwork
We work under a stack of standards. ADCI sets the training and equipment guidelines, IMCA publishes safety logs, and OSHA has its own rules under 29 CFR Part 1910. HSE is the British equivalent, and you'll see their references on international sites. Before every dive we write a JSA — job safety analysis — and we update the risk assessment as the weather changes or the tide turns. The dive plan covers everything from gas mix to emergency procedures. If we're near a chemical plant, we check for H2S and LEL. H2S is hydrogen sulfide, and LEL is the lower explosive limit of flammable gas. We keep a recompression chamber on standby, and the rescue diver is ready to go in if something goes wrong. That paperwork might look like a pain, but it's saved my life more than once. A dive log is the official record of every dive, and it's more than a memory book — it's your insurance if a question comes up years later. I keep mine in a waterproof box, and I sign off on every one of my divers' logs too. That's the discipline that keeps you working.
Frequently Asked Questions
What does 'pneumo' mean in commercial diving? Pneumo is short for pneumofathometer, the pneumatic depth gauge that measures the pressure in the dive hose and shows the depth to the surface tender. It's the primary way a tender knows your depth. Why do commercial divers use heliox or trimix instead of air? Air becomes narcotic at depth and the nitrogen can cause decompression sickness. Heliox and trimix replace part or all of the nitrogen with helium, which is less narcotic and easier to decompress from after deep dives. What's the difference between a dive supervisor and a dive tender? The supervisor plans and directs the dive, with responsibility for safety and the dive plan. The tender physically handles the diver's umbilical, maintains tension, and watches the pneumo and comms. The supervisor is the one in charge. How is 'bottom time' calculated? Bottom time is the total time from the moment the diver leaves the surface until the start of the final ascent, not counting any decompression stops. It's the core number for figuring out decompression schedules and gas consumption.