Dive Computer FAQ

Bühlmann (usually the ZHL-16C variant) is a "dissolved gas" model that tracks how much inert gas is loaded into a set of theoretical tissue compartments and lets you dial conservatism up or down with gradient factors. RGBM and VPM-B are "bubble" models that assume microscopic bubbles are present from the start of a dive and try to control their growth, which typically pushes for deeper, shorter initial stops and less time shallow compared to a Bühlmann profile at the same conservatism. In practice, forum consensus is that for recreational no-decompression diving the differences between these are small enough that they rarely change what a diver actually experiences. No-stop limits vary by only a minute or two between comparable settings.

Where divers do care is in technical and decompression diving, where the shape of the ascent (deep stops vs. shallow-weighted profiles) becomes a real philosophical debate, and where being able to adjust gradient factors on a Bühlmann computer is often valued more than the underlying model itself. Most experienced divers will tell a new buyer not to overthink the algorithm choice for recreational use, but to care more about whether the computer lets you adjust conservatism at all.

Sources: ScubaBoard: "RGBM vs Buhlmann"; ScubaBoard: "VPM-B vs Buhlmann GF"; r/scuba: "Buhlmann ZHL-16C vs Suunto rgbm"

libdivecomputer is an open-source library that a range of independent dive-log apps (Subsurface, and newer BLE-focused apps like Currents, which I also build) use to talk directly to dive computers over Bluetooth, instead of requiring the manufacturer's own app. Divers who bring it up on forums generally aren't anti-manufacturer-app on principle. The concern is about being stuck if a brand's app is discontinued, gets a bad redesign, or simply doesn't support a phone or OS a diver wants to use.

Having open BLE support means your dive log data isn't hostage to one company's software roadmap, and it opens the door to community dive-log tools that some divers simply prefer for cross-platform logging or trip analysis. The tradeoff some point out is that third-party support for a brand-new model can lag behind release by months, since libdivecomputer support has to be reverse-engineered or added by the manufacturer after the fact, so buyers who want day-one BLE compatibility with tools like Subsurface should check current support status rather than assume it.

Sources: ScubaBoard: "Suunto diving computer and PC sync issues"; r/scuba: "Looking for a dive computer with 3D dive mapping + Subsurface compatibility"

Opinions here split along a fairly predictable line. Divers who've had good luck with modern wireless transmitters (the kind that thread onto a first stage's HP port) report that battery awareness is really the only maintenance involved, and that once paired, reliability is a non-issue. They cite the value of always having tank pressure and SAC rate visible on the wrist without craning to find a console. Others report intermittent dropouts, especially with older or budget transmitters, and note that a transmitter is one more battery, one more O-ring, and one more point of failure compared to a hard-wired hose gauge.

A common piece of practical advice on the forums is to add a short (around 6 inch) high-pressure hose between the transmitter and the first stage rather than screwing it in directly, both to protect the transmitter from being grabbed or knocked and to make it easier to read. The general takeaway: wireless air integration is convenient and, on current-generation transmitters, reasonably reliable, but most divers still treat a mechanical SPG (or at least a backup gauge) as a sensible redundancy rather than a nice-to-have.

Sources: ScubaBoard: "I've had it with wireless air integration"; ScubaBoard: "Extension hose for wireless air transmitter"; r/scuba: "Transmitter users, do you still carry a classic SPG?"

The consistent advice from more experienced forum members is no: don't pay a premium for CCR support "just in case" you go to a rebreather someday. The reasoning is practical: CCR mode on a dive computer typically needs to be paired with a specific rebreather's electronics (often via a particular connector or protocol), so a CCR-capable computer bought years before you actually own a rebreather may not even be compatible with the unit you eventually choose. Divers who do end up going the CCR route are generally better served buying the computer their instructor or rebreather manufacturer recommends at that time.

Where CCR capability does make sense as a purchase driver is for divers who already dive open-circuit on a computer capable of running the full range of technical modes (like the Shearwater Peregrine/Perdix family) and like the idea of a single computer that can grow with them into CCR later as a backup or bailout display, but that's a different case from paying extra purely for a feature you won't use for years, if ever.

Sources: ScubaBoard: "Upgrading Computer Peregrine vs Perdix"; ScubaBoard: "Shearwater Computers and CCR"; r/scuba: "Advice for new Dive computer" (on not overbuying features you don't need yet)

This is one of the most argued-about specs on ScubaBoard, and it comes down to two different kinds of risk. User-replaceable batteries (usually a coin cell or AA) can be swapped in minutes anywhere in the world, which divers repeatedly point to as the deciding factor for liveaboards and remote trips where a dead battery could otherwise cost you days of diving. The tradeoff is that opening the battery compartment yourself introduces a small flood risk if the O-ring isn't seated or replaced correctly.

Rechargeable batteries are simpler day to day (no compartment to open, no spare cells to carry) and are what almost all color-screen, always-on-display computers use since a coin cell can't drive that kind of screen for long, but divers report the real risk is logistical: forgetting to charge between dive days, running out of charge mid-trip with no easy fix, and (over a period of years) the battery's maximum capacity fading, at which point some models need a service center to replace it rather than a diver doing it themselves. Neither option is considered universally "better"; the common framing is that user-replaceable suits travel and worst-case reliability, while rechargeable suits divers who dive locally and are disciplined about charging routines.

Sources: ScubaBoard: "User replaceable battery vs rechargeable?"; ScubaBoard: "Are replaceable batteries in a dive computer better than rechargeable?"; r/scuba: "User-replaceable battery vs sealed rechargeable battery life span"

Divers who've used both are fairly positive about color, particularly AMOLED, for underwater legibility: it's typically an emissive, always-lit display, so there's no fumbling for a backlight button, and the higher contrast makes small numbers and color-coded warnings (like a red ascent-rate alert) easier to read at a glance than on a traditional monochrome LCD. The cited downsides are battery life and, for some divers, glare or reduced contrast against very bright water or backscatter in bright sun. LCD screens are sometimes reported as being easier to read in direct sunlight at the surface despite generally lower contrast underwater.

Color screens also tend to correlate with larger housings ("watch style" computers) and rechargeable batteries, since a coin-cell-powered puck computer generally can't drive a large always-on color screen. For a lot of divers the honest answer is that color is a genuine comfort and readability upgrade rather than something that changes what you can safely do underwater. A well-designed monochrome display remains perfectly functional.

Sources: ScubaBoard: "Cons of Colour Screen Dive Computers?"; ScubaBoard: "LED LCD vs OLED screen"

For divers staying within recreational no-decompression limits (roughly 40m/130ft and shallower), most computers on the market comfortably cover the relevant range, and the difference between, say, a 100m-rated and a 200m-rated computer is largely irrelevant day to day. Where it starts to matter is at the margins: some budget recreational computers will display an error, lock out, or stop giving reliable no-stop guidance once you exceed their rated operating depth, which matters more to divers who occasionally push past standard recreational limits (deep specialty dives, wall dives, or the odd accidental deep excursion) than to divers who stick to typical reef and wreck depths.

For technical and deep divers, depth rating is a much bigger deal since decompression dives routinely exceed 40-50m and a computer that caps out or misbehaves near its rated limit isn't just an inconvenience, it's a safety issue. The general guidance from experienced forum members is to buy comfortably past whatever depth you actually plan to dive, since the cost difference between depth-rating tiers is usually small relative to the computer's overall price.

Sources: ScubaBoard: "Buying a Petrel 2 or Perdix"; ScubaBoard: "History behind Dive computer error mode"

On computers that expose Bühlmann gradient factors, the common default pairing discussed on forums is something like 40/85 (medium), with more conservative divers running lower numbers like 35/75 and more liberal, typically more experienced technical divers running settings closer to 45/95 for recreational no-stop diving.

The most frequently repeated advice for new divers is not to chase the most conservative setting by default, since overly conservative settings mostly just shorten no-stop times without a clear safety benefit for depths and profiles well within recreational limits, and not to obsess over the exact numbers at all until you're diving decompression or repetitive multi-day profiles where conservatism choices compound. Divers with a personal DCS risk factor (age, fitness, dehydration-prone destinations, cold water) commonly report choosing a slightly more conservative setting as a matter of personal margin rather than because the "correct" setting changed.

Sources: ScubaBoard: "Gradient Factors - What is Everyone Using?"; ScubaBoard: "Diving with gradient factors for a new recreational diver"; r/scuba: "gradient factor conservatism setting for new diver"

This is largely a matter of preference rather than function, but a few practical patterns show up repeatedly. Puck-style computers (worn on a hose console, or on the wrist with a strap kit) tend to be less expensive and use small, monochrome, low-power displays, which some divers like for reliability and long battery life but others find harder to read compared to a full watch face.

Watch-style computers are larger, can be worn day-to-day like an actual watch, and are the form factor that supports bigger, color, always-on displays, at a price premium and typically with a rechargeable battery. Divers with smaller wrists sometimes report that the largest watch-style computers feel bulky or catch on wetsuit cuffs, while puck-style computers sidestep that by living on a console or a simple wrist mount. Neither format changes the underlying dive-planning function, so the choice mostly comes down to how you want to wear it and what display/battery tradeoffs you're willing to accept for that.

Sources: ScubaBoard: "Puck vs. Dive Computer"; r/scuba: "Console Computer or Skip It?"