Dive computers and gauges: what they measure and what they model.

Pressure gauges and consoles

PADI describes the submersible pressure gauge (SPG) as the instrument that shows how much gas is left in the cylinder. It can be a mechanical gauge on a hose, reading in bar or psi, or built into a dive computer, and it comes in analogue and digital forms. PADI says it should be secured so that it does not dangle, add drag, hit marine life or get damaged.

A console, in PADI's description, combines the SPG, a dive computer or depth gauge and a compass in one unit. In hoseless air integration, a transmitter on the regulator's first stage sends cylinder pressure wirelessly to the computer.

What a dive computer does

PADI says a dive computer combines depth and time readings with a decompression model that estimates the body's nitrogen uptake and shows how much no-stop time remains. It can stand in for a separate depth gauge and timer, and sometimes for the SPG too.

PADI lists these as standard features:

• Depth, dive time and no-stop time remaining
• Ascent rate
• Emergency decompression information
• A record of previous dives
• A low-battery warning
• Nitrox compatibility

Optional features PADI names include air integration, watch formats, altitude adjustment, multi-gas and rebreather modes, a compass, data download and displays mounted on the mask. Reading and acting on any of these is taught in training; this page does not explain dive planning or decompression.

Algorithms

PADI's blog (Megan Denny, 2026) explains that computers model how gas enters and leaves the body using theoretical tissue compartments, some using 16, and take previous dives into account. PADI says computers replaced its Recreational Dive Planner table and The Wheel, and each computer uses its own algorithm. PADI dates the first recreational dive computers to the 1980s, with air integration, ascent-rate indicators and nitrox modes added in the 1990s.

Makers describe their models in their own terms. Suunto, for example, says:

• Its decompression modelling began in the 1980s with a Buhlmann M-value model, and in the late 1990s it adopted Dr Bruce Wienke's Reduced Gradient Bubble Model (RGBM).
• Its Fused RGBM 2 is a bubble model developed with Dr Wienke, using 15 tissue groups, tracking nitrogen and helium separately, for open and closed circuit to 150 m. That is the maker's claim.
• Its Buhlmann 16 GF option is based on Swiss physician Dr Albert A. Buhlmann's ZHL-16C, a dissolved-gas model with 16 theoretical tissue groups whose half-times run from about 4 to 635 minutes, with conservatism set by gradient factors. Suunto flags non-default settings as outside its recommendation.

Guidance, not guarantees

The bodies and makers agree on one point. PADI (2026) says a computer cannot account for a diver's age, fitness or alcohol intake, and quotes DAN that algorithms give guidance, not guarantees. Suunto states that all decompression models are theoretical and that none can guarantee a diver will not get decompression sickness.

What an independent study found

A 2018 study by Doug Fraedrich in Diving and Hyperbaric Medicine (volume 48, issue 4) tested four algorithms, ZHL-16C, VPM-B, Suunto-RGBM and the Cochran EMC-20H, against reference criteria. At factory-default settings, none of the four passed every test. The study classes VPM-B and Suunto-RGBM as bubble (dual-phase) models and ZHL-16C as a dissolved-gas model, and found that VPM-B's first stop was always deeper than its reference.

That links to an open question. Retailer and training-blog sources note that recent research questions the benefit of deep stops, which bubble models such as VPM-B tend to add. This site does not settle that debate; it is a matter for decompression research and for the reader's training.

EN 13319: what the standard covers, and what it does not

The European standard for these instruments is EN 13319:2000, "Diving accessories, Depth gauges and combined depth and time measuring devices, Functional and safety requirements, test methods", from committee CEN/TC 136. The Estonian catalogue lists it valid from 1 October 2000 and links it to the EU General Product Safety Regulation 2023/988.

It covers depth gauges, the depth function of other instruments, and depth-and-time functions where the instrument counts dive time automatically, for instruments that work out depth from the surrounding pressure. It expressly excludes any other information, including any decompression information a device shows. So EN 13319 says nothing about a dive computer's decompression calculations; only the depth and time readings fall within it.

Rental computers and care

DAN's Alert Diver (2023) says a diver using a rented computer should be able to read its key displays and alerts, and should check its gas setting and transmitter pairing, because some computers keep the previous user's settings. More is on renting or owning. For care, PADI says to rinse a computer in fresh water after use, keep it out of direct sun, protect it from drops, store it cool and dry, and handle batteries as the maker instructs.